Facts on File Dictionary of Weather and Climateunivpgri-palembang.ac.id/perpus-fkip/Perpustakaan/Kamus... · The Facts On File Dictionary of Weather and Climate Revised Edition

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The Facts On FileDICTIONARY

ofWEATHER

and CLIMATE

Revised Edition

FREEDOM PALESTINE


FREEDOM PALESTINE


The Facts On FileDICTIONARY

ofWEATHER

and CLIMATE

Edited byJacqueline Smith

Revised Edition

FREEDOM PALESTINE


The Facts On File Dictionary of Weather and ClimateRevised Edition

Copyright © 2001, 2006 by Market House Books Ltd

All rights reserved. No part of this book may be reproduced or utilized in anyform or by any means, electronic or mechanical, including photocopying,recording, or by any information storage or retrieval systems, withoutpermission in writing from the publisher. For information contact:

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This book is printed on acid-free paper

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PREFACE

This dictionary is one of a series designed for use in schools. It is intended for stu-dents of meteorology and climatology, but we hope that it will also be helpful toother science students and to anyone interested in science. Facts On File also pub-lishes dictionaries in a variety of disciplines, including astronomy, biology, chem-istry, computer science, Earth science, physics, mathematics, forensic science, andmarine science.

This second Facts On File edition has been extensively revised and extended. The dic-tionary now contains over 2,000 headwords covering the terminology of modernmeteorology and climatology. A totally new feature of this edition is the inclusion ofover 800 pronunciations for terms that are not in everyday use. The Appendix con-tains a chronology of important events and discoveries in the subject, a bibliography,and relevant Web sites, as well as a number of useful conversion tables. A guide tousing the dictionary has also been added to this latest version of the book.

We would like to thank all the people who have cooperated in producing this book.A list of contributors is given on the acknowledgments page. We are also grateful tothe many people who have given additional help and advice.

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ACKNOWLEDGMENTS

Editors (previous editions)

Stella Stiegeler B.Sc.John O. E. Clark B.Sc.

Contributors

B. M. Abbott B.Sc., Ph.D., F.G.S.Anna Clyde B.Sc.Andrew Hill B.Sc., Ph.D., F.G.S.I. P. Joliffe B.Sc., M.Sc, Ph.D., C.I.C.E., F.C.S.,

F.R.G.S.R. B. Lanwarne B.Sc.P. A. Smithson B.Sc., Ph.D., F.R.Met.S.T. J. Speechley B.Sc.F. A. Sultan B.Sc., M.Sc., D.I.C.S. D. Weaver B.Sc., Ph.D., F.G.S.A. C. Wornell B.A.

Pronunciations

William Gould B.A.

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CONTENTS

Preface v

Acknowledgments vi

Guide to Using the Dictionary viii

Pronunciation Key x

Entries A to Z 1

Appendixes

I. Chronology 245

II. Conversion Tables 253

III. Temperature Conversions 258

IV. Web Pages 259

Bibliography 261

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GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

HeadwordsThe main term being defined is in bold type:

aggregation The process by which icecrystals grow to form snowflakes throughcollision and clumping together.

VariantsSometimes a word has a synonym or alternative spelling. This is placed in brackets afterthe headword, and is also in bold type:

coreless winter (kernlose winter) Aunique feature of the climate in Antarctica,the condition in which surface tempera-tures remain almost the same from the au-tumn equinox to the end of winter.

Here, ‘kernlose winter’ is another term for ‘coreless winter’. Generally, the entry for thesynonym consists of a simple cross-reference:

kernlose winter See coreless winter.

AbbreviationsAbbreviations for terms are treated in the same way as variants:

actual evapotranspiration (AE) Thetrue amount of water vapor returned to theatmosphere through evaporation from aground surface and transpiration from itsplant cover where the water supply is lim-ited... .

The entry for the abbreviation consists of a reference:

AE See actual evapotranspiration.

Multiple definitionsSome terms have two or more distinct senses. These are numbered in bold type

offshore current 1. A current in the sea or ocean that flows parallel to thecoast.2. Any current that flows away from shore.

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Cross-referencesThese are references within an entry to other entries that may give additional usefulinformation. Cross-references are indicated in two ways. When the word appears in thedefinition, it is printed in small capitals:

anticyclonic rotation The direction ofthe flow around an ANTICYCLONE (high).

In this case there is a cross-reference to the entry for ‘anticyclone’.

Alternatively, a cross-reference may be indicated by ‘See’, ‘See also’, or ‘Compare’, usu-ally at the end of an entry:

degree 1. A unit of measure on a tem-perature scale. See Celsius scale; Fahren-heit scale; Rankine scale.

Hidden entriesSometimes it is convenient to define one term within the entry for another term:

pilot balloon A small meteorologicalballoon, inflated with hydrogen or helium,and used for visually observing the upperwind speed and direction above a station.... . A pilot balloon observation (pibal), themeasurement and calculation of the windspeed and direction, is made by reading theelevation and azimuth angles of a theodo-lite while ... .

Here, ‘pilot ballon observation’ and ‘pibal’ are hidden entries under ‘pilot baloon’, andare indicated by italic type. Individual entries consist of a simple cross-reference:

pibal See pilot balloon.

PronunciationsWhere appropriate pronunciations are indicated immediately after the headword, en-closed in forward slashes:

halocarbon /ha-loh-kar-bŏn/ A chemi-cal compound consisting of carbon, possi-bly hydrogen, and any of the halogens, i.e.chlorine, fluorine, bromine, or iodine. Seealso chlorofluorocarbon; halon; hydro-chlorofluorocarbon.

Note that simple words in everyday language are not given pronunciations. Also head-words that are two-word phrases do not have pronunciations if the component words arepronounced elsewhere in the dictionary.

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/a/ as in back /bak/, active /ak-tiv//ă/ as in abduct /ăb-dukt/, gamma /gam-ă//ah/ as in palm /pahm/, father /fah-ther/,/air/ as in care /kair/, aerospace /air-ŏ-

spays//ar/ as in tar /tar/, starfish /star-fish/, heart

/hart//aw/ as in jaw /jaw/, gall /gawl/, taut /tawt//ay/ as in mania /may-niă/ ,grey /gray//b/ as in bed /bed//ch/ as in chin /chin//d/ as in day /day//e/ as in red /red//ĕ/ as in bowel /bow-ĕl//ee/ as in see /see/, haem /heem/, caffeine//kaf-een/,/ baby /bay-bee//eer/ as in fear /feer/, serum /seer-ŭm//er/ as in dermal /der-măl/, labour /lay-ber//ew/ as in dew /dew/, nucleus /new-klee-ŭs//ewr/ as in epidural /ep-i-dewr-ăl//f/ as in fat /fat/, phobia /foh-biă/, rough

/ruf//g/ as in gag /gag//h/ as in hip /hip//i/ as in fit /fit/, reduction /ri-duk-shăn//j/ as in jaw /jaw/, gene /jeen/, ridge /rij//k/ as in kidney /kid-nee/, chlorine /klor-

een/, crisis /krÿ-sis//ks/ as in toxic /toks-ik//kw/ as in quadrate /kwod-rayt//l/ as in liver /liv-er/, seal /seel//m/ as in milk /milk//n/ as in nit /nit//ng/ as in sing /sing/

/nk/ as in rank /rank/, bronchus /bronk-ŭs//o/ as in pot /pot//ô/ as in dog /dôg//o/ as in buttock /but-ŏk//oh/ as in home /hohm/, post /pohst//oi/ as in boil /boil//oo/ as in food /food/, croup /kroop/, fluke

/flook//oor/ as in pruritus /proor-ÿ-tis//or/ as in organ /or-găn/, wart /wort//ow/ as in powder /pow-der/, pouch

/powch//p/ as in pill /pil//r/ as in rib /rib//s/ as in skin /skin/, cell /sel//sh/ as in shock /shok/, action /ak-shŏn//t/ as in tone /tohn//th/ as in thin /thin/, stealth /stelth//th/ as in then /then/, bathe /bayth//u/ as in pulp /pulp/, blood /blud//ŭ/ as in typhus /tÿ-fŭs//û/ as in pull /pûl/, hook /hûk//v/ as in vein /vayn//w/ as in wind /wind//y/ as in yeast /yeest//ÿ/ as in bite /bÿt/, high /hÿ/, hyperfine /hÿ-

per-fÿn//yoo/ as in unit /yoo-nit/, formula /form-yoo-lă//yoor/ as in pure /pyoor/, ureter /yoor-ee-ter//ÿr/ as in fire /fÿr//z/ as in zinc /zink/, glucose /gloo-kohz//zh/ as in vision /vizh-ŏn/

x

A consonant is sometimes doubled to prevent accidental mispronunciation of a syllableresembling a familiar word; for example, /ass-id/ /acid/,rather than /as-id/ and /ul-tră-/sonn-iks/ /ultrasonics/, rather than /ul-tră-son-iks/. An apostrophe is used: (a) betweentwo consonants forming a syllable, as in /den-t’l/ /dental/,and (b) between two letters whenthe syllable might otherwise be mispronounced through resembling a familiar word, as in/th’e-ră-pee/ /therapy/ and /tal’k/ /talc/. The symbols used are:

Pronunciation Key

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AAO See Antarctic Oscillation.

ablation The disappearance of snowand ice through the processes of melting,evaporation, or sublimation; the term mayalso be applied to the calving of glaciers,removal of snow and ice by the wind (de-flation), and avalanching. The rate of abla-tion is controlled principally by airtemperature; solar radiation, humidity,rainfall, and wind speed also contribute tothe process. Ablation usually refers to sur-face snow and glacier ice but is sometimesapplied to ice crystals and snowflakes inthe atmosphere.

absolute drought In the UK, aDROUGHT in which there is a period of atleast 15 consecutive days, none of whichhave recorded 0.25 mm (0.01 in) of rain ormore. This definition is inappropriate fordissimilar climates; it was originally de-fined in 1887 by the former British RainfallOrganisation, but is no longer much used.See also partial drought.

absolute humidity A measurement ofthe amount of moisture (WATER VAPOR) inthe atmosphere, usually expressed as themass (grams) of water vapor per cubicmeter of moist air. In contrast to RELATIVE

HUMIDITY, absolute humidity expresses theamount of water in the air regardless of thetemperature. It is sometimes referred to asvapor concentration. See also humidity;specific humidity.

absolute instability The atmosphericstate in which the observed decrease intemperature with height, as defined by theENVIRONMENTAL LAPSE RATE (ELR), isgreater than that of a dry air parcel coolingadiabatically, as defined by the dry adiabat

(9.8°C km–1). Consequently, at any heighth the air parcel will be warmer and lessdense than its surroundings, and so willcontinue to rise, leading to absolutely un-stable air, and often to the formation ofclouds of great vertical depth.

absolutely stable air See absolute sta-bility.

absolutely unstable air See absoluteinstability.

absolute momentum The sum of themomentum of a particle relative to theEarth and the momentum of the particledue to the Earth’s rotation.

absolute stability The atmosphericstate in which the observed decrease intemperature with height, as defined by theENVIRONMENTAL LAPSE RATE (ELR), is lessthan the SATURATED ADIABATIC LAPSE RATE

(SALR). Consequently, at any height h theair parcel will be cooler and thereforedenser than its surroundings. If an air par-cel is perturbed in this situation, it will re-

1

A

temperatureof environment

temperatureof air parcel

temperature

hei

gh

t h

h

DA

LR

ELR

Absolute instability

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absolute temperature

2

turn to its original position because verticalair motion cannot be sustained. Absolutelystable air typically accompanies a summerANTICYCLONE (high).

absolute temperature See thermody-namic temperature.

absolute vorticity See vorticity.

absolute zero The zero of THERMODY-NAMIC TEMPERATURE. This is the lowesttemperature theoretically possible and al-though it cannot be reached in practice,temperatures of 10–3 K are used in low-temperature physics. Absolute zero, is 0 K= –273.15°C. See kelvin.

absorption In the atmosphere, the con-version of electromagnetic radiation intointernal energy of molecules. The absorp-tion process depends on the energy levels ofthe molecule and on the wavelength of theincident radiation. For example, ozone ab-sorbs ultraviolet radiation; carbon dioxideand water vapor both absorb infrared radi-ation. Absorption occurs across the wholespectrum for electromagnetic wavelengths,although there are ‘atmospheric windows’in which little absorption occurs, notablyin the radio and visible regions of the spec-trum. Of the incoming radiation around19% is absorbed by the atmosphere: 16%by gases and dust and 3% by clouds.

accessory cloud A smaller cloud fea-ture that is related to a main cloud, depen-dent on it for development and

continuance, and can either be attached orseparate from it. Examples include PILEUS

(cap cloud), a smooth ‘cap’ cloud of smallhorizontal extent that forms just above cu-muliform clouds, PANNUS, and VELUM,which are all generally separate from themain cloud, and ARCUS (shelf and rollclouds), which are attached. See also cloudclassification.

acclimatization /ă-klÿ-mă-tă-zay-shŏn/ The process of adaptation made byorganisms, such as animals (including hu-mans) and plants, to a change in climateand/or environment, which may result, forexample, from a long-term CLIMATE

CHANGE or from the organism being trans-ferred to a new environment.

accretion In cloud physics, the processby which ice crystals in a cloud growthrough contact with supercooled waterdroplets, which freeze instantly on impactwith the ice crystals to form rime ice. Grau-pel (soft hail) forms through the accretionof small supercooled droplets onto fallingice crystals. If the graupel descendsthrough an area of plentiful droplets abovefreezing level, a coating of clear ice mayform through accretion to produce ice pel-lets. Hailstones form from successive ac-cretions of opaque and clear ice when thenuclei are swept up into areas of abundantsupercooled droplets.

accumulated temperature The sum ofdepartures (positive or negative) of temper-ature from a reference temperature for aspecific period. These departures may be,for example, on a daily, monthly, or yearlymean basis. The accumulated temperaturecan be represented by the number of de-gree-hours or the number of DEGREE-DAYS.For a number of specified hours (X), thetemperature exceeds the reference temper-ature by a number of degrees (Y), thusthere are X × Y degree-hours and (XY)/24degree-days.

accumulation The increase in the sur-face area of an environment by a coveringof snow, firn, and ice. Accumulationsources include precipitation in the form of

temperatureof air parcel

temperatureof environment

temperature

hei

gh

t h

h

ELR

SALR

Absolute stability

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snow and freezing rain; hoar frost from ra-diative cooling at the surface; rime associ-ated with supercooled water droplets in theatmosphere; and mass movement transportprocesses, such as avalanches and land-slides.

acid deposition The deposition ofacidic substances from the atmosphereonto the Earth’s surface. This may bethrough wet deposition of acidic sub-stances from precipitation that is moreacidic than ‘normal’ (the background levelof acidity of rain is around pH 4.8 to 5.6),through dry deposition in which dry acidicparticulates settle directly onto the surface(e.g. acid soot), or through acidic vapors orgases reacting with plants, water, andother surfaces. The sources of acidic pollu-tants are generally emissions from the com-bustion of fossil fuels and other industrialactivities. See also acid rain; occult deposi-tion.

acid rain Precipitation in which acidityis above the natural level. Normal rainfallis slightly acidic with a pH of about 5.5 asa result of chemical reaction between natu-rally occurring CARBON DIOXIDE and watervapor in the atmosphere forming weak car-bonic acid. Precipitation with a pH valueof less than 5 is considered to be acidic.The burning of such fossil fuels as coal andoil in power stations, motor vehicles, andmany industrial processes releases largequantities of SULFUR DIOXIDE and NITROGEN

OXIDES into the atmosphere. These pollu-tants can be carried considerable distancesfrom their original source and react withwater vapor in the presence of sunlight toform weak solutions of sulfuric and nitricacids.

When deposited, acid rain can cause se-vere environmental problems, includingkilling fish, damaging forests and crops,and accelerating the decay of buildings; itmay also be of harm to humans. First rec-ognized as an environmental problem inthe 1950s, when increased acidity was ob-served in lakes in Canada and Scandinavia,it had reached serious levels by the late1970s. Since the 1980s various measureshave been introduced to reduce polluting

emissions, especially by the use of catalyticconverters in vehicle exhaust systems andflue-gas desulfurization plants in powerstations. In the US, Phase I of the Acid RainProgram of the 1990 Clean Air ActAmendments set out to reduce SO2 emis-sions by 10 million tons and nitrogen oxideemissions by 2 million tons by 2000. Thistarget has been reached. Phase II, begun in2000, tightens permitted limits and encom-passes a wider range of plants. The AcidRain Program is noteworthy for its re-placement of traditional command regula-tions by an allowance trading systemmaking use of market forces to reduce pol-lution.

actinometer /ak-tă-nom-ĕ-ter/ An in-strument used for measuring electromag-netic radiation, especially that transmittedby the Sun, scattered by the atmosphere,and reflected by the Earth’s surface. Thereare a number of different designs includingthose that use bimetallic strips which, de-pending on their heat capacity, surfacearea, and reflectivity, bend differently as aresult of being heated by radiation.

active layer The upper surface layer inperiglacial areas that thaws and freezesseasonally and lies above permanentlyfrozen ground (see permafrost). The depthof the active layer varies from a few cen-timeters to around 5 m (16 ft). Frost heav-ing (frost heave) occurs when the activelayer freezes in winter and when it thaws insummer the layer becomes unstable; inpoorly drained silts and clays the soil mayflow above the permafrost table.

active remote sensing The methods inREMOTE SENSING of collecting information,for example, the measurement of atmos-pheric properties, using instruments thatemit an artificial signal, which is thenpartly returned by the phenomena to theinstrument. Such an instrument is said to‘active’ because it actually generates the ar-tificial beam of radiation; LIDAR and RADAR

are examples of active remote sensing.Compare passive remote sensing.

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actual evapotranspiration (AE) Thetrue amount of water vapor returned to theatmosphere through evaporation from aground surface and transpiration from itsplant cover where the water supply is lim-ited. Compare potential evapotranspira-tion.

adiabat /ad-ee-ă-bat/ A line drawn on aTHERMODYNAMIC DIAGRAM along which anair parcel moves as it ascends or descendsthrough the atmosphere, cooling or warm-ing adiabatically. There are two types ofadiabat: the DRY ADIABAT and the SATU-RATED ADIABAT.

adiabatic /ad-ee-ă-bat-ik/ Denoting anyprocess in which there is no change of heat.In meteorology it describes a thermody-namic process in which the temperature ofan air parcel changes without any ex-change of energy with the surroundings. Bythe first law of thermodynamics:

∆Q = ∆U + ∆W,where ∆Q = heat added to the system (inthis case the air parcel), ∆U = change in in-ternal energy (a function of temperature)and ∆W = work performed against the sur-roundings. In an adiabatic process, ∆Q = 0;hence ∆U = –∆W. As an air parcel expands,its volume increases, so performing workagainst the surroundings. By the first lawof thermodynamics, the internal energy(and therefore the temperature) of the airparcel must decrease during an adiabaticexpansion process; conversely the airparcel’s temperature increases during anadiabatic compression process. Two typesof adiabatic processes take place: (1) thedry adiabatic process, when the air parcelis unsaturated; and (2) the saturated adia-batic process, when the air parcel is satu-rated. A rising saturated air parcel coolsmore slowly with height than a dry parcelbecause the release of latent heat at thechange of state from gas to liquid acts tobuffer the temperature decrease that occursas a result of the adiabatic expansionprocess.

adiabatic change The changes in thetemperature, pressure, and volume of an

air parcel that occur as a result of an adia-batic process. See adiabatic.

adiabatic chart See thermodynamic di-agram.

adiabatic lapse rate The rate at whicha parcel of air changes temperature adia-batically as it moves vertically. In this caseheat does not leave or enter the system: asan air parcel rises the reduction in sur-rounding pressure enables the air is to ex-pand, which results in its temperaturefalling; conversely, if the air parcel sinksand is compressed its temperature rises.The SATURATED ADIABATIC LAPSE RATE

(SALR) and the DRY ADIABATIC LAPSE RATE

(DALR) apply to the rate of temperaturechange of an air parcel, depending on themoisture content of the parcel.

adsorption The process by which com-pounds, molecules, or ions are attached tothe surface of a particle by chemical orphysical means to form a thin layer of liq-uid or gas.

advanced microwave sounding unit(AMSU) An instrument carried on polar-orbiting satellites to measure global at-mospheric temperature profiles andatmospheric humidity. It replaced the orig-inal microwave sounding unit (MSU) thatwas carried on NOAA satellites. AMSUcomprises two separate complementaryunits: AMSU-A is a 15-channel microwavesounding instrument that measures atmos-pheric temperature, even in cloudy condi-tions; AMSU-B is a 5-channel microwaveinstrument that measures profiles of at-mospheric humidity. NOAA-15 was thefirst satellite to fly AMSU; it has since beencarried on NOAA-16 and NOAA-17 (seePOES).

advanced very high resolution ra-diometer (AVHRR) The imaging scan-ner widely used on many meteorologicalsatellites. It has been used on board the USNational Oceanic and Atmospheric Ad-ministration’s (NOAA) polar-orbitingsatellites (see POES) since the launch ofTIROS–N (1978).

actual evapotranspiration

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Advanced Weather Interactive Pro-cessing System (AWIPS) A technologi-cally advanced interactive computersystem for information processing, display,and telecommunications developed for useby the US National Weather Service(NWS). The system integrates meteorolog-ical, hydrological, satellite, and radar in-formation and data enabling forecasts tobe prepared more accurately and swiftly.

advection /ad-vwk-shŏn/ In meteorol-ogy, the transfer of a property, such as heator humidity, by motion within the atmos-phere, usually in a predominantly horizon-tal direction. Thermal advection, forexample, is the transport of heat by thewind. Advection is most often used to sig-nify horizontal transport but can alsoapply to vertical movement, for example ina HYDRAULIC JUMP. Large-scale horizontaladvection of air is a characteristic of mid-dle-latitude zones and leads to markedchanges in temperature and humidityacross boundaries separating air masses ofdiffering origins. See also cold advection.Compare convection.

advection fog A fog produced when rel-atively warm moist air cools as it flowsacross a cooler surface. It occurs most com-monly over the sea when water tempera-ture is still cool in the spring and early tomid summer. Outbreaks of warm moistmaritime tropical air stream poleward oc-casionally along the English Channel oracross the Grand Banks off Newfound-land, for example. The chilly sea can coolthe lowest layer of air to its dew-point tem-perature; further chilling will lead to con-densation of the fog droplets. Californiafog is advection fog that occurs along thecoast of California when warm moist airfrom over the Pacific Ocean blows acrossthe cooler California Current. Sea fog is atype of advection fog, which is alwayscharacterized by windy conditions. Overland, advection fog occurs sometimeswhen mild moist air blows across coolersurfaces. For example, it occurs in winterover the central US when moisture-ladenair from the Gulf of Mexico flows north-ward inland.

AE See actual evapotranspiration.

aeolian /ee-oh-lee-ăn/ See eolian.

aerial photography Photography ofthe Earth’s surface from airborne plat-forms, such as satellites, spacecraft, andaircraft; it is the oldest method of REMOTE

SENSING. The photographs may be verticalor oblique (high-oblique or low-oblique)and are frequently taken in runs of over-lapping prints, which can be assembledinto mosaics (print lay-down). Three-dimensional images can be viewed whenpairs of photographs taken with a stereo-scopic camera are viewed through astereoscopic viewer (stereoscope). Aerialphotography has widespread applicationswithin the earth sciences, e.g. in mapmak-ing and the revision of existing maps, andthe production of three-dimensional mod-els of the land surface; in land-use planningand mapping; and in providing data ontopography, geology, hydrology, and vege-tation and soils. Color infrared film, whichis sensitive to near-infrared wavelengthsand visible wavelengths, is used, for exam-ple, in vegetation and land-use studies.

aeroallergen /air-roh-al-er-jĕn/ A sub-stance or gas in the atmosphere capable ofinducing an allergy in humans. For exam-ple, nitrogen dioxide is associated with al-lergen-induced nasal inflammation inasthmatics.

aerodynamic roughness The propertyof a surface, which may be solid or a layerof different density, that determineswhether the flow of a fluid (e.g. air in theatmosphere) across it is turbulent orsmooth. A surface is defined as aerody-namically rough when the fluid flowingover it is turbulent right down to the sur-face itself. In the atmosphere, this condi-tion is very common and all surfaces are‘rough’ to some degree; how much dependson the height and spacing of the roughnesselements.

aerological diagram /air-rŏ-loj-ă-kăl/ See thermodynamic diagram.

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aerological diagramFREEDOM PALESTINE


aerology /air-rol-ŏ-jee/ The study of theatmosphere through its vertical extent,generally above the surface layers.

aeronomy /air-ron-ŏ-mee/ The study ofthe upper atmosphere of a planet, such asthe Earth, in which it is usually the study ofthose layers above the lower stratosphere.Dissociation and ionization are the mainprocesses within these levels in the Earth’satmosphere; phenomena include AIRGLOW

and the AURORA. Aeronomy is sometimesused for studies of all the atmospheric lay-ers.

aerosol A dispersion of solid or liquidparticles suspended in a gas. In meteorol-ogy, the gas is the atmosphere and theprime sources (both natural and artificial)of the particles are at the Earth’s surface,including sea salt particles generated bybreaking waves, carbon particles in smokeand soot, fine wind-blown soil and clayparticles, and ash from volcanic eruptions.They are important meteorologically be-cause some of these particles can act ascondensation nuclei (see cloud condensa-tion nucleus) and FREEZING NUCLEI; in sig-nificant concentration they can influencethe RADIATION BUDGET of the atmospherethrough ABSORPTION or SCATTERING of solarradiation.

afforestation /ă-fô-rĕ-stay-shŏn/ Theplanting of trees to establish new forest,often for commercial purposes but also toimprove the environment, for conservationpurposes, or to provide a CARBON SINK.Compare deforestation.

African monsoon The predominantlynortheastward invasion of moist air fromthe Gulf of Guinea during the summermonths (May–September) across WestAfrica. The MONSOON air is accepted asbeing present as soon as daytime relativehumidity has risen to 50%. Elsewhere overtropical Africa, complex topography andthe pattern of land and sea, leads to muchmodified seasonal airflow patterns. Thismeans that only some components of theseasonal reversal of winds may be detected.

afterglow 1. A broad high arch of radi-ance sometimes observed in the westernsky after sunset and during the deepeningTWILIGHT.2. See alpine glow.

ageostrophic wind /ay-jee-oh-stroff-ik/ A theoretical wind; the vector differ-ence between the actual observed wind andthe GEOSTROPHIC WIND at a given level. It isnot, therefore, an actual wind that blowsbut is a very important concept associatedwith CONVERGENCE, DIVERGENCE, and VER-TICAL MOTION, for example. These are sig-nificant in the production of cloud and,ultimately, precipitation. Surface friction isone property that leads to an ageostrophicwind.

aggregation In cloud physics, theprocess by which ice crystals clump to-gether, the branched shapes of the crystalsreadily linking, to form snowflakes. Theprocess is especially likely to occur at tem-peratures between –5 and 0°C (23 and32°F). See also Bergeron–Findeisenprocess.

agroclimatology /ag-roh-klÿ-mă-tol-ŏ-jee/ The study of the effects of CLIMATE,which may include climate variability andchange, on agriculture. It may include, forexample, the effects of climate on crop pro-duction, the length of the growing season,the soil climate, and the location and as-pects of agricultural units, such as farms inrelation to climatic variables.

agrometeorology /ag-roh-mee-tee-ŏ-rol-ŏ-jee/ The study and use of weather andclimate information in relation to agricul-ture. Such information may be used, forexample, to expand and improve agricul-tural production, to assess the impact ofweather on crop response and survival,and forecast the occurrence of frosts.

Agulhas Current /ă-gull-ăss/ A warmsurface ocean current, originating in the In-dian Ocean from the South EQUATORIAL

CURRENT and also fed by the MozambiqueCurrent. The Agulhas Current flows south-westward along the South-African coast-

aerology

6

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line and joins the eastward flow betweenAfrica and Australia; some may continueto flow westward round the Cape of GoodHope to join the Atlantic. The AgulhasCurrent is one of the fastest flowing cur-rents in any of the oceans, capable ofreaching a maximum velocity of 5 knots(5.8 mph) off the southeast coast of SouthAfrica.

air The mechanical mixture of gases thatmake up the Earth’s atmosphere. The twomost abundant constituents are NITROGEN

(78.08%) and OXYGEN (20.95%); also pre-sent are variable amounts of water vapor(the amount depending on location, tem-perature, and time), and argon (0.93%),while the TRACE GASES include significantamounts of CARBON DIOXIDE and METHANE.

Aircraft Meteorological DataRelay See AMDAR.

AIREP (aircraft report) An internation-ally agreed type of SYNOPTIC CODE used totransmit a message relating to dry-bulbtemperature as well as wind speed and di-rection from a commercial aircraft inflight.

air frost The condition in which the airtemperature of the lower atmosphere, mea-sured at Stevenson screen level of 1.25 m (4ft) above the ground surface is at or below0°C (32°F), the freezing point of water. Seealso frost; ground frost.

airglow A faint radiant emission thatoccurs in the ionosphere of the upper at-mosphere. It results from the ionization ofatoms and molecules by radiation from theSun.

air mass An extensive homogeneous re-gion of air, typically a few thousand kilo-meters across, that possesses broadly thesame temperature and humidity character-istics, with generally weak horizontal gra-dients of these properties. Air masses havetheir SOURCE REGIONS in the locations of thepermanent or semi-permanent ANTICY-CLONES (high-pressure zones) where windsare light and from which surface air spirals

out to influence more extensive regions.The nature of an air mass depends on thelocation of the anticyclone with which it isassociated, the season, and, for areas out-side this source region, the nature of theair’s track once it moves away. Air massesare classified according to temperature (i.e.tropical, polar, and Arctic or Antarctic)and on whether the surface where it origi-nates is maritime or continental. The SUB-TROPICAL ANTICYCLONES over oceans act assource regions for warm and humid airthat is known as a MARITIME TROPICAL (mT)air mass. The wintertime CONTINENTAL AN-TICYCLONES act as source regions for coldand dry CONTINENTAL POLAR (cP) airmasses, while the highest latitude anticy-clones are sources of very cold and dryARCTIC (A) and ANTARCTIC (A or AA) airmasses. CONTINENTAL TROPICAL (cT) airmasses originate in the continental sub-tropical anticyclones. Additional classifica-tions sometimes include equatorial (E) andMediterranean air masses; suffixes are alsosometimes used to indicate whether an airmass is colder than the surface over whichit is moving (k) or whether it is warmer(w).

The air that flows away from the anti-cyclonic source regions is modified to vary-ing degrees, depending on its track. Thatmoving out from northern wintertime con-tinents (continental polar), for example,

7

air mass

COMPOSITION OF AIR(% by volume)

nitrogen (N2) 78.08oxygen (O2) 20.95carbon dioxide (CO2) 0.03argon (Ar) 0.93neon (Ne) 1.82 × 10–3

helium (He) 5.24 × 10–4

methane (CH4) 1.5 × 10–4

krypton (Kr) 1.14 × 10–4

xenon (Xe) 8.7 × 10–5

ozone (O3) 1 × 10–5

nitrous oxide (N2O) 3 × 10–5

water (H2O) variable, up to 1.00

hydrogen (H2) 5 × 10–5

FREEDOM PALESTINE


often blows across the North Atlantic andNorth Pacific Oceans to be strongly heatedand moistened, and transformed into MAR-ITIME POLAR (mP) air. Where two differentair masses are juxtaposed, the horizontalgradients of temperature and humidity be-tween them are steep forming FRONTS.

air-mass thunderstorm A short-livedand generally isolated thunderstormformed by local differential heating of anair mass, not associated with frontal orsynoptic features. It forms in warm,humid, and unstable air masses, for exam-ple the maritime tropical (mT) air that ex-tends from the Midwest to the Gulf Coaststates in the US. Air-mass thunderstormsare typically 1–10 km in diameter and maylast for up to about 60 minutes.

air parcel A concept used most often inthe context of small volumes of air thatmay or may not ascend or descend throughtheir surrounding ‘environmental’ air. The‘parcel’ is a distinct and ‘isolated’ bubble ofair that may cool or warm (if ascending ordescending respectively) at either the DRY

ADIABATIC LAPSE RATE or the SATURATED ADI-ABATIC LAPSE RATE. Such an UNSTABLE parcelmay well appear as a small CUMULUS cloud,for example.

air pollution The presence in the at-mosphere of gases, aerosols, particles, orenergy above background levels in suchquantities and over such timescales that arelikely to cause harm to all or part of thebiosphere, deterioration in the fabric ofbuildings and structures, or changes to theway the atmospheric system operates. Seealso pollution.

air pressure See atmospheric pressure.

Air Quality Index (AQI) In the US, ameasure of the daily air quality of an area.The system was developed by the Environ-mental Protection Agency (EPA) and is arevision of the earlier Pollution StandardsIndex (PSI), which it replaced in 1999. TheAQI is calculated for five pollutants: ozone(ground level), particulates (particulatematter or PM), carbon monoxide, sulfur

dioxide, and nitrous oxide. The index hasa scale of 0 to 500: the higher the value, thegreater the level of pollution. Six levels arespecified: Good, 0–50; Moderate, 51–100;Unhealthy for Sensitive Groups, 101–150;Unhealthy, 151–200; Very Unhealthy, 201–300; Hazardous, 301–500; each level isalso assigned a color for easy recognition.

air-quality model A mathematical orconceptual attempt to simulate the path-ways of pollutants through the atmospherein order to form a first step in understand-ing the likely impact of air pollutants onhumans and their environment. The mod-els may incorporate components ofphysics, fluid dynamics, and chemistrywithin and above the BOUNDARY LAYER ofthe atmosphere.

air-quality standard The prescribedconcentrations of air pollutants thatshould not legally be exceeded during agiven period at a specified location, usuallywithin an urban environment. Recom-mended guidelines for short-term exposureto pollutants are also provided. See alsoNational Ambient Air Quality Standards.

air–sea interaction See atmosphere–ocean interaction.

Aitken nuclei counter (Aitken dustcounter) A laboratory instrument for de-termining the dust or cloud condensationnuclei content of a sample of air. The sam-ple is drawn into an expandable chambercontaining air almost saturated with watervapor. Sudden expansion of the chambercauses the air to cool adiabatically tobelow its dewpoint, and water droplets toform on the dust or nuclei. A proportion ofthe droplets settle onto a graduated plateand can be counted. The instrument wasinvented by the Scottish physicist and me-teorologist, John Aitken (1839–1919).

Aitken nucleus /ayt-kin/ The smallestand most numerous type of solid particle inthe atmosphere (AEROSOL), with a radius ofless than about 0.1 µm, originating mainlyfrom deliberate combustion. Aitken nucleiare most highly concentrated in continen-

air-mass thunderstorm

8

FREEDOM PALESTINE


tal air. The nuclei are named for the Scot-tish physicist and meteorologist, JohnAitken (1839–1919).

Alaska Current A comparatively warmcurrent that flows counterclockwise in theGulf of Alaska; it is an example of a GYRE.

albedo /al-bee-doh/ The ratio, usuallyexpressed as a percentage, of the reflectedsolar radiation beam to the total energy ofthe beam falling on to a surface as mea-sured across the total spectrum of incom-ing solar radiation. The greater the albedovalue, the less energy is available for otherenergy transformations (e.g. sensible andlatent heat). Bright surfaces, such as freshsnow, reflect more than dark surfaces, suchas a coniferous forest, and consequentlyhave a higher albedo value.

albedometer /al-bĕ-dom-ĕĕe-ter/ An in-strument used for measuring the reflectingpower, i.e. the ALBEDO, of a surface (e.g.clouds, snow, water, or crops).

Alberta clipper (Alberta low) A gener-ally wintertime fast-moving low-pressuresystem that forms or redevelops to the leeof the Canadian Rocky Mountains, to theeast of the mountain chain under westerlyflow. These LEE DEPRESSIONS track generallyeastward to often produce strong gustywinds and snowy weather in the vicinity ofthe Canadian/US border. Alberta clipperscan travel as far as the Maritime Provincesof Canada and the northeast US, occasion-ally intensifying off the east coast whenthey encounter the relatively warm moistair over the Atlantic Ocean, and spreadingheavy snow over land.

Alberta low See Alberta clipper.

Aleutian Current /ă-loo-shŏn/ (Sub-Arc-tic Current) A comparatively cold oceancurrent that flows between latitudes 40°and 50°N in the north Pacific Ocean.

Aleutian low A semi-permanent regionof low pressure that appears on averagemonthly MEAN-SEA-LEVEL PRESSURE charts,located over the Aleutian Islands chain thatstretches westward across the North Pa-cific Ocean from Alaska. It is deeper in thewinter than the summer and is not neces-sarily present from day-to-day. It is the lo-cation in which, on average, the travelingFRONTAL CYCLONES in the North Pacificreach their minimum mean-sea-level pres-sure value.

Alliance of Small Island States(AOSIS) A coalition of small island andlow-lying countries that share concernsabout their vulnerability to sea-level riseassociated with global CLIMATE CHANGE.The alliance was initiated in 1990 tostrengthen the states’ negotiating positionwhen voicing their opinion on greenhouseemissions within the UNITED NATIONS

FRAMEWORK CONVENTION ON CLIMATE

CHANGE (UNFCC). It has a membership of43 states and observers, from all regions ofthe world.

almwind /ahm-wind/ A warm wind thatblows across the plains of southern Polandand occurs mainly during the spring. Thealmwind is a FÖHN-type wind originatingfrom the forced ascent of a southerlyairstream across the Tatra Mountains,which form the Polish/Slovakian borderand reach heights of up to 2655 m (8711ft). This air stream warms rapidly on its de-scent from the mountain range, often initi-ating avalanches in southern Poland.

along-slope wind system A wind sys-tem that circulates air up, along, and downthe slopes of a valley. The system is com-prised of ANABATIC and KATABATIC compo-nents. Such flows are generally shallow,with the maximum velocities occurringwithin a few meters of the slope surface.

9

along-slope wind system

ALBEDOSurface type % Reflected

fresh snow 90%water (Sun high in sky) 12–10%

water (Sun low in sky) 10–80%

grass 15–25%forest 15–10%

FREEDOM PALESTINE


along-valley wind system A wind sys-tem that occurs in mountainous regions. Itis generated as a result of the differentialheating of surfaces during the solar radia-tion cycle, this being induced by variationsin slope aspect. An up-valley wind is mostlikely to develop on calm clear days. Valleywind systems may have ANABATIC or KATA-BATIC components, but will always be con-ditioned by the dynamics of thesynoptic-scale GRADIENT WIND flow in thefree atmosphere.

alpine glow The series of colors that ap-pears on mountains, especially when snow-covered, as the Sun nears the horizon atsunset. The mountain tops exposed to theSun are first yellow, then become pinkish,and finally purple. The series of colors, butin reverse order, is also visible at sunrise onmountains facing the rising Sun. A purplishglow – the afterglow – is sometimes visibleafter the Sun has gone below the horizon.

alternative energy Energy derived fromnontraditional sources or sources that arerenewable and do not deplete finite min-eral resources. The majority of countriesare heavily dependent on fossil fuels assources of energy, but with the decline inreserves and contribution made to the en-hanced GREENHOUSE EFFECT by releases ofcarbon dioxide from fossil fuel combus-tion, alternative energy sources are beingincreasingly used and developed. Examplesinclude wind power, solar power, waveand tidal power, biomass energy, geother-mal power, and hydroelectric power.World primary energy sources include onlyabout 7% from renewable sources, most ofwhich is hydroelectric.

altimeter An instrument used to mea-sure the altitude of an object with respectto a fixed level, such as sea level. There aretwo main types. The pressure altimeter, themost common of which is the ANEROID

BAROMETER, is used in aircraft; it works onthe principal of measuring falls in atmos-pheric pressure with altitude and shows theheight, in meters or feet, above sea level.The radio altimeter (electronic altimeter) isan absolute altimeter, providing an altitude

over any surface; it measures the time ittakes for a radio pulse or continuous signalto be reflected by a surface and convertsthis into a geometric height.

altithermal /al-tă-ther-măl/ (hypsi-thermal) A period during which theEarth’s climate was warmer than it is cur-rently. In particular, altithermal refers to aperiod about 8000–4000 years ago duringwhich, it is believed, zones of higher pre-cipitation were shifted poleward of theirpresent position.

altitude 1. The vertical height of an ob-ject (e.g. an aircraft, meteorological bal-loon, or atmospheric layer) above areference level, which is usually mean sealevel.2. (elevation) A horizontal coordinate; it isthe angular displacement of an object orastronomical body above the horizon. It ispositive if the object is above the horizon,negative if below.

alto- In meteorology, a prefix relateduniquely to MIDDLE CLOUDS that have basestypically between 2000 m and 7000 m(6500 ft and 23,000 ft) above the surface.See also cloud classification.

altocumulus /al-toh-kyoo-myŭ-lŭs/ Atype of MIDDLE CLOUD commonly consist-ing of parallel bands or small roundedmasses that have angular widths between1° and 5°. The cloud can appear as a patchor a more extensive sheet of cloud, white orgray in color, and typically partly shaded.Altocumulus clouds usually form by con-vection in an unstable layer aloft.

altocumulus castellanus /kas-tĕ-lay-nŭs/ A type of MIDDLE CLOUD; a species orspecial form of ALTOCUMULUS in which thecumuliform components are much tallerthan they are wide and so appear to be tur-reted or castellated. They express the pres-ence of overturning in their layer and areoften known to be useful harbingers ofthundery weather.

altostratus /al-toh-stray-tŭs/ A type ofMIDDLE CLOUD, most often with a grayish

along-valley wind system

10

FREEDOM PALESTINE


or bluish tone. It is usually a sheet or layerof fairly uniform appearance with no opti-cal phenomena, and totally or partly cov-ers the sky. It is sometimes thin enough toview the Sun through it in which case thesolar disk appears as if through groundglass. The cloud may contain waterdroplets only, or a mix of water dropletsand ice crystals.

Ambient Air Quality Standards(AAQS) See National Ambient Air Qual-ity Standards.

ambient pressure The ATMOSPHERIC

PRESSURE, at any point in the atmosphere,which surrounds a physical entity, such asa cloud. Ambient pressure is most fre-quently referred to in discussions of airSTABILITY, when the temperature and pres-sure of a theoretical air parcel are com-pared to the pressure and temperature(AMBIENT TEMPERATURE) of the surroundingair.

ambient temperature The temperatureof the area surrounding a specified object,for example the temperature of the sur-rounding atmosphere, soil, or water.

AMDAR /am-dar/ (Aircraft Meteorologi-cal Data Relay) The internationallyagreed scheme for relaying automaticallysensed and transmitted weather data fromcommercial aircraft in flight. The relay ishandled by the telecommunications capa-bility of GEOSTATIONARY SATELLITES.

AMIP /am-ip/ See Atmospheric ModelIntercomparison Project.

amorphous cloud A usually continu-ous layer of featureless low cloud fromwhich rain falls.

amplitude In a wave, the height be-tween the crest of the wave and the meanlevel, or the depth between the lowest pointof the trough of the wave and the meanlevel.

AMSU See advanced microwave sound-ing unit.

anabatic wind /an-ă-bat-ik/ A windthat blows up the slope of a valley, some-times attaining a velocity of 12 knots (14mph). It develops as a result of the greaterinsolation warming of air that is in contactwith the valley slopes during the day incomparison to that of the air at the sameheight but vertically above the valley floor.The heated air above the slope rises in theform of a convection current, with the airof the anabatic wind moving in to take itsplace. An anabatic wind typically begins30 minutes after sunrise and ceases 30 min-utes before sunset. Compare katabaticwind.

ana-front /an-ă-/ A WARM or COLD

FRONT characterized by ascent of the warmmoist air over the colder drier air; it is thusactive in terms of deep cloud and PRECIPI-TATION. Compare kata-front.

analog data Data represented in a con-tinuous form. In radar, a scheme in whichthe radiative power emitted from a scenebeing imaged is converted into a proxy sig-nal, such as an electrical current, beforebeing transmitted for display on theground.

analog image In remote sensing, animage in which the continuous variationsin the scene being sensed is recorded ascontinuous variations in image tone. Forexample, in a photograph the variations inthe scene are recorded directly as varia-tions in tone by the grains of the photosen-sitive film. Compare digital image.

analog weather forecasting A methodof weather forecasting in which pastweather situations similar to the presentare used to predict that the weather will de-velop in a similar way. The method is diffi-cult to use and is now little used.

analysis In synoptic meteorology, an ex-amination of the state of the atmospherebased on synoptic weather observationsand used as the basis for weather forecasts.The process includes the preparation ofcharts, such as isobaric charts, from thedata obtained from many locations at the

11

analysisFREEDOM PALESTINE


same time, together with the interpretationof the weather elements plotted and dis-played on the weather chart. See also isen-tropic analysis; isobaric analysis.

anchor ice See ground ice.

andhi /an-dee/ In the northwestern re-gion of the Indian subcontinent, a duststorm that accompanies a sudden increasein wind speed; it is caused by strong con-vection.

anemogram /ă-nem-ŏ-gram/ See anem-ograph.

anemograph /ă-nem-ŏ-graf -grahf/(recording anemometer) An ANEMOME-TER that continuously records wind speedand sometimes wind direction. The result-ing record of wind speed is known as ananemogram.

anemometer /an-ĕ-mom-ĕ-ter/ An in-strument for measuring the velocity of afluid; in meteorology this is usually windspeed or force and wind direction. Themost commonly used type is the CUP

ANEMOMETER in which revolving cups drivean electric generator that operates an elec-tric meter calibrated in terms of windspeed. This type of anemometer uses the ki-netic energy of the wind to measure itsspeed. Another type is the hot-wireanemometer, which works on the basis ofthe cooling ability of a stream of air. Anelectrically heated wire is cooled as the airflow increases. Alternately, it is kept at aconstant temperature and the electric out-put required to maintain that temperatureis measured and then converted to an air-speed. See also pitot tube anemometer;pressure-plate anemometer; pressure-tubeanemometer.

anemometry /an-ĕ-mom-ĕ-tree/ The sci-ence of the measurement of the speed of afluid, especially wind speed or force.

aneroid barometer /an-ĕ-roid/ A non-liquid BAROMETER used to measure atmos-pheric pressure and widely used in aircraftALTIMETERS due to its compact size and

convenience. This type of barometer con-sists of a flexible capsule (the aneroid cap-sule or cell), which is usually metallic andpartially exhausted of air. The capsule isheld extended by an internal spring and asatmospheric pressure changes, the walls ofthis capsule expand and contract. Thesemovements are transmitted either mechan-ically to a pointer on a dial, which registersthe atmospheric pressure on a scale, orelectrically to a digital read-out device.

angle of incidence The angle between aline perpendicular to a surface and thesolar beam at the point of incidence. Ingeneral, it is the angle between the ZENITH

and the declination of the solar beam.

Ångstrom /ang-strŏm/ (Å) A unit oflength equal to 10–10 meter formerly usedas a measure of the wavelength of radia-tion. The nearest SI unit, the nanometer(nm), equal to 10–9 meter, is now used, to-gether with the micrometer (µm; 10–6

meter) to describe the wavelengths of elec-tromagnetic energy.

angular momentum The momentumpossessed by a body that rotates about afixed axis. The Earth has rotational angu-lar momentum, which is also possessed byair that is rotating with the Earth; it isequal to its linear velocity multiplied by itsdistance from the axis of rotation. The an-gular momentum for a rotating air mass isthe product of the angular velocity (ω) andthe radius of curvature (r), i.e. ωr.

angular velocity The rate of rotation ofa body about an axis. For the Earth, the an-gular velocity for any point on its surfacedepends upon its latitude. A parcel of air atany latitude Φ shares the angular velocityof the Earth as ΩsinΦ, where Ω is the rateat which the planet rotates about its polaraxis (i.e. through 360° or 2π radians everyday or every 86,400 seconds). This ΩsinΦrepresents the angular velocity of air, notmoving with respect to the surface, aboutthe local vertical, i.e. in the local horizontalplane. It is an important component inlarge-scale atmospheric motion. Air thatmoves around a CYCLONE or an ANTICY-

anchor ice

12

FREEDOM PALESTINE


CLONE (i.e. in a more-or-less circular path)also possesses angular velocity. It is thetime rate of change of the angle subtendedfrom a parcel of air to the pressure centeras it circulates around the center.

anisotropy /an-ÿ-sot-rŏ-pee/ The prop-erty of a diffuse surface to reflect incidentsolar radiation in a slightly imperfect wayso that the incident intensity of the solarbeam does not equal the reflected intensityas the ANGLE OF INCIDENCE varies. This typeof reflection is termed anisotropically dif-fuse.

annual snowline See firn line.

anomalous propagation A type ofBACKSCATTER to a RADAR in which the emit-ted beam is not angled upward at a shallowangle but is bent downward to intersect theEarth’s surface. This occurs when there is amarked atmospheric temperature INVER-SION not too far above the surface that re-fracts the emitted beam back down to thesurface. The received signal is then animage of surface features, and of no opera-tional use.

anomaly The deviation of a property,such as precipitation or temperature, in agiven location from the normal long-termvalue.

Antarctic Denoting the south polar re-gion of Antarctica, which comprises theAntarctic Ocean and the Antarctic ice-cap.It usually refers to the region to the southof and within the Antarctic Circle at 66°32′S, along which the Sun does not set on De-cember 22 (the southern hemisphere sum-mer solstice) or rise on June 21 (thesouthern hemisphere winter solstice).

Antarctic air mass A year-round airmass that originates from the ANTICYCLONE

over the intensely cold Antarctic continent.The frigidly cold and very dry air oftenflows strongly down valleys toward andacross the coast as KATABATIC WINDS. Thisair streams toward milder damper air tothe north, meeting it along the ANTARCTIC

FRONT that stretches as an elongated fea-ture around the circumpolar ocean.

Antarctic bottom water A bottom-water flow of cold and very saline waterfound in the SOUTHERN OCEAN, which canspread as far north as 40°N. Antarctic bot-tom water is the main source for the bot-tom waters of the South Pacific, southernIndian Ocean, South Atlantic and parts ofthe North Atlantic. Cooling of the oceanduring winter and formation of sea ice re-leases salt, which increases the density ofthe water causing a mass of cold water tosink from the surface to the bottom of theAntarctic Ocean. The cold dense wateroriginating on the edge of the continentalshelf of Antarctica spills off the shelf andflows northward along the sea floor, belowother water masses, as the Antarctic bot-tom water. It is the most dense water of theglobal oceans, with salinity at around34.65 parts per thousand and temperatureoriginating at –0.5°C (31.1°F). See alsoAntarctic intermediate water; Arctic bot-tom water.

Antarctic Circumpolar Current Anocean GYRE located in the Southern Oceanthat flows west to east, creating a ringaround the Antarctic. The current is cre-ated from the wind stress produced by thecontinuously strong westerly winds foundaround the Antarctic. Variations in thewidth and path of the Antarctic Circumpo-lar Current are caused by the influence oflandmasses, sea-floor topography, and pre-vailing winds; it is most restricted where ifflows through the Drake Passage betweenSouth America and Antarctica. The currentis alternatively known as West Wind Driftalong its surface portion.

Antarctic Coastal Current (East WindDrift) A narrow current that flows in awestward direction around Antarctica inresponse to the easterly wind blowing offthe ice cap.

Antarctic Convergence (Antarctic polarfront) A convergence line in the oceansurrounding Antarctica, located betweenlatitudes 50° and 60° S. The cold Antarctic

13

Antarctic ConvergenceFREEDOM PALESTINE


water sinks below the warmer and lessdense water from the middle latitudes. Theresulting water mass formed where theconvergence occurs is known as theANTARCTIC INTERMEDIATE WATER.

Antarctic front A zone that separatesthe very cold dry CONTINENTAL POLAR airblowing off Antarctica from the milderdamper MARITIME POLAR air to the north. Itstretches around the circumpolar ocean be-tween roughly 55° and 65° S and is associ-ated with the ROARING FORTIES.

Antarctic intermediate water A coldintermediate water flow within the oceansthat originates where cold surface waterfrom Antarctica sinks below warmer andless saline surface water at the ANTARCTIC

CONVERGENCE. It is less saline thanANTARCTIC BOTTOM WATER.

Antarctic Ocean See Southern Ocean.

Antarctic Oscillation (AAO, SouthernAnnular Mode) A periodic fluctuation inatmospheric pressure that occurs in thesouthern hemisphere between the SouthPole and 55–60° S. In its high-index phasethere is low pressure above the Antarcticand near the Southern Ocean and highpressure north of about 50°S. See also Arc-tic Oscillation; North Atlantic Oscillation.

Antarctic polar front See AntarcticConvergence.

anthelion (counter sun) An optical phe-nomenon in which a bright spot of light isvisible at the same level as the Sun on theparhelic circle, but at 180° in azimuthaway from the Sun.

anthropogenic Resulting from or in-duced by the activities of humans.

anthropogenic source /an-throp-ŏ-jen-ik/ An identifiable origin of a pollutantdue to human activity. In most cases this isassociated with the use of energy either inindustry, in transport, or the production ofelectricity. Sources may also be attribut-able to acts of sabotage and warfare.

anticyclogenesis /an-tee-sÿ-klŏ-jen-ĕ-sis/ The process of the formation of anew ANTICYCLONE (high) or the strengthen-ing of the circulation around an existinganticyclone. The formation of a new anti-cyclone can be related to the processes ofRADIATIVE COOLING, which is responsiblefor COLD ANTICYCLONES, or dynamic SUBSI-DENCE, which is linked to WARM ANTICY-CLONES. Compare anticyclolysis.

anticyclolysis The process of the dissi-pation of an ANTICYCLONE (high), or theweakening of the circulation around such afeature. Compare anticyclogenesis.

anticyclone (high) An extensive region,typically a few thousand kilometers across,in which barometric pressure is relativelyhigher than in the surrounding air. Anticy-clones are SOURCE REGIONS for AIR MASSES

from which the air at low levels spirals outclockwise in the northern and counter-clockwise in the southern hemisphere.They are characterized by deep sinkingmotion and generally dry weather, al-though very widespread STRATIFORM cloudcan often be trapped underneath the tem-perature inversion that typifies such fea-tures leading to ANTICYCLONIC GLOOM.

The world’s principal anticyclones arethose semi-permanently over the subtropi-cal oceans and those that appear in thewinter season deep in the interiors of themore extensive northern-hemisphere conti-nents. See cold anticyclone; warm anticy-clone. See also Siberian high; Azores high;Bermuda high; North Pacific high.

anticyclonic gloom /an-tee-sÿ-klon-ik/ Weather associated with stationaryanticyclonic conditions in which extensiveSTRATUS or STRATOCUMULUS persists formany days – sometimes for more than aweek – without breaking up. Conditionsare therefore dull with occasional drizzle.The air in the BOUNDARY LAYER beneath theINVERSION is often polluted in such condi-tions of persistent gloom.

anticyclonic rotation The direction ofthe flow around an ANTICYCLONE (high).The air circulates out clockwise in the

Antarctic front

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FREEDOM PALESTINE


northern and counterclockwise in thesouthern hemisphere.

antitrades /an-tee-traydz/ The windsthat occur in the upper troposphere abovethe surface easterly TRADE WINDS. Typicallyoccurring above 2000 m (6500 ft), the an-titrades blow in the opposite direction tothe surface trades. In the past, antitradeswere defined as high-altitude return windstransporting, to higher latitudes, rising airfrom the Intertropical Convergence Zone(ITCZ). This theory has now been dis-proved and the earlier definition of anti-trades has consequently been largelyabandoned.

antitriptic wind /an-tee-trip-tik/ A typeof LOCAL WIND that is topographically gen-erated; examples include the ANABATIC

WIND and the KATABATIC WIND. A term ini-tially used by the British astronomer andgeophysicist Sir Harold Jeffreys (1891–1989), antritriptic winds are so called be-cause frictional effects dictate both thewind’s dynamics and its thermodynamics.Such winds are diurnal in character, andmost develop on calm and clear days.

anti-wind The intermediate layer of anALONG-VALLEY WIND SYSTEM, in which theflow is reversed from the layer below. Theanti-wind acts to balance the flow of thelowest layer of the system. Since the AN-ABATIC and KATABATIC components accountfor only a few meters, the remainder of thedepth of the valley is taken up by the re-turning anti-wind.

anvil (anvil cloud) A more-or-less hori-zontal sheet of icy CIRRUS and CIRROSTRA-TUS that flows out of the upper reaches of adeep CUMULONIMBUS cloud. It is distortedby the upper winds to stream away in ananvil-like shape.

anvil dome A large OVERSHOOTING TOP

or penetrating top of a SUPERCELL THUN-DERSTORM cloud that is a sign of consider-able thermal activity and updraft withinthe cloud.

AO See Arctic Oscillation.

AOSIS See Alliance of Small IslandStates.

aphelion /ă-fee-lee-ŏn/ The point in aplanet’s orbit at which it is furthest fromthe Sun. In the Earth’s orbit around theSun, the planet is furthest from the Sun(about 152 million km or 1.017 astronom-ical units) currently on about July 4. Vari-ations in this orbit are associated with theMILANKOVITCH CYCLES and the astronomi-cal theory of climate change.

apogee /ap-ŏ-jee/ In remote sensing, thepoint along a satellite’s orbit at which it isfurthest from the Earth or the point that itis orbiting. More generally, in astronomyany Earth satellite, including the Moon, issaid to be in apogee when it is at its great-est distance from the Earth. Compareperigee.

apparent temperature See heat index.

Appleton layer /ap-’l-tŏn/ See F-layer.

applied climatology The analysis ofCLIMATE data for useful applications. Ex-amples of potential applications includeagriculture (see agroclimatology), urbanstudies, industry, aviation, water re-sources, disaster management, and any cli-mate-sensitive areas of society.

applied meteorology The applicationof meteorological information to problemsfacing, for example, agriculture, com-merce, and planning (e.g. the location anddesign of airports, factories, and other con-structions).

APT See automatic picture transmis-sion.

AQI See Air Quality Index.

Aqua See Earth Observing System.

Arctic Denoting the north polar regionof the Arctic. In its strictest sense it is de-fined as the region to the north of the Arc-

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tic Circle at 66°32′ N, along which the Sundoes not set on June 21 (the northern hemi-sphere summer solstice) or rise on Decem-ber 22 (the northern hemisphere wintersolstice). See also Arctic warming.

Arctic air mass (A) An air mass that re-sides across the Arctic Basin and can leadto anomalously cold weather to its south.In winter, when the Arctic Ocean is frozen,the continental Arctic (cA) air mass is verycold and dry. In summer, when the basin ismore like a cold ocean and the air above itis cool and damp, it is defined as a mar-itime Arctic (mA) air mass.

Arctic bottom water A bottom-waterflow found in the ARCTIC OCEAN. It origi-nates when sea water freezes to form seaice and releases salt, forming salt-free iceand a high-density residual brine. The in-crease in density causes a mass of coldwater to sink to the bottom of the ArcticOcean. This bottom water is restrictedmainly to the Arctic due to submarinetopographic barriers, such as the BeringSill, which blocks flow into the PacificOcean. Some bottom water does escapeinto the western Atlantic Ocean from theregion around Greenland. See also Antarc-tic bottom water.

Arctic Climate System Study(ACSYS) A former core project of theWORLD CLIMATE RESEARCH PROGRAM

(WCRP) that sought to understand ArcticOcean variability and change, such as sea-ice processes, and the role that the Arcticplays in the global climate It was particu-larly concerned with establishing theglobal consequences of anthropogenicchange in the Arctic climate system, andwhether the climate of the Arctic is as sen-sitive to increased GREENHOUSE GASES as cli-mate models predict. The organization ofresearch was expanded into the CLIMATE

AND CRYOSPHERE (CliC) initiative followingapproval received in 2000 from theWCRP. The Arctic Climate System Studyended in 2003.

Arctic front A gently sloping zone sepa-rating cold Arctic air that has flowed south

from the ice- and snow-covered ArcticBasin, from CONTINENTAL POLAR and MAR-ITIME POLAR air masses. These latter twoAIR MASSES have different properties overhigh-latitude continents and oceans inwhich they reside. In the winter and springthe Arctic front is often found between thesouth of Greenland and northern Norwayas well as over central and northernCanada, for example.

Arctic Ocean The smallest of theEarth’s oceans, covering an area of 14 mil-lion sq km (5.4 million sq miles). It is lo-cated to the north of North America andAsia, and entirely within the Arctic Circle,surrounding the North Pole. The averagedepth is approximately 1200 m (3900 ft),with a maximum depth of approximately5500 m (18,000 ft). The Bering Strait con-nects the Arctic Ocean with the PACIFIC

OCEAN and the Greenland Sea is its linkwith the ATLANTIC OCEAN. Three submarineridges – the Alpha Ridge, LomonosovRidge, and Arctic Mid-Ocean Ridge, splitthe ocean floor. The Arctic Ocean is almostentirely covered with ice, around 0.5–4 m(1.5–13 ft) thick, for most of the year ex-cept in fringe areas. For this reason it wasonce also known as the Frozen Ocean. Icecover reduces the energy exchange betweenthe surface and the atmosphere, and re-duces the amount of sunlight that can pen-etrate surface waters, thus reducing theability of marine life to photosynthesize.Cold Arctic currents cool the shores ofnortheast North America and northeastAsia, contrasting with the coastlines of Eu-rope and North America, which arewarmed by the NORTH ATLANTIC DRIFT andthe KUROSHIO CURRENT, respectively. TheArctic currents tend to be much less salineand less dense than the warmer currentsand remain at the surface while Atlanticcurrents tend to be deeper.

Arctic Oscillation (AO) Periodic fluc-tuations in atmospheric pressure that occurbetween the mid-latitudes and Arcticabove about 45° N. In a ‘positive phase’ ofthe oscillation there is a strengthening ofthe counterclockwise polar vortex extend-ing from the lower stratosphere to the sur-

Arctic air mass

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face. The Arctic surface air pressure de-creases while there is higher pressure atmid-latitudes. The North Atlantic stormtracks bring rain and mild temperatures tonorthern Europe while conditions are drierin the western USA and Mediterranean. Inthe ‘negative phase’ the strength of the vor-tex decreases and there is comparativelyhigh pressure over the Arctic region andlow pressure in the mid-latitudes. Cold airfrom the Arctic extends down into the USAand Europe and Asia. During the1980sand 1990s the positive phase was domi-nant. The Arctic oscillation is closely re-lated to the NORTH ATLANTIC OSCILLATION.

Arctic sea smoke (Arctic frostsmoke) A type of STEAM FOG that occursin high latitudes when very cold air passesover relatively warm water. Water evapo-rates into the air, increasing the dew-point,and the air above may become saturated.The cold air directly above the warmerwater is heated and rises through convec-tion carrying the moisture, which almostimmediately condenses to form thin verti-cal columns of condensed water vapor, up-wards like small pillars of smoke. If thetemperature of the air is below the freezingpoint it forms Arctic frost smoke.

Arctic warming The warming of theArctic region during recent years. It is be-lieved by many scientists that the polar re-gions, and in particular the Arctic, willshow evidence of GLOBAL WARMING beforeother regions, a situation supported bymany GENERAL CIRCULATION MODELS. Al-though climate records are sparse in thisregion, data, including climate reconstruc-tions from tree-ring studies, ice cores, andlake cores, appear to support this theory.Evidence from the last 400 years indicatesa gentle warming in the Arctic region butduring the period 1976 to 1995 climatedata show a more rapid rise. For example,winter temperatures in central Siberia roseby 6°C (11°F) during this period. Satelliteobservations of the Arctic have revealedthe extent of warming in the region. In2002 sea-ice coverage was the lowest in the20 years of observations.

arcus /ar-kŭs/ A type of low ACCESSORY

CLOUD formation that is produced when athunderstorm’s GUST FRONT scoops upmoist air at the ground. One form of arcusis the shallowly sloping shelf cloud, whichhas an elongated smooth shelf-like appear-ance, sometimes layered and terracedalong the leading edge, and a dark turbu-lent base. It is usually attached to the thun-derstorm base. It is accompanied by gustywinds when it passes overhead. Anothertype of arcus is the ROLL CLOUD, an elon-gated cloud that appears to spin slowlyabout its horizontal axis. Roll clouds arefound behind gust fronts that flow outacross the surface from thunderstorms;their formation is not currently well under-stood. See also cloud classification.

area source In air pollution studies, pol-lutants entering the atmosphere from azone, such as an industrial complex cover-ing several square kilometers, in which itmay be impossible to identify the exactsources of individual pollutants. It mayalso refer to a power park incorporating anumber of power stations, or to an area inwhich pollution permits may be traded.Compare linear source; point source.

arid climate See dry climate.

aridity index An index that measuresthe degree of aridity or water deficiencybelow the level needed at a given location.The American climatologist Charles War-ren Thornthwaite (1889–1963) devised thefirst aridity index, which was used in defin-ing climate classifications. The aridityindex = 100 d/n, where d = water deficit(the sum of the monthly differences of PRE-CIPITATION and POTENTIAL EVAPOTRANSPIRA-TION when the precipitation is less than thepotential evapotranspiration), and where nis the sum of the monthly values of poten-tial evapotranspiration for deficientmonths.

artificial rain Rainfall that is producedby human modification of cloud micro-physical processes, mainly by CLOUD SEED-ING with dry ice (frozen carbon dioxide)pellets, silver iodide, or other suitable sub-

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stances. Seeding increases the number ofcondensation nuclei and/or freezing nucleiwithin the cloud. While this increases theefficiency of precipitation generationmechanisms, attempts at cloud seedinghave not always been successful, leading insome cases to a reduction in precipitation,especially when the cloud has been over-seeded.

Asian high (Asian anticyclone) SeeSiberian high.

Asian monsoon The invasion by themoisture-laden southwest airflow overeastern Asia, including China, from Maythrough to October with warm humid airlying over much of central China by midJune (See Mai-yu season). Rainfall ismostly associated with surges in the flowover Eastern China. Mountain ranges con-siderably disrupt the southwest tosoutherly flow over western China. Seealso monsoon.

ASOS See Automated Surface Observa-tion System.

aspect The direction faced by a slopingsurface. It is the point of the compass (usu-ally approximated to ‘north’, etc) indicatedby a line projected on a horizontal planenormal to a slope. Aspect is important inmicroclimate studies since it is a major de-terminant of the receipt of solar radiation,south-facing slopes receiving most in themiddle-latitude northern hemisphere.

astronomical theory of climatechange See Milankovitch cycles.

Atlantic conveyor (Atlantic Ocean ther-mohaline circulation system) A thermo-haline circulation system within the oceansthat is important in the global transfer ofheat from high to low latitudes. The down-welling of salt water takes place in theNorth Atlantic, near Labrador and nearGreenland, when warm waters are cooledby chill winds. Together with the flow ofcold water from the Norwegian Sea thatspills over sills, the cold saline water sinksto the ocean floor and flows south as the

North Atlantic Deep Water, eventually tothe Indian and Pacific Oceans and theSouthern Ocean. The cold dense watereventually warms and returns to the sur-face of the worlds oceans. The occurrenceof a strong Atlantic conveyor has beenlinked by scientists to the increased proba-bility of major storms and hurricanes dur-ing a season.

Atlantic Ocean The world’s secondlargest ocean, covering an area of approxi-mately 82 million sq km (31.8 million sqmiles), about 20% of the Earth’s surface.Its average depth is approximately 3500 m(11,500 ft) with a maximum depth ofroughly 8500 m (28,000 ft) at the PuertoRico Trench, north of Puerto Rico. Theocean extends from the Arctic to theAntarctic between North and South Amer-ica in the west and Europe and Africa inthe east. The ocean floor consists of a seriesof ridges. The Mid-Atlantic Ridge, approx-imately 500–1000 km (300–600 miles)wide, extends 16,000 km (10,000 miles)from Iceland to the Antarctic Circle. Theridge is produced by volcanic activity aswell as sea-floor spreading; it is constantlywidening causing the ocean to expandpushing apart the Americas in the westfrom Europe and Africa in the east. Otherridges include a shallow submarine ridgeacross the Strait of Gibraltar, which sepa-rates the Mediterranean basin from the At-lantic.

Atlantic period A period betweenabout 8000 and 5000 years BP, which en-compasses most of the warmest postglacialtimes and was characterized by oceanic cli-matic conditions in northwest Europe. Itwas preceded by the BOREAL PERIOD andfollowed by the Sub-Boreal period.

Atlantic polar front The gently slopingzone that separates MARITIME POLAR andMARITIME TROPICAL air masses across mid-dle- and high-latitudes of both the Northand South Atlantic. The tropical air origi-nates from the SUBTROPICAL ANTICYCLONES

of both basins while the polar air streamsacross the oceans from the cold CONTINEN-TAL ANTICYCLONES that reside over the ad-

Asian high

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jacent wintertime continents. The front di-vides two regimes of weather associatedwith the tropical and polar air, and is thezone along which FRONTAL CYCLONES

(frontal depressions) form and evolve.

atmometer /at-mom-ĕ-ter/ See evapor-imeter.

atmosphere 1. The gaseous layer thatsurrounds the Earth, or another planet.The atmosphere of the Earth is composedof a mixture of gases, called the AIR, whichis predominantly composed of oxygen andnitrogen. See also atmospheric structure.2. (atm) A unit of pressure, equal to101,325 Pa (1013.25 hPa). It is used in ex-pressing pressures in excess of standard at-mospheric pressure, such as the highpressures used in some chemical processes.

atmosphere–ocean interaction (air–seainteraction) The interactions occurringbetween the atmosphere and oceans; theyinclude the input of energy to the oceanfrom wind, transfers of heat, the transfer of

water through precipitation and evapora-tion, and gas exchange (e.g. carbon diox-ide, dimethyl sulfide (DMS), and nitrousoxide). The interactions are of vital impor-tance to the understanding of climate. Ona global scale, changes within theocean–atmosphere system impact onglobal sea level, sea-surface temperaturedistribution, sea-ice concentrations, theheat transferred by the ocean currents, andthe distribution of heat moisture and mo-mentum in the atmosphere. Studies of theinteractions on a global scale use datagathered from satellite remote sensing, and employ large-scale coupled ocean–atmos-phere models.

atmospheric boundary layer See plan-etary boundary layer.

atmospheric chemistry The study ofthe chemical composition, interactions,and processes of the Earth’s atmosphere. Itincludes the study of aerosols and pollu-tants, distributions of trace gases, changesin stratospheric ozone concentrations, and

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atmospheric chemistry

maritimepolar

continentalpolar

maritimetropical

maritimepolar

low

low

Atlantic polar front

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the chemical reactions that take place inthe atmosphere. Processes include photo-chemistry and dissociation.

atmospheric circulation See generalcirculation.

atmospheric energetics The study ofthe energy content and energy transforma-

tions, especially in the lowest 6 km (4miles) of the troposphere. It is the study ofheat, latent heat, potential energy, and ki-netic energy. Analysis of the first three iscarried out using THERMODYNAMIC DIA-GRAMS (e.g. TEPHIGRAMS).

atmospheric heat engine A concept inwhich the atmosphere is regarded as a heat

atmospheric circulation

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thermosphere

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engine. The Sun’s radiation provides theprimary source of energy. Temperature ishighest in the vicinity of the equatorialtrough and lowest in the upper atmosphereabove the poles. The engine is said to ex-tract energy from the warm source and per-form work in driving the atmosphericGENERAL CIRCULATION.

atmospheric lifetime The residencetime of a pollutant in the atmosphere; thisrefers to the length of time that the mole-cules of a gas, liquid, or particle remain inthe atmosphere at above its natural levelswithout being deposited at the surface orremoved via a SINK, or altered or chemi-cally changed into another substance in theatmosphere. The length of time depends onthe source, the sinks, and the reactivity ofthe pollutant.

Atmospheric Model IntercomparisonProject (AMIP) A project shared be-tween many atmospheric research centersthat was organized by the Working Groupon Numerical Experimentation (WGNE)as a contribution to the WORLD CLIMATE RE-SEARCH PROGRAM. The project aimed to de-termine systematic errors in atmosphericMODELS using the decade 1979–88 as thetest period for the simulation of climatefrom the participating atmospheric mod-els, under specified conditions. The projectprovides a standard experimental basis foratmospheric GENERAL CIRCULATION MOD-ELS, as well as infrastructure in support ofclimate model diagnosis, validation, inter-comparison, documentation, and data ac-cess. This framework allows scientists tocompare and analyze models with a viewto model improvement and climate studies.See also climate model.

atmospheric moisture The WATER

VAPOR in the atmosphere; all three states ofwater occur in the atmosphere and thestate should be taken into account when re-ferring to atmospheric moisture. Most at-mospheric moisture occurs below 5000 m(16,500 ft). Water in the atmosphere mayoccur in its liquid state at below 0°C (32°F,the freezing point of water at sea level) in

the upper atmosphere due to its lower at-mospheric pressure. All forms of PRECIPITA-TION result from the condensation ofatmospheric moisture.

atmospheric optics (atmospheric opticalphenomena) The optical phenomena vis-ible within the atmosphere at various timesas a result of the processes of diffraction,reflection, refraction, and scattering.

atmospheric pollution See air pollu-tion.

atmospheric pressure (air pressure,barometric pressure) The PRESSURE expe-rienced at any point above the Earth’s sur-face as a result of the weight of the aircolumn, extending to the outer limit or topof the atmosphere, above that point. Con-sequently, pressure declines exponentiallywith height, the rate of decrease being afunction of the temperature of the atmos-phere. Atmospheric pressure is generallymeasured, in meteorology, either in the SIunit HECTOPASCALS (hPa) or in the c.g.s.unit of the same size, the MILLIBAR (mb)using a mercury or aneroid BAROMETER, ora BAROGRAPH. In the US surface atmos-pheric pressure is measured in INCHES OF

MERCURY (in Hg). Global mean sea levelpressure is 1013.25 hPa (or mb) = 29.29 inHg.

atmospheric structure The layers orregions into which the Earth’s ATMOSPHERE

is divided. Based on differences in the rateof change of temperature with height (theLAPSE RATE) these layers, in ascending orderabove the Earth’s surface, are the TROPOS-PHERE, STRATOSPHERE, MESOSPHERE, andTHERMOSPHERE. In addition, the zone be-yond about 700 km (430 miles) is some-times called the EXOSPHERE. Based onchemical composition, two divisions arerecognized: the HOMOSPHERE and HETEROS-PHERE. Another region, the IONOSPHERE, inwhich there is a high concentration of freeelectrons, extends through the mesosphereand thermosphere to indefinite heights.The OZONE LAYER extends up from about10 km (6 miles), through the stratosphere.

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atmospheric tide Fluctuations in the at-mosphere, mainly of atmospheric pressureand wind, that are produced by the gravi-tational influences of the Sun and Moon,and stronger thermal influence of the Sunalone, and are analogous to the oceanictides. The greatest effect is observed at theEquator where a small but distinct varia-tion in atmospheric pressure occurs ever 12hours (i.e. semi-diurnal) with a maxima ataround 10:00 and 22:00 local time. At-mospheric tides in the wind increase withdistance from the surface of the Earth withthe greatest magnitude in the upper atmos-phere.

atmospheric window The small band-widths in the electromagnetic spectrumwhere the atmosphere is nearly transparentand transmittances of radiation approachunity both for incoming and outgoing radi-ation. The window for the visible range ofsolar radiation, 0.4–0.7 µm, lets most solarradiation through to the surface of theEarth; the longwave radiation window of8–12 µm lets some terrestrial radiation es-cape to space.

attenuation /ă-ten-yû-ay-shŏn/ A reduc-tion in energy of radiation as it passesthrough a medium; in meteorology it is theloss of energy of a beam of solar radiationby the solar beam SCATTERING and ABSORP-TION as it passes through the atmosphere.The amount attenuated may be calculatedby Lambert’s law (Beer’s Law) using ex-tinction or attenuation coefficients, eitherfor individual gases or for mixed gases.

Aura See Earth Observing System.

aureole /or-ee-ohl/ The luminous bluish-white disk with a brown outer ring thatsometimes surrounds the Sun or Moon,and is the inner part of a CORONA.

aurora /aw-ror-ă, -roh-rră/ A luminousphenomenon in the sky, typically at night,that is primarily visible in high geomag-netic latitudes of both hemispheres near tothe Earth’s magnetic poles. At times ofgreater solar activity the aurora may be vis-ible at lower latitudes. The phenomenon

takes place at altitudes generally above 100km (62 miles) in the ionosphere and mayextend as high as 1000 km. It occurs whenhigh speed charged solar particles (elec-trons and protons), are captured by theEarth’s MAGNETOSPHERE, and are deflecteddownward toward the magnetic poles. Inthe upper atmosphere the charged particlescollide with atmospheric oxygen and nitro-gen atoms or molecules causing electronsto be released leaving the atom or moleculewith a net positive charge. The ionizedoxygen and nitrogen leads to the emissionof light, especially in the red and greenish-blue colors. The display of the aurora maytake the form of luminous colored rays,arcs, bands, patches, and curtains, the arcbeing the more persistent form of aurora,which may last for several hours.

The aurora is known as the aurora bo-realis or the northern lights in the northernhemisphere, and as the aurora australis orsouthern lights in the southern hemisphere.

automatic picture transmission(APT) The continuous transmission ofANALOG IMAGES from satellites. The systemstarted with TIROS–8, launched in Decem-ber 1963, in which cloud images from thesatellite were broadcast live. This enabledany properly equipped ground station,within the field of view of the satellite, toreceive and reproduce images for local use.

Automatic Surface Observation Sys-tem (ASOS) In the US, an automatedweather observation system jointly imple-mented by the Federal Aviation Adminis-tration (FAA), National Weather Service(NWS), and the Department of Defense(DoD). The first ASOS was installed inTopeka, Kansas, since when over 900 unitshave been planned throughout the US. TheASOS provide observations that includetemperature, dew point, wind speed anddirection, visibility, sky condition, and pre-cipitation to users, such as the FAA, NWS,and DoD, and in addition, to members ofthe public, local authorities, air traffic con-trol, and the media.

automatic weather station (AWS) Anunmanned weather station at which all

atmospheric tide

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meteorological measurements are madeautomatically by various sensors. Thesestations are frequently positioned in remote places. Data acquired by the auto-matic weather stations (e.g. air tempera-ture, relative humidity, wind speed anddirection, rainfall, and solar radiation) areoften transmitted automatically to a con-trolling station, either at set time intervalsor continuously. Alternatively, the datamay be collected from the station by an ob-server.

autumn (fall) The SEASON of the year be-tween summer and winter occurring as theSun approaches the winter solstice. It ap-plies only in the middle-latitude areas thatexperience the typical four seasons. In theUS, it is generally regarded as being themonths September to November. It is a pe-riod of adjustment in the general circula-tion of the atmosphere, characterized byincreasing storminess and more frequentoutbreaks of air of polar or arctic origin.Declining temperatures combined with rel-atively high moisture content in the atmos-phere, may lead to an increase in theincidence of fog. The first air frost marksthe end of the GROWING SEASON.

avalanche wind A blast of air that pre-cedes a descending avalanche. Resultingfrom the sudden change in air pressure,such shock waves are powerful enough tocause buildings to ‘explode’ before they areoverwhelmed by snow and/or ice. Theshock waves experienced are equivalent toair blasts of magnitude 0.5 tonnes/m2.

AVHRR See advanced very high resolu-tion radiometer.

aviation meteorology The study ofmeteorological phenomena that affect air-craft in flight and the use of weather datafor flight planning.

AWIPS See Advanced Weather Interac-tive Processing System.

AWS See automatic weather station.

axial tilt The tilt of the Earth’s axis ofrotation, which can be expressed as theangle between the axis or rotation and aline drawn perpendicular to the plane ofthe ecliptic. The Earth’s axial tilt variesfrom 22.1° to 24.5° on a cycle of 41,000years and is currently about 23.45°. Thetilt is one of the variables that determinethe distribution of solar radiation and theseasons.

azimuth /az-ă-mŭth/ The directional lo-cation of an object around the horizon,measured in degrees clockwise from truenorth (0°) to a point on the horizon verti-cally below the object.

azimuth angle The angle in the hori-zontal plane between north and the direc-tion of a particular object when its locationis projected vertically onto the horizon.The azimuth angle of an object that is dueeast of the observer has an azimuth of 90°,for example.

Azores high (Azores anticyclone) Anextensive region of semi-permanent highpressure across the subtropical North At-lantic. It is a year-round feature that in thenorthern hemisphere winter and earlyspring tends to be centered over the Azores;during the summer months it migrateswestward to near the island of Bermuda,when it is also known as the BERMUDA

HIGH. The Azores high is the source regionfor MARITIME TROPICAL air flowing gener-ally northward and of the TRADE WINDS

that blow toward lower latitudes, into theINTERTROPICAL CONVERGENCE ZONE. Incontrast to cold anticyclones such as theSIBERIAN HIGH, the Azores high is TROPOS-PHERE deep.

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back-bent occlusion (bent-back occlu-sion) An occlusion (see occluded front)in which the direction of motion of thefront is reversed behind its cold front. Thismay be as the result of the development ofa secondary cyclone near the point of theocclusion, or of movement of the originalcyclone.

back-building thunderstorm A THUN-DERSTORM that appears to remain station-ary or propagate backward as a result ofnew growth occurring on the upwind sideof the storm, instead of on the more nor-mal downwind side. In the northern hemi-sphere the upwind side is generally on thewest or southwest side.

back-door cold front In the US, espe-cially in New England and further southalong parts of the Atlantic seaboard, a coldfront that sweeps in from the east or north-east, rather than the much more commondirection between west and north, andmoves westward over the continental US.Its passage is associated with flow aroundthe southeastern flank of an ANTICYCLONE

to the north over Canada.

background level The concentration ofa pollutant that would be present in a loca-tion without any enhancement of a partic-ular contaminant from local sources. Itmay therefore also refer to concentrationsof a pollutant far removed from its source.In some cases in which a pollutant also oc-curs naturally, it refers to the backgroundlevel from natural sources only.

backing The change of the wind direc-tion in a counterclockwise fashion. For ex-ample, suppose a northwesterly (NW)wind (315°) at a particular location at

12:00 noon is replaced by a southwesterly(SW) wind (225°) twelve hours later, thenthe wind is said to have backed during this12-hour period. Backing is the oppositechange of direction to VEERING.

back radiation See counterradiation.

backscatter In radar, the fraction of asignal emitted by an instrument that is re-flected back to the radar antenna, whichcan therefore be compared to the power ofthe original signal. Precipitation radars, forexample, measure the power scatteredback to the antenna by raindrop-size parti-cles in the atmosphere.

baguio /bag-ee-oh, bah-gee-oh/ Thelocal Philippine name for a TROPICAL CY-CLONE (also known in the Pacific region asa TYPHOON). Severe tropical cyclones devel-oping annually in the western PacificOcean sometimes take a route across the is-lands of the Philippines and are particu-larly prevalent between July andNovember. See also tropical storm.

Bai-u season /bÿ-oo/ See Mai-yu sea-son.

ball lightning An iridescent sphere ofgas that is very occasionally observed nearthe Earth’s surface, associated with electri-cally disturbed weather. There are manyanecdotal reports of phenomena describedas ball lightning, which vary in their relia-bility. Ball lightning phenomena are oftenreported to have a blue or orange color,and are said to float, move around, andsometimes hiss, but descriptions vary as towhether the gaseous ball is actually hot. Itremains unclear where the energy arises tosustain such a plasma system for the

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timescales of tens of seconds commonly de-scribed: microwave energy from a parentthundercloud has been postulated as a pos-sible source.

balloon sounding A technique employ-ing lighter-than-air hydrogen or heliumfilled balloons to explore and record con-ditions in the Earth’s atmosphere. Severaltypes of meteorological balloon are used.The largest, a sounding balloon, carriesmeteorological instruments in a smalltransmitter – a RADIOSONDE – to transmitdata continuously to a ground receiver.These balloons are used to examine thecomposition of the atmosphere, for exam-ple ozone concentrations, relative humid-ity, temperature, wind speeds anddirections, etc., by attaching the appropri-ate meteorological instruments. Soundingballoons are released twice daily at 0000and 1200 UTC from a network of sitesaround the world. Wind speed and direc-tion can also be determined by using aPILOT BALLOON, which is tracked by atheodolite or radar. The smaller ceilingballoon is used to determine cloud-base al-titude.

band In remote sensing, a limited part ofthe electromagnetic spectrum over whichradiation is sensed, for example, by satel-lites. Sensors on meteorological satellitesmeasure radiation in a number of bands,including the visible band and the thermalinfrared band.

banner cloud A fixed cloud that ap-pears to be attached to a sharp mountainpeak, and streams downwind like a pen-nant or banner. Strong wind and relativelymoist air are important ingredients. Thecloud is believed to form in rising airwithin a large eddy that is often found tothe lee of isolated mountain peaks, andmay also be related to a pressure drop totheir lee. The Matterhorn and Mount Ever-est occasionally generate these clouds.

bar (b) A c.g.s. unit of ATMOSPHERIC

PRESSURE equal to 1 million dynes persquare centimeter; equivalent to 750.1 mmof mercury at 0°C at latitude 45°. 1 million

dynes per square centimeter is equal to 105

pascals (Pa), the fundamental SI unit ofpressure. The bar is slightly less than 1standard atmosphere (which equals1.01325 million dynes per square centime-ter = 760 mm of mercury = 1013.25 mb =29.29 in Hg).

barat /bă-raht/ A LOCAL WIND experi-enced on the northern coast of the Indone-sian island of Sulawesi (formerly Celebes).The barat is a squally and occasionallyfierce northwesterly wind that blows on-shore from the Celebes Sea to the north. Itoccurs most frequently between Decemberand February.

baroclinic /ba-rŏ-klin-ik/ In a waterbody, denoting a state in which the sur-faces of constant pressure intersect surfacesof constant density.

baroclinic atmosphere An atmospherewithin which surfaces of constant pressure(ISOBAR) and constant density (ISOPYCNIC)intersect. The result is an atmosphere char-acterized by a marked horizontal tempera-ture gradient; a frontal zone is an exampleof a baroclinic atmosphere. Comparebarotropic atmosphere.

baroclinic instability A kind of dy-namical instability associated with a BARO-CLINIC ATMOSPHERE that often occurs in thestrong westerly current associated withFRONTAL ZONES. It is linked to the develop-ment of middle- and high-latitude FRONTAL

WAVES, associated with ascending warmand descending cold air. This typical verti-cal motion pattern means that the system’sPOTENTIAL ENERGY decreases, while kineticenergy is released.

baroclinic zone A confined regionwithin the atmosphere typified by a rela-tively steep horizontal temperature gradi-ent. A middle- or high-latitude FRONTAL

ZONE is an example of a troposphere-deepgently sloping baroclinic zone.

barograph /ba-rŏ-graf, -grahf/ An in-strument, usually the ANEROID BAROMETER,used to continuously record atmospheric

25

barographFREEDOM PALESTINE


pressure. The pressure-sensitive element ofthe barometer is attached mechanically bylevers to a pen marker, which transfers theaneroids movements and records thechanges in atmospheric pressure on apaper chart mounted around a slowly ro-tating drum or cylinder. This enables theproduction of a time series of changes in at-mospheric pressure, usually for up to aweek.

barometer An instrument used for mea-suring atmospheric pressure. There are twomain types: the MERCURY BAROMETER andANEROID BAROMETER. The mercury barom-eter works on the basis that atmosphericpressure can support the weight of a col-umn of mercury; as pressure alters so willthe height of the column. Mercury barom-eters are often used to calibrate aneroidbarometers. Other forms of barometer in-clude the use of solid-state sensors; onetype measures changes in a piezoresistivetransducer due to changes in barometricpressure. See also Fortin barometer; Kewbarometer.

barometric gradient See pressure gra-dient.

barometric pressure See atmosphericpressure.

barometric tendency See pressure ten-dency.

barothermograph /ba-rŏ-ther-mŏ-graf, -grahf/ An instrument combining a BARO-GRAPH and a THERMOGRAPH used to recordatmospheric pressure and temperature si-multaneously and continuously.

barotropic /ba-rŏ-trop-ik/ In a waterbody, denoting a state and type of motionin which pressure is constant or parallel onsurfaces of constant density.

barotropic atmosphere An atmos-phere within which isobaric and isopycni-cal surfaces coincide. It is an idealizedatmosphere with no horizontal gradient oftemperature and is therefore an approxi-mation to conditions across an AIR MASS.Compare baroclinic atmosphere.

barrier jet A low-level core of higherwind speeds that sometimes occurs at aheight of 1000–1500 m (3000–5000 ft) inthe vicinity of a mountain range. A barrierjet is a product of the deceleration of the

barometer

26

Barometer

Torricellianvacuum

760

mm

air pressure

mercury levers transmitpressure change

evacuatedmetal box

aneroid barometermercury

barometer

air pressure

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airflow on crossing such a topographicbarrier and the associated release of latentheat changing the local thermodynamicsand dynamics of the flow. For example, asouth–north aligned barrier jet often devel-ops along the west side of the SierraNevada mountain range in California aswinter storms track eastward from the Pa-cific. Bands of heavy snow may developduring the evolution of a barrier jet.

baseline emission The long-term emis-sions, for example of greenhouse gases,that would occur if no policy interventiontook place. This provides a ‘business asusual’ scenario, which may be used as abaseline to show the effectiveness of poten-tial, in the case of MODEL experiments, andactual greenhouse-gas emission programs.

beaded lightning (chain lightning, pearl-necklace lightning) A rare form of light-ning channel which, on its dissipation,produces separate sections of iridescencereminiscent of a beaded necklace, probablydue to variations in thermal losses from theheated channel.

beam radiation See direct radiation.

Beaufort notation /boh-fert/ (Beaufortletters) A code consisting of a system ofletters originally introduced by Admiral SirFrancis Beaufort (1774–1857) to describepresent (ww) or past (WW) weather condi-tions. It has been much modified since; ex-amples of the letters include c for cloudy, ofor overcast, r for rain, p for passing show-ers, d for drizzle, s for snow, rs for sleet, hfor hail, tl for thunderstorm, and f for fog.Capital letters were introduced to the sys-tem to indicate an event’s intensity (e.g. S isheavy snow) and persistence (e.g. SS is pro-longed heavy snow). Beaufort’s alphanu-meric system is now rarely used, havingbeen replaced by an internationally recog-nized system of 100 geometrically basedsymbols (0–99) that are in current usage onofficial weather maps. For example, differ-ing intensities of rain (60–69) in the wwsystem are represented by geometricarrangements of solid black circles.

Beaufort scale A numerical scale rang-ing from 0 (calm) to 12 (hurricane) thatwas introduced by Admiral Sir FrancisBeaufort (1774–1857) in 1805 to describevariations in wind force at 10 m (30 ft)above the ground and originally based onthe effect of wind on a fully rigged man-of-war. Beaufort developed his index at atime when instruments for measuring windspeed were unavailable; approximate windspeeds and effects of each Beaufort numberon land and sea surfaces were only addedat a much later date. The International Me-teorological Committee adopted the scalein 1874 for use in weather telegraphs. Seetable overleaf.

belt A linear feature related to a specificweather system, for example a CONVEYOR

BELT is a characteristic aspect of most EX-TRATROPICAL CYCLONES. There are no strictscale criteria for the feature.

Bénard convection (Rayleigh–Bénardconvection) A type of CONVECTION thatdevelops in a shallow fluid that is heateddifferentially at its bottom and top. For ex-ample, longwave emission at the top of thestratiform anvil of a mature CUMULONIM-BUS cloud and absorption at the base desta-bilizes the anvil layer. Under suchconditions, the fluid evolves into a regulararray of hexagonal cells with a preferredhorizontal scale. The conversion of the sys-tem’s potential energy into kinetic energybecomes more efficient as the horizontaldimension of the cells decreases. This en-ergy conversion process is eventually offsetby frictional dissipation (e.g. momentum ismixed across the cell’s walls), which be-comes important when the horizontal di-mension of the cells is comparable to thedepth of the fluid. Consequently, the hori-zontal dimension of a typical cumulonim-bus cell is comparable to the depth of theTROPOSPHERE. It was named for the Frenchphysicist Henri Bénard and the Britishphysicist Baron Rayleigh (1842–1919).

Benguela Current /ben-gwel-ă, beng-/ A cold current in the South Atlantic Oceanthat flows northward along the west coastof southern Africa. It is a branch of the

27

Benguela CurrentFREEDOM PALESTINE


WEST WIND DRIFT in the southern hemi-sphere and it merges with the westward-flowing Atlantic South EQUATORIAL

CURRENT just before it reaches the Equator.

bent-back occlusion See back-bent oc-clusion.

Bergen School (Norwegian School) Afamous Norwegian school of meteorolo-gists, led by Vilhelm Bjerknes (1862–1951)and including his son Jacob Bjerknes(1897–1975), Halvor Solberg, and theSwedish meteorologist Tor Bergeron

(1891–1977). It produced ground-break-ing work on the nature of AIR MASSES andFRONTAL CYCLONES, based on brilliant ob-servational and theoretical work. Many fa-mous papers appeared soon after WorldWar I and through the 1920s from theirbase at the Bergen Geophysical Institute(now part of the University of Bergen).

Bergeron–Findeisen process /ber-jer-onfin-dÿ-zĕn/ (Bergeron process, ice-crystaltheory) A theory of the process of rain-drop growth in MIXED CLOUDS (composedof both ice crystals and supercooled liquid

bent-back occlusion

28

THE BEAUFORT SCALE ON LAND

Beaufort Descriptive Wind speed Visualnumber term knots mph effect

0 calm < 1 < 1 smoke from factories risesvertically

1 light air 1 – 4 1 – 4 wind direction shown bysmoke, but not by aweather vane

2 light breeze 4 – 6 4 – 7 wind felt on face3 gentle breeze 7 – 10 8 – 12 broken twigs from trees are

in constant motion4 moderate breeze 11 – 16 13 – 18 dust and loose paper raised

from sidewalk5 fresh breeze 17 – 21 19 – 24 swaying of small trees6 strong breeze 22 – 27 25 – 31 large branches of trees in

motion7 near gale 27 – 33 32 – 38 whole trees vibrate8 gale 34 – 40 39 – 46 twigs detached from trees9 strong gale 41 – 47 47 –54 chimney pots and roof tiles

fall from houses10 storm 48 – 55 55 – 63 trees uprooted; much struc-

tural damage11 violent storm 56 – 63 64 – 72 considerable damage. Usu-

ally experienced along thecoast; very infrequent in-land

12 hurricane > 64 > 73 mostly confined to the trop-ics. Disastrous; loss oflife, towns flattened

(As the Beaufort scale is subjective, wind speeds assigned to each Beaufort numbervary; as a result some values appear in more than one wind force class.)

FREEDOM PALESTINE


water drops), which proposes that all rain-drops begin as ice crystals at upper levels.The theory is based around the differentialin saturation vapor pressure (SVP) over iceand over supercooled water surfaces atcloud temperatures of between –10°C(14°F) and –30°C (–22°F). The SVP withrespect to ice is less than that with respectto supercooled liquid water at the sametemperature. Given this, evaporation cantake place from supercooled water dropletsand ice crystals can grow rapidly by directsublimation from water vapor on to the icecrystals at the expense of the supercooledwater droplets. When the ice crystals be-come heavy enough to overcome updraftsand fall through the cloud they may collidewith supercooled water droplets whichfreeze on contact in a process known as ac-cretion or riming. The ice particles formed,known as GRAUPEL, may collide with clouddroplets and fracture, producing more icecrystals, which can in turn grow into moregraupel. During their descent the ice crys-tals may stick or clump together, in aprocess known as aggregation, to formSNOWFLAKES. This process is most commonwhen the temperature is near 0°C (32°F). Ifheavy enough, the snowflakes fall throughthe atmosphere and, dependent on the tem-peratures in the lower atmosphere, fall assnow or melt to form raindrops. The the-ory depends on an ice/water mix in a cloudthus the mechanism does not operate intropical clouds in which temperatures areabove the freezing level.

The theory was proposed by theSwedish meteorologist, Tor Bergeron(1891–1977) and subsequently modifiedby Walter Findeisen. See also collision–coalescence process.

Berg’s index of continentality Seecontinentality.

berg wind A hot, dry, and occasionallydusty mountain wind of the FÖHN type thatblows from South Africa’s interior plateaudown to the coast, generally flowing in asoutherly direction. It occurs most fre-quently in winter when the pressure gradi-ent is directed offshore between highpressure over the plateau and low pressureover the ocean. A berg wind may persist forseveral days, giving oppressive weatherconditions with temperatures in excess of35°C (95°F) and sometimes causing severedamage to crops.

Berlin mandate A mandate concludingthat the commitment under the UNITED NA-TIONS FRAMEWORK CONVENTION ON CLI-MATE CHANGE (UNFCCC) to reducegreenhouse-gas emissions to 1990 levels bythe year 2000 was not adequate for thepurpose of halting potential anthro-pogenic-induced climate change. The man-date was negotiated in Berlin, Germany, inMarch 1995 at the First Conference of Par-ties (COP 1) and included commitments toundertake action beyond the year 2000.

Bermuda high (Bermuda anticyclone)Another name for the AZORES HIGH, thesemi-permanent subtropical anticycloneover the North Atlantic Ocean, especiallywhen located in the west part of the oceannear Bermuda and near 30°N. It is alsosometimes seen as a separate western highpressure cell within the broad subtropicalanticyclone over the North Atlantic.

billow clouds Wave-like cloud protu-berances that form on the upper surface ofa CLOUD STREET line in a layer where thewind speed increases sharply with heightand is just underneath an INVERSION.

29

billow clouds

satu

rati

on

vap

or

pre

ssu

re (

hP

a)

temperature (°C)

10

5

1

-30 -20 -10 0 100.5

ice

water

Bergeron–Findeisen process

FREEDOM PALESTINE


bimetallic thermograph A THERMO-GRAPH in which the continuous recordingof temperature is based on the differingheat capacity of two metals (typically ironand brass) welded together to form a singlestrip. The different expansion rates of thetwo metals causes the strip to bend whenheated. This strip is attached via gears andlevers to a pen that records the changes incurvature with time on a paper chart.

bioclimatology /bÿ-oh-kl;ÿ-mă-tol-ŏ-jee/The study of the relationships between liv-ing organisms, such as animals (includinghumans) and plants, and climate. This in-cludes the effects of climate on the organ-ism and also the effects the organism mayhave on climate; this includes, for example,the effects of human activities in contribut-ing to global warming.

biogeochemical cycle /bÿ-oh-jee-oh-kem-ă-kăl/ (nutrient cycle) The naturalprocesses that recycle elements betweenliving organisms (the biotic phase) andtheir non-living environment (the abioticphase). Examples of biogeochemical cyclesinclude the CARBON CYCLE, HYDROLOGIC

CYCLE, NITROGEN CYCLE, and oxygen cycle.Climatic and atmospheric changes canhave implications for these cycles.

biomass /bÿ-oh-mass/ 1. The total dryweight of all the living organisms of a giventype (e.g. species) or of all types within agiven area, such as a BIOME. It is typicallymeasured in terms of dry mass per squaremeter.2. Organic material (living and nonliving)from above and below ground, such astrees and tree litter, agricultural crops,grasses, and animals and animal waste.Biomass energy is produced by combustingorganic material, such as wood from trees,and has been proposed as an alternative toburning fossil fuels. A current theory holdsthat the release of carbon dioxide fromburning biomass as a fuel will not affectoverall atmospheric carbon dioxide levelsas long as replanting is maintained to ab-sorb carbon dioxide. Biomass burning thatoccurs as a result of burning forests, grass-lands, and cleared agricultural land

through land clearance or agriculturalpractices, for example, releases a variety ofcompounds including carbon dioxide,methane, and nitrous oxide, all of whichare GREENHOUSE GASES.

biome /bÿ-ohm/ A major biotic commu-nity, extending over a large geographicalarea and having a characteristic type ofdominant vegetation that is adapted to theclimatic conditions of the area. Biomes in-clude DESERT, grassland, SAVANNA, forest,and TUNDRA. In the mapping of biomes,areas that are now under agricultural culti-vation or urbanized are mapped accordingto the biome that is assumed to have beenpresent.

biometeorology /bÿ-oh-meet-ee-ŏ-rol-ŏ-jee/ The study of the relationships be-tween living organisms, such as animals(including humans) and plants, and atmos-pheric conditions, such as temperature andhumidity.

biosphere /bÿ-ŏ-sfeer/ That part of theEarth that contains living organisms; it in-cludes the lower part of the atmosphere inaddition to the Earth’s surface, seas, andoutermost part of the lithosphere.

bise /beez/ (bize) A cold dry wind thatblows from the northwest, north, or north-east to affect mountainous regions ofsouthern France and Switzerland duringthe winter. The strong outbreak of cold dryair associated with the bise is often accom-panied by heavy cloud.

bishop’s ring A pale whitish circle witha reddish-brown outer ring centered on theSun or Moon. It has been observed follow-ing volcanic eruptions, the first recordedobservation being after the eruption ofKrakatau in 1883. The cause is believed tobe the diffraction of light by fine volcanicdust or sulfur dioxide particles.

bispectral technique /bÿ-spek-trăl/ Aremote-sensing method used in meteorol-ogy for automatically recognizing and clas-sifying cloud type and cloud-free surfacetype by pairing the value of a PIXEL in both

bimetallic thermograph

30

FREEDOM PALESTINE


the VISIBLE and the THERMAL INFRARED im-ages of the same scene at the same time. If,for example, the two pixels are such thatthere is a strong signal in the visible (verybright reflection) and a weak signal in thethermal infrared (very cold surface), it isvery likely to be a CUMULONIMBUS cloud. If,in contrast, the visible displays a weak sig-nal (poor reflection) and the thermal in-frared quite a strong one (warm surface),the pixel is likely to be located over acloud-free sea surface.

bize /beez/ See bise.

black body A hypothetical body thatabsorbs and emits radiation at all thewavelengths for a given temperature, i.e. ithas an emissivity of 1. The land, ocean, andmost clouds are said to act as near blackbodies and emit the maximum amount ofradiation at their respective temperatures.

black-body radiation The radiationemitted from a surface across the full rangeof the electromagnetic spectrum for a givensurface temperature. The rate of emissionis expressed by the STEFAN–BOLTZMANN

LAW.

black-bulb thermometer An instru-ment that is sometimes used to measuresolar radiation by recording the maximumtemperature reached. It is basically a MER-CURY-IN-GLASS THERMOMETER with the bulbblackened and mounted in an evacuatedglass sheath.

black frost (hard frost) A frost that oc-curs when the atmosphere is very dry andice crystals fail to develop as hoar frost onexternal surfaces. In the absence of a pro-tective layer of hoar frost, internal freezingof moisture within the plants occurs andconsiderable damage results, includingblackened leaves and stems.

black ice A type of GLAZE that formswhen rain falls on to a surface, such as aroad, if the temperature is below 0°C(32°F). It is a potential hazard for highwayusers since the thin sheet of ice formed isdark in appearance and often hard to see.

blizzard 1. A storm of falling and/orblowing powdery SNOW, sometimes withsmall ice crystals, that is accompanied byhigh winds of at least 30 knots (35 mph),low temperatures of –7°C (20°F) or below,visibility of less than 1/4 mile, and lastingfor at least 3 hours. A severe blizzard is ac-companied by winds in excess of 39 knots(45 mph), temperatures below –12°C(10°F), and near zero visibility. Blizzardsare common in northern and central partsof the US, especially the N Great Plains,during winter. See also buran.2. A general term for a severe snowstormaccompanied by strong winds.

blocking An interruption or obstructionin the normal west-to-east flow of surfaceCYCLONES, ANTICYCLONES, TROUGHS, andRIDGES that occurs in middle or high lati-tudes and may last from a few days to sev-eral weeks. Blocking occurs in bothhemispheres but is more common in thenorthern hemisphere, especially over theeastern Pacific Ocean to the west of NorthAmerica and over the eastern AtlanticOcean to the west of northern Europe.Winter and spring are the times of greatestincidence and it is most pronounced nearthe tropopause. The upper-level flowchanges from predominantly zonal tomeridional eventually leading to the for-mation of CUT-OFF HIGHS and CUT-OFF

LOWS. Blocking can take one of a numberof forms. In the omega block, so named be-cause the meridional flow about this blockresembles the Greek letter Ω, the strongestflow is diverted to lower latitudes. A ridgeoften forms off the west coast of NorthAmerica in winter, which can steer Pacificstorms far to the north to keep British Co-lumbia and Pacific Northwest dry andmild. See also meridional circulation; zonalindex.

blocking anticyclone (blocking high) A large-scale stationary, or very slow-mov-ing, ANTICYCLONE (high), often aloft, thatextends down throughout the troposphere.This can occur when there is a split in thejet stream into two branches and the for-mation of a cut-off low south of the anti-cyclone. The mobile pressure systems are

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blocking anticycloneFREEDOM PALESTINE


diverted either north or south (or both) totrack around the flank of the block. Theblocking anticyclone can persist for a weekor more, diverting the usual zonal track offrontal depressions into a more meridionaltrack. See blocking. See also meridionalcirculation; zonal index.

blood rain RAIN containing fine red dustparticles derived from desert regions. Onceentrained into the atmosphere, these parti-cles are often transported over long dis-tances (e.g. Saharan red dust sometimesoccurs in rainfall as far north as Finland)by upper-level winds before being washedout in PRECIPITATION, which leaves a redstain on exposed surfaces.

blue Moon (blue Sun) A rare occur-rence in which the Moon or Sun appears tohave a bluish color. This arises when parti-cles of a particular size, produced typicallyby forest fires, dust storms, or volcaniceruptions, are suspended within the atmos-phere. The dust particles scatter the lightleaving only the blue end of the spectrum;sometimes a green or orange color occurs.The rarity of the event is reflected in thephrase ‘once in a blue moon’, to mean al-most never.

bomb see explosive cyclogenesis.

bora /bor-ă, boh-ră/ A regional name fora strong, cold, and often very dry north ornortheasterly wind that is funneled by thevalleys of the Karst and Dinaric Alps in theBalkan mountain range down to the east-ern (Dalmatian) coast of the Adriatic Sea,between Trieste and Albania. It occursmainly in winter, when air blows from acontinental anticyclone (high) establishedover central Europe to low pressure overthe Mediterranean. If a deep region of lowpressure is located in the Adriatic at thetime, then considerable precipitation canaccompany the bora. The wind may con-tinue for several days, sometimes with vio-lent gusts and squalls exceeding 50 knots(57 mph). Generically, the name bora canbe applied to a cold squall that movesdownhill from any upland area (e.g. fromthe Caucasus Mountains to the Black Sea);

also to a cold northeast wind from Siberiathat is experienced in the Gulf of Finland.

boreal /bor-ee-ăl, boh-ree-/ 1. Denotingthe north or the north wind.2. Denoting the coniferous forests in thenorth of the northern hemisphere. See alsoboreal climate.

boreal climate The climate that extendsover large areas of Eurasia and NorthAmerica and is associated with the borealforest zone. In Eurasia it extends from theScandinavian peninsula at about 70°Neastward across Siberia to the Pacific andas far south as about 50°S. In North Amer-ica it extends from Alaska across theYukon and Northwest Territories to theAtlantic coast of Canada. Winters are longand cold and summers short, cool, andhumid; there is a wide annual range of tem-peratures, e.g. at Yakutsk, Siberia, therange is 41°C (64°F). Much of the borealforest zone is moist but parts of westernCanada and Siberia have low annual pre-cipitation and are cold and dry. See alsoKöppen climate classification.

Boreal period A period in the post-glacial times between 9500 and 7500 yearsBP during which the Earth’s climate beganto warm. It was preceded by the Pre-Borealperiod and followed by the ATLANTIC PE-RIOD.

borehole In climatology and geo-physics, a hole drilled into the Earth’s sur-face, which is used for explorationpurposes to gather data for studies of pastclimate or for geophysical purposes. Exam-ples include ice cores drilled in the Antarc-tic ice sheet and sediment cores taken fromocean-floor surfaces. See also GreenlandIce Core Project; Greenland Ice SheetProject 2.

bottom water The deepest layer ofwater in the oceans, especially those watermasses that move slowly in deep waterareas. Bottom water is comparatively coldand dense. See Antarctic bottom water;Arctic bottom water.

blood rain

32

FREEDOM PALESTINE


Bouguer’s halo (Ulloa’s ring; Ulloa’s cir-cle) An optical phenomenon that appearsas a white arc or ring of light, with a radiusof 39°, centered on the antisolar point (i.e.the point on the celestial sphere that, to theobserver, is opposite the Sun). It is namedfor the French mathematician and hydrog-rapher Pierre Bouguer (1698–1758).

boundary current An ocean currentthat flows either at the eastern or the west-ern side of the subtropical GYRES in theoceanic basins, parallel to a continentalmargin. The eastern boundary currents areproduced by westerly winds being de-flected by the EKMAN EFFECT and the conti-nental boundaries. This often leads toupwelling of cold water from deeper levels.The currents are broad and generally cold;the CALIFORNIA and PERU CURRENTS in thePacific as well as the BENGUELA and CA-NARIES CURRENTS in the Atlantic are exam-ples. The western boundary currents arefaster, deeper, and narrower (less than 100km or 62 miles wide) than the easternboundary currents. They carry warm wateralong the western margins of the oceansand include the GULF STREAM in the AtlanticOcean and the KUROSHIO in the PacificOcean.

boundary layer 1. The thin layer offluid in contact with the surface of a solid;it is stationary as a result of the adhesionbetween the molecules of the fluid and thesurface of the solid.2. In microclimatology, the layer of the at-mosphere that is closest to the surface, suchas the ocean or land, or it may refer to avery thin layer of about a millimeter cover-ing the surfaces of vegetation throughwhich fluxes of energy and water vapormust pass. It is the layer immediately abovethe active layer.3. See planetary boundary layer.

boundary layer climate The integra-tion over time of all the flows and trans-formations of energy and mass that affectthe layer adjacent to the surface. In a re-strictive sense, it may refer principally totemperature and evaporation at or belowthe height at which most meteorological

measurements are made: the screen height,which is about 1.25 m (4 ft) above groundsurface. Boundary layer climates are the re-sult of the way in which many differentsurfaces receive and emit energy, and theninteract with the physical processes, in-cluding precipitation and airflow, of thefree air lying above the boundary layer.

bounded weak echo region (BWER,vault) A feature of the part of the veryrapidly ascending air within a THUNDER-STORM or SUPERCELL that has low RADAR re-flectivity and gives a weak radar echo. Theupward-rushing air moves so rapidly thatraindrop-size particles form only in theupper parts of the CUMULONIMBUS cloud.Radar that views the intense updraft of thispart of the cloud will see only a weak re-turn that is often surrounded (or bounded)by stronger returns from precipitation-sizeparticles – the bounded weak echo region.

bow echo In radar, the return signal orecho from PRECIPITATION that is associatedwith some SQUALL LINES that have intenserainfall and trailing STRATIFORM precipita-tion. As these evolve, they gradually takeon a ‘bow’ shape that is convex in the di-rection of motion. A little later in their lifecycle, they change into a ‘comma’ form - toproduce a COMMA ECHO. Bow echoes areoften associated with damaging surfacewinds near the apex of the bow; short-livedtornadoes may develop along the apex orat the north end of the bow (in the north-ern hemisphere).

Bowen ratio /boh-ĕn/ The ratio of theamount of SENSIBLE HEAT to LATENT HEAT

for a surface. The major use of solar energyat the Earth’s surface is either to heat thesurface (H) or to evaporate water (LE).The Bowen ratio is expressed as β = H/LE,where H is the sensible heat flux and LE isthe LATENT HEAT FLUX. Values rise fromabout 0.5 in humid middle latitudes to 5.0in deserts where available water for evapo-ration is a limiting factor. It is named forthe American astrophysicist Ira SpragueBowen (1898–1973).

33

Bowen ratioFREEDOM PALESTINE


box model A computer model used inatmospheric pollution studies to calculateconcentrations of pollutants. It is a blackbox volume in which pollutant concentra-tions from a number of small sources withassumed emission strengths per unit timeand area are treated as a single unit sourceof uniform concentration within theBOUNDARY LAYER, such as an urban area.The box-model solution assumes steady-state conditions and BACKGROUND LEVELS

of zero.

Boyle’s law A law that describes the be-havior of gases: at constant temperature,the volume of a given mass of an ideal gasis inversely proportional to the pressureupon it. The law was named for the Irishphysicist and chemist Robert Boyle(1627–91). See also Charles’ law; Dalton’slaw; ideal gas laws.

brave west winds A nautical term forthe westerly winds that occur over theoceans of the southern hemisphere be-tween approximately 40°S and 65°S. Thelow percentage of land relative to oceanmeans that such winds blow with greatforce and regularity; they are associatedwith rough seas. See also roaring forties.

Brazil Current A warm current in theSouth Atlantic Ocean that flows southeast-ward along the east coast of Brazil to about30°–40°S, where it is deflected to the east bythe northward-flowing Falkland Current.

breeze 1. A wind speed of force 2, 3, 4,5, or 6 on the BEAUFORT SCALE.2. A local-scale air movement that is con-vectively forced; for example a LAND

BREEZE or SEA BREEZE.3. Any generally light wind.

brickfielder (brickfelder) A hot, dry,and often dust-laden northerly wind expe-rienced along the southeastern coastlandsof Australia (for example, in Victoria) dur-ing the summer. It is caused by a polewardadvance of tropical air from the deserts ofthe interior. The brickfielder is usually as-sociated with the leading edge of a troughof low pressure, and precedes the

SOUTHERLY BUSTER. Prolonged hot spellsoften occur when the brickfielder blows;temperatures can exceed 40°C (104°F) ona daily basis.

bright band (melting band) In a radardisplay, a layer of enhanced radar reflectiv-ity that indicates the level where the icecrystals in snowflakes are melting to formraindrops. It is caused by the difference inradar reflectivity between ice crystals andwater particles.

brocken specter An optical phenome-non in which an observer’s shadow is seenagainst a bank of cloud, mist, or fog asgreatly magnified and sometimes sur-rounded by colored lights. It results fromthe diffraction of light. The phenomenon isnamed after the Brocken, the highest peakin the Harz Mountains of central Ger-many, where it was first noted.

bromofluorocarbon /broh-moh-floo-ŏ-rŏ-kar-bŏn/ See halon.

Brückner cycle /brûk-ner/ A cycling ofperiods of cold and damp conditions alter-nating with conditions that are warm anddry in northwestern Europe. In 1890, theGerman geographer and climatologist Ed-uard Brückner (1862–1927) calculatedthat there is a mean period of 34.8 yearsbetween such cycles, with cycles varyingfrom 25 to 50 years.

bubble high A mesoscale dome of highpressure that builds up from rain-cooledair in THUNDERSTORM downdrafts. Bubblehighs can be large enough and persist longenough to be identified on surface charts.The edges of these high-pressure regionsare marked by the GUST FRONT created bythe contrast between the warm thunder-storm air and the cooler bubble-high air.

Buchan spell /buk-ăn/ In the UK, aperiod of unseasonal weather during theyear that was believed to occur with regu-larity. In 1869, the Scottish meteorologist,Alexander Buchan (1829–1907), attemp-ted to construct a calendar of periods ofweather that recurred with sufficient regu-

box model

34

FREEDOM PALESTINE


larity at about the same time in the year tobe termed ‘spells’ in Scottish climate. Theinitial listing distinguished six cold periods,with temperatures regularly below the av-erage temperature curve, and three warmperiods. Statistical analysis has subse-quently cast doubt on the theory. Themethod was later refined by the British cli-matologist Hubert Horace Lamb (1913–97) and reinterpreted in terms of atmos-pheric circulation types. See singularity.

bulk Richardson number (BRN) A di-mensionless number derived from an equa-tion containing vertical STABILITY andvertical shear terms. The BRN is generallydefined as the stability term divided by theshear term. High values indicate an unsta-ble and/or weakly sheared atmosphere;low values being indicative of weak insta-bility and/or strong vertical shear. BRNvalues of 50–100 are favorable for SUPER-CELL development; thunderstorms are un-likely if BRN is <10. It was named for theBritish physicist Lewis Fry Richardson(1881–1953).

buoyancy force A force that acts uponan AIR PARCEL that has a different tempera-ture (T′) from that of the surrounding en-vironmental air (T) within which it occurs.The force is defined as positive (upward)when T′>T and negative (downward) whenT′<T.

buoy weather station A fixed or float-ing buoy in the ocean that carries instru-ments for sensing a number ofmeteorological parameters. The data is au-tomatically transmitted by radio.

buran /bû-rahn/ A strong cold north tonortheasterly wind experienced in Russiaand central Asia. Although the buran canoccur in any season, it is most frequent inthe winter when it is exceptionally cold,with temperatures below –30°C (–22°F)accompanied by BLIZZARDS that are dan-gerous to human and animal life, especiallyon the open steppes. The winter buran isalso known as the purga. Occurring in therear of a depression, the buran interruptsthe comparatively calm anticyclonic condi-

tions associated with the Siberian high. Seealso blizzard.

Bureau of Meteorology, Australia The national meteorological agency inAustralia, one of the leading meteorologi-cal services of the southern hemisphere,with its headquarters in Melbourne. TheBureau commenced operations in 1908and aims to observe and understand Aus-tralia’s weather and climate, and to pro-vide meteorological, hydrological, andoceanographic services for national useand also to support the country’s interna-tional obligations. It participates in theWORLD METEOROLOGICAL ORGANIZATION’s(WMO) WORLD WEATHER WATCH programwith one of the three World Meteorologi-cal Centers being based in Melbourne.

burst of the monsoon The onset of thesouthwest MONSOON over the Indian sub-continent and southeast Asia which, bytradition, is reputed to start in a dramaticway. It marks the end of the period of greatheat and DROUGHT that usually precedesthe invasion of moisture-laden air overIndia. Indian meteorologists tend to referto the ‘timetable’ of the burst to describethe progression of this change in tempera-ture and humidity across the subcontinent.It is recorded at Cochin (far southwest) onJune 1, Hyderabad (central east India) onJune 5, Calcutta on June 10, New Delhi onJune 15, and Lahore about July 1.

bush fire See fire.

butterfly effect See chaos theory.

Buys Ballot’s law A rule, proposed in1857 by the Dutch meteorologist Christo-pher Buys Ballot (1817–90), that relatesthe wind direction to the locations of high-and low-pressure centers. It states that ifone stands (in an exposed location) withone’s back to the wind, then (in the north-ern hemisphere) low pressure will lie to theleft and high pressure to the right. The op-posite is true for the southern hemisphere.

BWER See bounded weak echo region.

35

BWERFREEDOM PALESTINE


calibration The process of making andmeasuring instruments so that measure-ments can be made in specified units.

California Current A cold current inthe Pacific Ocean, a southward extensionof the NORTH PACIFIC CURRENT, that flowssoutheastward along the west coast ofNorth America. In the summer months es-pecially, the surface current is deflected tothe west through the EKMAN EFFECT, whichresults in the upwelling of colder water richin nutrients.

California fog See advection fog.

calm A state in the atmosphere in whichthere is virtually no horizontal motion, forexample smoke from a factory rises verti-cally. It corresponds to force 0 on the BEAU-FORT SCALE, with a wind speed of less than1 knot (1.15 mph). Calm conditions canoccur at any place and at any time, but aremost common in the subtropical high-pres-sure belts and in the doldrums along theEquator.

calvus /kal-vŭs/ A species of CUMULO-NIMBUS cloud in which some of the protu-berances across its upper surface are losingtheir sharp CUMULIFORM character but arenot CIRRIFORM in nature. See also cloudclassification.

Campbell–Stokes sunshine recorder A type of SUNSHINE RECORDER that uses theheating action of the Sun to record the du-ration of bright sunshine. A spherical glasslens focuses the Sun’s rays onto a recordingcard graduated in time intervals, usuallymarked in hours. Duration and intensity ofthe sunshine is determined by the lengthand width of the burnt track on the card.

Canaries Current (Canary Current) Acold current in the North Atlantic Oceanthat branches south from the NORTH AT-LANTIC DRIFT to flow southward past Spain,Portugal, and northwest Africa beforeturning westward to join the AtlanticNorth EQUATORIAL CURRENT. The tempera-ture of the current is relatively cool, a prod-uct of upwelling caused by continentaloffshore winds, and helps to reduce theheating effect of the Sahara to the east.

Cancer, Tropic of See Tropic of Can-cer.

canyon wind (jet-effect wind, mountaingap wind, ravine wind) A wind gener-ated as a result of variations in local topog-raphy. Airflow may, for instance, befunneled through a canyon, mountainpass, valley, or even a street in a city linedwith high buildings, resulting in local-scalewind velocities being higher than the re-gionally averaged mean wind speed. Exam-ples include the dusenwind (Aegean Sea,eastern Mediterranean), KOSAVA, tehuante-pecer (Central America), and WASATCH.

cap See inversion.

capacity In winds, the capability of awind current to transport material. Thewind’s capacity is measured by the maxi-mum amount of detritus (e.g. silt, sand,and/or gravel) carried past a specific pointin unit time. Capacity increases with windvelocity and decreases as the particle size ofdebris to be transported increases.

cap cloud 1. See pileus.2. A stratiform cloud above a hill or moun-tain and formed through orographic lift.

36

C

FREEDOM PALESTINE


CAPE See convective available potentialenergy.

capillatus /kap-ă-lay-tŭs, kă-pil-ă-tŭs/ Aspecies of CUMULONIMBUS cloud that ischaracterized in its upper reaches by CIRRI-FORM strands or sheets of fibrous cloudhaving the form of an anvil, a plume, or adisorganized mass of hair. This type of cu-mulonimbus is often accompanied by ashower or by a thunderstorm. It also fre-quently has distinct VIRGA. See also cloudclassification.

capping inversion See inversion.

Capricorn, Tropic of See Tropic ofCapricorn.

carbon cycle The global-scale exchangeof carbon substance between its differentreservoirs. These include the relativelyrapid continuous flow from the atmos-phere (where carbon is chiefly in the formof carbon dioxide) into plant life throughphotosynthesis. It can then move to ani-mals that consume plants (or other ani-mals), returning to the atmosphere inrespiration. Dead animals are decomposed

by bacteria and fungi, which return the in-gested carbon by respiration. Dead plantsfossilize into fossil fuels, some of which re-turn the carbon dioxide to the atmosphereon combustion (see diagram). Study of thecarbon cycle involves the estimation of themagnitudes and distribution of the reser-voirs, and the rates of flow of carbon be-tween them.

carbon dating (radiocarbon dating) Amethod, used in paleoclimatology for ex-ample, of estimating the age of organic ma-terial; it is based on the ratio of radioactivecarbon atoms to stable carbon atoms in thematerial. The radioactive carbon-14 (14C)is formed by the interaction of cosmic rayswith atmospheric nitrogen. Together withthe other two carbon isotopes, it is presentin living organisms; once an organism diesit ceases to absorb 14C, which steadily de-creases, decaying at the known rate of thehalf-life of 5730 ± 30 years. An estimate ofthe age of a material can be obtained fromthe residual 14C present. It gives reasonablyaccurate results for about 40,000–50,000years. However, since the method wasoriginally developed in 1946–47 byWillard F. Libby (1908–80), uncertainties

37

carbon dating

carbon fluxand reservoirs

shown in giga-tons

atmosphere750

fossil fuels andanthropogenic

sources

sediments

ocean40,000

90

5.5

92

1.6

60.0

0.5

61.4

vegetation, soilsand detritus

2190

global net primaryproduction and

respiration

changingland use

The global carbon cycle

FREEDOM PALESTINE


have arisen as to the actual 14C content ofthe atmosphere during past ages. It is nowaccepted that the content varies in line withfluctuations in the cosmic ray activity thatproduces the 14C. Corrections to dateswithin the recent past are now madethrough calibration with dates obtainedthrough DENDROCHRONOLOGY.

carbon dioxide (CO2) A TRACE GAS inthe Earth’s atmosphere; it is colorless,odorless, and noncombustible, and isformed by the oxidization of carbon andcarbon compounds. Carbon dioxide actsas an important GREENHOUSE GAS: it ab-sorbs longwave radiation emitted from theEarth’s surface and contributes to main-taining the Earth’s temperature at a level atwhich life can be supported. Carbon diox-ide concentrations vary in the range0.3–0.4% with location and season – thenorthern hemisphere has higher concentra-tions than the southern as a result ofgreater number of anthropogenic sources.Levels of carbon dioxide in the atmosphereare rising in line with increased emissionsfrom human activity, especially burningfossil fuels and deforestation, which is be-lieved to be an important cause of GLOBAL

WARMING. See also carbon cycle.

carbon dioxide equivalent (CDE) Ameasure used to compare the emissions ofdifferent GREENHOUSE GASES: the amount ofcarbon dioxide by weight that would causethe same amount of radiative forcing as agiven weight of the other greenhouse gasbeing measured. Carbon dioxide equiva-lents are generally calculated by multiply-ing the mass of the gas of interest (inkilograms) by its estimated GLOBAL WARM-ING POTENTIAL.

carbon equivalent (CE) A metric mea-sure used to compare the emissions of dif-ferent GREENHOUSE GASES. In the US,greenhouse gas emissions are usually ex-pressed as million metric tons of carbonequivalents (MMTCE). Carbon equivalent(CE) units can be converted from CARBON

DIOXIDE EQUIVALENT (CDE) units by multi-plying the CDE by the ratio of the molecu-

lar weight of carbon to carbon dioxide (i.e.12/44).

carbon monoxide (CO) A colorlessodorless gas that is toxic to humans. It isreleased into the atmosphere from naturalsources that include volcanoes and forestor bush fires, and from the incompletecombustion of fossil fuels through humanactivities, especially in vehicle exhaustemissions, and is a pollutant gas. It occursin the atmosphere in very low concentra-tions and is short-lived with a lifespan inthe atmosphere of a few months.

carbon sequestration The long-termstorage of carbon dioxide as a means of re-ducing harmful levels of the gas in the at-mosphere that contribute to the enhancedGREENHOUSE EFFECT and thus to globalwarming. The sites where it may be se-questered include plants (especially exten-sive forests), soils, the ocean, orunderground within depleted oil and gasreservoirs.

carbon sink A reservoir that absorbs ortakes up carbon that has been released else-where in the carbon cycle. In the generalcase, the atmosphere, the terrestrial bios-phere, oceans, and sediments can be re-garded as carbon sinks. Forests and oceansare potential large carbon sinks, whichmay help to offset greenhouse gas emis-sions.

carbon tetrachloride (tetrachloro-methane) A colorless volatile liquid,CCl4, that readily evaporates. It was for-merly widely used in the production ofchlorofluorocarbons and as a solvent. It istoxic to humans and a contributor to thedepletion of ozone in the stratosphere and,as such, was phased out in developed coun-tries by 1996, and is to be phased out in de-veloping countries by 2010 under therequirements of the MONTREAL PROTOCOL.

Caribbean Current A warm surfaceocean current, formed by the convergenceof the North EQUATORIAL CURRENT and theGuiana Current, that flows clockwise inthe Gulf of Mexico. The Caribbean Cur-

carbon dioxide

38

FREEDOM PALESTINE


rent flows through the Caribbean Sea andinto the Straits of Florida via the YucatánChannel to form the FLORIDA CURRENT.

castellanus /kas-telan-ŭs/ (castellatus) Aspecies of clouds that have cumuliformprotuberances that appear on their uppersurfaces. These features appear like tallturrets of a castle and can form on, for ex-ample, CIRRUS, CIRROCUMULUS, ALTOCUMU-LUS, and STRATOCUMULUS. ALTOCUMULUS

CASTELLANUS indicates instability in themiddle TROPOSPHERE and is often the pre-cursor of the development of thunder-storms. See also absolute instability; cloudclassification.

CAT See clear-air turbulence.

catchment area The area from which ariver, river system, groundwater system, orwater-supply system gathers its water. Thewater originates from precipitation andrunoff over the land surface into rivers andtributaries, and from groundwater flowbelow the surface.

CCL See convective condensation level.

CCN See cloud condensation nucleus.

CDE See carbon dioxide equivalent.

CE See carbon equivalent.

ceiling The height above the Earth’s sur-face of the base of the lowest cloud layer.The cloud layer can be scattered or over-cast, but must not simply be a thin layer.

ceiling balloon See balloon sounding.

ceiling light See cloud searchlight.

ceilometer /see-lom-ă-ter/ (cloud-baserecorder, cloud-height sensor) An auto-mated measuring and recording instrumentused to determine the height of the cloudbase. A beam of light is emitted verticallyinto the sky and the reflected light pickedup by a detector, which measures the time-lag between the emitted and detected light.Trigonometry is used to determine the alti-

tude of the cloud base. Modulating thelight beam so that it can be recognizedwhen it is reflected reduces interferencefrom other light sources. More advancedceilometers are a form of LIDAR in which alaser pulse is projected into the sky.

celestial sphere The imaginary spherein the heavens of infinite radius on whichthe stars and other celestial bodies appearto lie. The Earth and the observer are imag-ined to be at the center of the sphere.

cell In meteorology, an atmospheric cir-culation feature that is more-or-less closed.Cells occur on different scales; for examplethe massive troposphere-deep HADLEY CELL

that occurs in the north–south plane withinthe tropics, the cellular motion that char-acterizes open and closed cells in CELLULAR

CONVECTION, or the cell formed by the UP-DRAFT and/or DOWNDRAFT currents withina THUNDERSTORM.

cellular cloud /sell-yŭ-ler/ The organi-zation of CUMULUS and STRATOCUMULUS

cloud across, typically, a few tens of kilo-meters horizontally. They can be dividedinto open and closed cells: the open cellsconsist of a ring of cumulus with a clearcenter, and closed cells are filled with stra-tocumulus with a clear surrounding rim.

cellular convection Convection thattakes the form of localized convective cells,which typically develop as a result of thestrong heating of the land surface, for ex-ample during the summer months in themiddle latitudes. Low temperatures in theupper TROPOSPHERE, which aid the releaseof CONDITIONAL INSTABILITY or CONVECTIVE

INSTABILITY, encourage the development ofconvective cells, for example as seen in tor-nadoes in the US. This can lead to intenseprecipitation events, with THUNDER, LIGHT-NING, and HAIL. See also Bénard convec-tion.

Celsius scale A temperature scale rang-ing from 0 to 100 degrees in which 0°Ccorresponds to the freezing point of water,and 100°C to the boiling point of water. Itis internationally the most widely used

39

Celsius scaleFREEDOM PALESTINE


temperature scale although in the US, theFahrenheit scale remains in widespreaduse. It is named for the Swedish as-tronomer Anders Celsius (1701–44) whoseoriginal scale (in which freezing point was100° and boiling point, 0°) was proposedin 1742. The scale was known, until 1948,as the centigrade scale, because of the 100-degree interval between the freezing andboiling points; this name is still frequentlyused as a synonym for the Celsius scale.The Celsius scale can be converted fromthe FAHRENHEIT SCALE using the followingformula:

°C = 5(°F – 32) ÷ 9.

centigrade scale See Celsius scale.

Central England Temperature (CET) A continuous series of mean temperaturerecords for a roughly triangular-shapedarea of central England, enclosed by Bris-tol, Lancashire, and London. The series be-gins in 1659 and is the longest instrumentalrecord of temperature in the world. Mostof the monthly series was originally com-piled by the British geographer and meteo-rologist Gordon Manley (1902–80); thedaily figures from 1772 to date have sincebeen added. The Hadley Centre of the MetOffice in the UK updates the data which,since 1974, have been adjusted for globalwarming.

centrifugal force A fictitious force, thata body constrained to move in a circlearound a central point can be said to expe-rience. It acts outward radially from thecenter and has a magnitude, per unit mass,that is the body’s linear velocity squared di-vided by the radius of curvature of its cir-cular motion. In the atmosphere, this forceacts upon a particle of air that follows acurved path around a center of low or highpressure, for example. It appears to beequal in magnitude to the CENTRIPETAL

FORCE, which acts in the opposite direction.However, the use of the concept of a cen-trifugal force balancing a centripetal forcecan cause confusion because, in fact, thecentrifugal force is hypothetical and thecentripetal force is the gravitational force.

centripetal acceleration The radialcomponent of acceleration of a body that isconstrained to move in a circular path. It isthe acceleration needed to keep an objectmoving in a circular path.

centripetal force The radial force expe-rienced by a body that is required to con-strain it to move in a circular or curvedpath; the force is directed toward the cen-ter of rotation. It appears to be balanced bythe equal and opposite CENTRIFUGAL FORCE,but this latter force is hypothetical.

CET See Central England Temperature.

CFC See chlorofluorocarbon.

chain lightning See beaded lightning.

chaos theory A mathematical theoryused to describe the erratic behavior of thedynamical atmospheric–oceanographic sys-tem. According to this theory a smallchange in the initial state leads to a rapidand unpredictable change in subsequentstates. It is known as the butterfly effect asit has been suggested that the system is sosensitive that a butterfly flapping its wingsin one part of the world could be the impe-tus for a tornado occurring in another partof the world. Whether or not such an effectactually exists, it is true that in the contextof weather forecasting a small change inthe initial conditions can have a dramaticeffect on the outcome.

Chapman cycle of ozone formation A scheme proposed in the 1930s to explainthe natural formation and destruction ofstratospheric OZONE by a continuousprocess of photoassociation and photodis-sociation. The interaction of ultravioletsolar radiation with oxygen (O2) moleculeswould split them into two atoms, some ofwhich would combine with other mole-cules to form ozone (O3). In turn, theozone molecules are split into one oxygenmolecule (O2) and one oxygen atom (O).The reactions constitute a steady state inwhich the rate of ozone formation equalsthat of ozone destruction. It is named forthe British geophysicist Sydney Chapman

centigrade scale

40

FREEDOM PALESTINE


(1888–1970). See also ozone layer; ozonedepletion.

Charles’ law A law that describes thebehavior of gases at constant pressure. Itstates that all gases expand by 1/273 oftheir volume at 0°C for a temperature in-crease of 1°C; i.e. the volume of a givenmass of gas at constant pressure is directlyproportional to its THERMODYNAMIC TEM-PERATURE. The law is named for the Frenchscientist Jacques Charles (1746–1823). Seealso Boyle’s law; Dalton’s law; ideal gaslaws.

chart See synoptic chart.

chemistry model A mass-balancemodel in which most of the importantchemical kinetics present in a reactive at-mosphere are modeled using rate equations(speed of reaction) and controlled usingobserved rate parameters. The method hasbeen applied chiefly to the modeling ofphotochemical SMOG.

chilled-mirror hygrometer See dew-point hygrometer.

chinook /chă-nook, -nûk/ (snow eater) 1. A warm dry southwesterly FÖHN-typewind experienced along the eastern side ofthe Rocky Mountains in parts of Canadaand the US. The arrival of the wind is fre-quently heralded by the line of cloudknown as the CHINOOK ARCH. The chinookblows on the southern side of a depressiontracking eastward across the North Amer-ican continent. The airflow is compressedand warmed adiabatically on its descentfrom the Rocky Mountains. Sometimes thewarming process is so sudden that it causesthe air temperature to increase by 20°C injust 15 minutes, leading to rapid snow meltduring the winter and spring. Economi-cally important in pastoral regions, fre-quent and strong chinooks result in mildwinters; the absence of chinooks leads toseverely cold winters and potentially heavylosses of livestock.2. A southwesterly wind that brings warmand moist air to the Pacific coastal regionsof Washington and Oregon, US.

chinook arch A line of cloud that over-hangs the Front Range of the North Amer-ican Rocky Mountains when the CHINOOK

blows. A great deal of cloud develops onthe upslope side of the mountains and dis-sipates as the air subsides along the easternslope onto the High Plains, producing anarch-like feature that marks the line alongwhich the cloud evaporates.

chlorofluorocarbon /klor-ŏ-floo-ŏ-rŏ-kar-bŏn, kloh-roh-/ (CFC) Any one of anumber of artificial gases composed of theelements chlorine, fluorine, and carbon,which were originally developed in the1930s as nontoxic and nonflammablerefrigerants. They include dichlorodifluo-romethane (CFC-12). Chlorofluorocar-bons have also been used as propellants inaerosol spray cans, as foaming agents inthe production of expanded plastic foams,and as solvents for oil and grease. Theycontribute to the depletion of stratosphericOZONE above the Antarctic, and are pow-erful GREENHOUSE GASES. Their productionand use is being phased out as part of thedirectives from the MONTREAL PROTOCOL.See also ozone layer; ozone depletion.

CIN (CINH) See convective inhibition.

circulation In meteorology, a generalterm related to the organized motion of theatmosphere on a variety of space scalesfrom global down to that, for example, ofa small CUMULUS cloud. See also general cir-culation.

circulation index A numerical valuethat expresses the strength or magnitude ofone of the large-scale atmospheric circula-tion components. It measures, for example,the strength of the ZONAL FLOW (east–west)or MERIDIONAL CIRCULATION (north–south)components of wind at the upper levels.

circumhorizontal arc An optical phe-nomenon, a form of HALO, that appearsas a brightly colored arc of light, parallel tothe horizon, and below the level of the Sunor Moon. It is produced by the reflectionand refraction of light through ice crystalswith a 90° prism; the light enters through a

41

circumhorizontal arcFREEDOM PALESTINE


vertical face and exits through a horizontalface. It is only visible when the elevation ofthe Sun is higher than 58°. Compare cir-cumzenithal arc.

circumpolar vortex Two separateplanetary-scale circulation features that arecentered more or less on the North andSouth Poles. They affect regions at middleand higher latitudes in the middle andupper TROPOSPHERE and the STRATOSPHERE.In the troposphere the vortex consists ofthe complete pattern of LONG WAVES

around the hemisphere, with westerly flowabout an axis on the pole. In the stratos-phere the vortex is westerly in the winterand easterly in the summer, in response tothe reversal in the low- to high-latitudetemperature gradient at that level.

circumzenithal arc An optical phe-nomenon, a form of HALO, that appears asa short arc of bright colorful light, centeredon the zenith. It appears as a result of therefraction of light as it passes through sus-pended ice crystals with a 90° prism; thelight enters through a horizontal face andexits through a vertical face. Compare cir-cumhorizontal arc.

cirriform /si-ră-form/ Of or relating toclouds consisting entirely of ice crystalsand which have a fibrous structure, such asCIRRUS, CIRROSTRATUS, and CIRROCUMULUS.

cirro- A prefix related to HIGH CLOUDS,which have bases, in the middle latitudes,above about 5000 m (16,000 ft) and arecomposed almost exclusively of ice crys-tals. See also cloud classification.

cirrocumulus /si-roh-kyoom-yŭ-lŭs/ Atype of HIGH CLOUD, one of the cloud gen-era (see cloud classification), in the form ofa thin white sheet made up of small ele-ments that are ripples or patches with anapparent width of less than one degree,viewed from the surface.

cirrostratus /si-roh-stray-tŭs/ A type ofHIGH CLOUD, one of the cloud genera (seecloud classification), in the form of a thinsheet or whitish transparent smooth veil

that normally produces optical phenom-ena, such as halos around the Sun andMoon. It is composed of ice crystals.

cirrus /si-rubreve;s/ A type of HIGH

CLOUD, one of the cloud genera (see cloudclassification), with a delicate white fi-brous, wispy, patchy, or banded appear-ance. It occurs as detached areas often withthe appearance of feathers or streamersacross the sky. It is composed of ice crys-tals. [Latin: ‘wisp of hair’]

CISK See conditional instability of thesecond kind.

CLAMP See Climate-Leaf AnalysisMultivariate Program.

clathrate A solid compound in whichmolecules of one substance are trappedwithin a lattice of molecules of anothersubstance. Examples include molecules ofgases such as methane, carbon dioxide, orhydrogen sulfide trapped within a water–ice lattice. See also methane clathrate.

Clausius–Clapeyron relation /klow-zee-ŭs kla-pay-rawn/ The non-linear relation-ship between pressure and temperaturewhen two phases of a substance (for exam-ple, liquid water and water vapor) are inequilibrium. The equation gives the SATU-RATION VAPOR PRESSURE over a flat surfaceof pure water as a function of temperature.It was named for Rudolph Clausius(1822–88), a German physicist, andBenoit-Pierre-Emile Clapeyron (1799–1864), a French engineer.

Clean Air Act 1. In the US, the two actspassed in 1965 and 1970 affirming the au-thority of the Federal Government to limitinterstate pollution and to set national airpollution standards.2. In the UK, the Parliamentary Act of1956, which controlled the emission ofsmoke, together with the extension of thatact in 1968 controlling industrial emis-sions, and the Parliamentary Act of 1993.

clear-air turbulence (CAT) A layer,normally within the upper troposphere but

circumpolar vortex

42

FREEDOM PALESTINE


also occurring at lower levels, in which theairflow is very choppy or turbulent, and apotential danger for jet aircraft. Much tur-bulent weather is made visible by the pres-ence of CUMULUS and CUMULONIMBUS cloudwith which it is associated but clear-air tur-bulence, in general, has no visible indica-tors and cannot be seen. It is oftengenerated by the marked vertical and hori-zontal WIND SHEAR around the core of thePOLAR FRONT JET STREAM. Such rapidchanges of speed over a short distance leadto very turbulent flow. The phenomenoncan also occur in clear air much lowerdown in the troposphere over topography,for example in lee waves.

clear ice See glaze.

clear slot A localized area of clearerskies or reduced cloudiness caused by theintrusion of drier air that may be visible onthe west or southwest side (in the northernhemisphere) of the wall cloud below a SU-PERCELL thunderstorm.

CliC See Climate and Cryosphere pro-ject.

climate The average long-term prevail-ing WEATHER, including seasonal extremesand variations, for a particular region andtime interval. It is often represented as astatistical collection of weather conditionsfor that region with a description of suit-able boundary conditions (epoch, season,physical boundaries). Properties of interestinclude temperature, dynamics (wind, ver-tical motion, or ocean currents), thermody-namics, and hydrology (humidity,cloudiness, total column moisture, groundand surface water), and large-scale systems(pressure and density of the atmosphere,salinity of the ocean), as well as precipita-tion, evaporation, infrared radiation, con-vection, advection, and turbulence.

Climate can be seen to be the result ofthe redistribution of solar energy, the im-balance of the greater heating of the tropi-cal latitudes driving the atmospheric andoceanic motions that redistribute the en-ergy globally. The local climate for a par-ticular location is determined by latitude,

altitude, topography, and continentality oroceanicity. See also climate change; clima-tology. See illustration overleaf.

Climate Analysis Center See ClimatePrediction Center.

Climate and Cryosphere project(CliC) A core project of the WORLD CLI-MATE RESEARCH PROGRAM (WRCP). It de-veloped from the ARCTIC CLIMATE SYSTEM

STUDY (ACSYS) and was established in2000. The project aims to assess and quan-tify the impacts of climate change on thecomponents of the cryosphere, and theirconsequences for the climate system. TheCliC project examines the entire cryos-phere (e.g. snow cover; ice masses such asglaciers, ice sheets, ice caps, and ice shelves;ice on water bodies (sea, lake, and river);and frozen ground including permafrost)and its relation to climate.

climate change In general, the long-term fluctuations in the Earth’s climate.More particularly the term is used to de-scribe a significant change from one cli-matic condition to another on a globalscale; for example the shifts from ice agesto warmer periods that have taken placeduring the Earth’s geologic history. Suchchanges occur naturally, sometimesabruptly but at other times over consider-able timescales, through CLIMATE FEED-BACKS from changes, for example, in theEarth’s orbit, the energy output from theSun, volcanic eruptions, orogeny (moun-tain building), meteor strikes, or lithos-pheric motions. Over considerably shortertimescales more recent climate changes in-clude the periods of warmer-than-averagetemperatures, such as the Holocene cli-matic optimum and the medieval climaticoptimum (see climatic optimum), andcooler than average periods, such as theLITTLE ICE AGE.

More recently, the study of climatechange has sometimes been dominated bythe concept of GLOBAL WARMING and otherclimate changes caused by human activi-ties. There is evidence that the increase ingreenhouse gases, in particular of carbondioxide, as a result of human acitivity is af-

43

climate changeFREEDOM PALESTINE


44

sem

i-ar

id

tro

pic

alsu

b-t

rop

ical

tem

per

ate

arct

ic

des

ert

equ

ato

rial

c

limat

e

tro

pic

of

cap

rico

rn

tro

pic

of

can

cer

equ

ato

r

Clim

ate

FREEDOM PALESTINE


fecting climate by enhancing the naturalGREENHOUSE EFFECT thereby contributingto an increase in the Earth’s average sur-face temperature. See also radiative forc-ing.

climate classification A method to de-limit and describe the major types of cli-mate. The earliest were those of theclassical Greeks, divided along latitudinallines and the length of day into the FRIGID,TEMPERATE, and TORRID ZONES. Many dif-ferent modern schemes have since been de-veloped: (1) Generic classifications relate

climate limits to plant growth or vegeta-tion groups and use the basic criteria oftemperature and aridity. Aridity is usuallyexpressed as effective precipitation (precip-itation minus evaporation), based on theratio of rainfall/temperature. The widelyused climate classifications of WladimirKöppen (1846–1940) are examples of thistype. (2) Energy and moisture budget clas-sifications, such as that of the American cli-matologist Charles Warren Thornthwaite(1889–1963), which is based on the mois-ture budget and potential evapotranspira-tion. (3) Genetic classifications are basedon the causal elements of the climates, suchas the atmospheric circulation in terms ofwind systems and air masses. An exampleof this approach is that of the German cli-matologist Hermann Flohn (1912–97),proposed in 1950, and based on the globalwind belts and precipitation characteris-tics. Another genetic classification is thatof the American geographer-climatologistArthur N. Strahler (1918–2002) in 1951,which is based on air masses. (4) Climaticcomfort classifications, are based on com-fort indices to reflect how the climates ef-fect human comfort. An example of such ascheme is that of the American geographerWerner H. Terjung in 1966, which wasbased on the parameters of temperature,relative humidity, wind speed, and solarradiation. See also Köppen climate classifi-cation; Flohn climate classification;Strahler climate classification; Thornth-waite climate classification.

climate feedback A feedback associ-ated with climate processes; if the processincreases the effect of climate change it is apositive feedback, while if it diminishes theeffect it is a negative feedback. Generally,the interactions between the CLIMATE SYS-TEMS produce multiple feedbacks, whichsmooth out very long-term climate change.

Climate–Leaf Analysis MultivariateProgram (CLAMP) A method of recon-structing past climates, in particular the es-timation of mean annual temperatures, bystudying the fossil leaves of dicotyledonousplants.

45

Climate–Leaf Analysis Multivariate Program

0° 5° 10° 15° 20°| | | |1000

2000 –

3000 –

4000 –5000 –

7000 –

–10 000 –

–

–

––––––

–

8000 –

100 000 –

–

–

––––––

1 000 0000° 5° 10° 15° 20°

| | | |

year

sb

efo

rep

rese

nt

Flandrian

Devensian

Ipswichian

Wolstonian

Hoxnian

AnglianCromerian

Beestonian

Paztonian

Baventian

Antian

Changes in July mean temperature in lowlandEngland

FREEDOM PALESTINE


climate model A MODEL that uses quan-titative methods to represent dynamicaland physical processes and interactions inthe global system, which may include rep-resentation of the atmosphere, land, ocean,and ice. Climate models provide an impor-tant tool in the understanding and predic-tion of climate changes and can be used tomeasure anthropogenic climate change.See also general circulation model.

climate modeling program A programthat seeks to develop powerful computerclimate MODELS of the global atmosphereand of the combined atmosphere–oceansystem. These models are used to investi-gate climate variability, CLIMATE CHANGE

associated with the enhanced GREENHOUSE

EFFECT, and the likely impacts of climatechange.

climate prediction Forecasting the av-erage values of the properties of the atmos-phere some years into the future. Theaveraging is usually for a season, a particu-lar year, or for a group of years. Climateprediction models, for example, are used toassess the most likely impact of globalwarming over the coming decades and cen-tury. Their product might, for example, bea global chart of annual mean surface tem-perature for the year 2050, or for Decem-ber–February precipitation totals. Bysubtracting the same fields for the currentday, meteorologists can then quantify thebest estimate of the impact of, for instance,increasing the atmospheric carbon dioxideconcentration every year (in the model run)by 1 part per million.

Climate Prediction Center A US orga-nization, based in Camp Springs, Mary-land that fulfils the role of producingoperational predictions of climate variabil-ity, the real-time monitoring of climate,and the investigation of how major climateanomalies evolve. The operational fore-casts span the period from a few weeksahead to subsequent seasons. For example,it issues extended range outlook maps for6–10 and 8–14 days, and climate outlookmaps for 1–13 months ahead.

climate system The system that con-nects the entire interrelated processes of theatmosphere, oceans, land, biosphere, andcryosphere. Processes within this systemare divided into thermal, kinetic, and sta-tic, which are interconnected by the move-ment of water, radiation, and turbulence.

Climate Variability and PredictabilityProgram (CLIVAR) An internationalresearch program, begun in 1995 as part ofthe World Climate Research Program(WCRP), that examines issues of naturalclimate variability and anthropogenic cli-mate change, including the response of cli-mate systems to anthropogenic forcing.

climatic geomorphology The branchof geomorphology concerned with the as-sociation of types of landform with differ-ent climates. An example would be theassociation of periglacial landforms (thoseassociated with glacial conditions) in tun-dra climates. While there is some consis-tency in this association there are manyinstances in which this is not the case sothat factors other than climate need to beconsidered.

climatic optimum A period duringwhich global surface temperatures are sig-nificantly higher than the average. The pe-riod between 7000 and 4500 years BP,when global surface temperatures were sig-nificantly warmer than at present and attheir highest since the last ice age, is knownas the Holocene climatic optimum. It is be-lieved that during this time sea levels werearound three meters higher than their cur-rent level. The period of about 900–1200AD is known as the little climatic optimumor medieval climatic optimum.

climatic region See climatic zone.

climatic snow line See snow line.

climatic zone (climatic region) An areaof the Earth’s surface in which the climateis generally similar. Many CLIMATE CLASSI-FICATIONS have been devised to delimit cli-matic zones, ranging from simplistic tocomplex.

climate model

46

FREEDOM PALESTINE


climatograph /klÿ-mat-ŏ-graf, -grahf/ See climograph.

climatological station /klÿ-mă-tŏ-loj-ă-kăl/ An observing station where meteor-ological data is collected. The WorldMeteorological Organization classificationsinclude ordinary climatological stationsand principal climatological stations. Atthe ordinary stations observations aremade of maximum and minimum tempera-tures and precipitation at least once a day.At principal stations readings are takenhourly or at least three times a day of acomprehensive range of climate variables(weather, wind, cloud amount, type andheight of cloud base, visibility, humidity,temperature, atmospheric pressure, sun-shine or solar radiation, and soil temp-erature).

climatology /klÿ-mă-tol-ŏ-jee/ The studyand description of climate, i.e. the meanstate of the atmosphere and oceans, to-gether with statistical CLIMATE variability.Climate variability may encompass thoseassociated with seasonal variation, changesto external forcing, and internal interac-tions and feedbacks. Many scientists areforecasting the likelihood of climatechange due to GLOBAL WARMING and there-fore the study of climates is gaining greaterimportance.

climograph /klÿ-mŏ-graf, -grahf/ (cli-matograph, climatic diagram) A graphor diagram that displays two or more cli-

mate variables, for example mean temper-ature and mean precipitation, for a givenlocation; the data may be displayed overdifferent timescales but usually by month.

CLIVAR /klÿ-var/ See Climate Variabil-ity and Predictability Program.

closed low 1. See cut-off low.2. A CYCLONE (low) that is defined bymore-or-less circular isobars.

cloud A visible volume of very minutewater droplets or ice crystals, or a mixtureof both, that occurs above the Earth’s sur-face. The constituent droplets are no largerthan 200 µm in diameter and condense outfrom moist air principally when it ascendsand cools adiabatically in convection.Cloud can form also through orographiclift when moist air is lifted over hills andmountains or when it is forced to rise inhuge volumes within the circulation oflow-pressure systems. There are otherforms of cloud related to smaller-scaleforcing, for example the BANNER CLOUD.

Clouds are composed almost entirely ofsupercooled water droplets from 0°C(32°F) to –10°C (14°F) for layer clouds and–25°C (–13°F) in convective clouds. It isonly when the temperature is –40°C(–40°F) or colder that a cloud is made upentirely of ice crystals. See also cloud clas-sification.

cloud amount The fraction of the skythat is covered by either an individual typeof cloud, or the total amount of all cloudtypes present. Both are reported. The frac-tional unit employed internationally is theOKTA, or eighth of the sky, so that zeroeighths means completely cloud-free whileeight oktas is overcast. There is a possibil-ity of nine eighths, which means that thesky is obscured (e.g. by fog). The totalcloud amount in oktas is represented by in-ternational convention as an increasinglyblackened-in station circle that forms acomponent of the STATION MODEL. Aslightly different scale is used in NorthAmerica where it is based on tenths inplace of the eighths of oktas.

47

cloud amount

JulyAugust

September

OctoberApril

November

DecemberJanuary

February

March

May

June 30°C30°C30°C

20°C20°C20°C

10°C10°C10°C

0°C0°C0°C

Climograph: mean monthly maximum temperatures at Kew (London)

FREEDOM PALESTINE


cloud base The height above the Earth’ssurface of the base of a cloud. Technicallyit is the lowest zone in which the obscura-tion changes from that associated withclear air or haze to that linked to waterdroplets and/or ice crystals.

cloud-base recorder See ceilometer.

cloudbow See fogbow.

cloudburst A general term for brief butexceptionally heavy precipitation, usuallyof rain but also hail, that may be associatedwith a thunderstorm. Cloudbursts canoccur anywhere but are most frequent inmountainous areas, are often associatedwith violent upward and downward aircurrents, and may cause considerable dam-age. Temperate cloudbursts are short-livedsince the available supply of water vapor issoon exhausted.

cloud ceiling See ceiling.

cloud classification The most com-monly used classification of clouds,adopted by international agreement by theWorld Meteorological Organization, isbased on the height of a cloud’s base abovethe Earth’s surface and divides the types ofclouds into four main groups: low, middle,and high, and clouds of great vertical ex-tent. The approximate height ranges forthe low, middle, and high groups is shownin the Table above. The clouds of great vertical extent havetheir bases in the low cloud group. The lev-

els are linked to the ten cloud genera thatform the most basic cloud types: STRATUS,STRATOCUMULUS, and NIMBOSTRATUS (LOW

CLOUDS); ALTOCUMULUS and ALTOSTRATUS

(MIDDLE CLOUDS); CIRRUS, CIRROSTRATUS,and CIRROCUMULUS (HIGH CLOUDS); and CU-MULUS and CUMULONIMBUS (clouds of greatvertical extent).Further refinements to the classificationare the division of the genera into cloudspecies based on their differences in shapeand internal structure, as well as the divi-sion into varieties, based on their arrange-ment and transparency. In addition, anumber of supplementary descriptiveterms and ACCESSORY CLOUDS are used.Clouds can also be classified according toair motion that led to their formation:STRATIFORM (layer) clouds that form eitherthrough ascent of the air or through turbu-lence; CUMULIFORM that form through con-vection; and OROGRAPHIC CLOUD, in whichthe air is forced to rise over a hill or moun-tain barrier. The original classification ofthe cloud genera was proposed in 1803 bythe British pharmacist and amateur meteo-rologist Luke Howard (1772–1846) and,with modifications, this still forms thebasis for cloud classification. The types ofclouds in the classification are illustrated inthe first INTERNATIONAL CLOUD ATLAS,which was published in 1896. The WorldMeteorological Organization published anew two-volume International Cloud Atlasin 1956, with subsequent new editionssince (volume 1 is a loose-leaf manual forthe observation of clouds and other mete-ors; volume 2 contains photographs of

cloud base

48

CLASSIFICATION OF CLOUDS

Cloud Group Polar Latitudes Temperate Latitudes Tropical Latitudes

High 3000 – 8000 m 5000 – 13,000 m 6000 – 18,000 m10,000 – 26,000 ft 16,000 – 43,000 ft 20,000 – 60,000 ft

Middle 2000 – 4000 m 2000 – 7000 m 2000 – 8000 m6500 – 13,000 ft 6500 – 23,000 ft 6500 – 26,000 ft

Low Surface – 2000 m Surface – 2000m Surface – 2000 mSurface – 6500 ft Surface – 6500 ft Surface – 6500 ft

FREEDOM PALESTINE


49

1500 m

3 km

6 km

12 km0

cirrus

altocu

mu

lus

nim

bo

stratus

cirrocu

mu

lus

stratus

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clouds); a single-volume, abridged versionhas also been published.

cloud cluster A specifically tropicalphenomenon in which a region of deep vig-orous cumulus and cumulonimbus cloudsare clustered together over the warmocean. Cloud clusters are very common inthe humid tropics and may dominate theIntertropical Convergence Zone (ITCZ),when 10–15 clusters can occur in a month.They form through the amalgamation ofareas of mesoscale convection. The clustersmay persist for several days, accompaniedby thunderstorms and heavy rain. Theseareas represent concentrations of cyclonicflow at low levels and anticyclonic flow athigh levels. The release of latent heatthrough condensation plays a significantpart in weakening the TRADE-WIND INVER-SION of the tropics.

cloud condensation nucleus (CCN) Ahygroscopic substance (i.e. a substancethat has an affinity for water) or aerosolthat provides the surface on to which con-densation of water or ice takes place toform a CLOUD DROPLET. The radius of thenuclei ranges from less than 0.2 µm(AITKEN NUCLEI), from 0.2 to 1.0 µm (largenuclei), and larger than 1.0 µm (giant nu-clei). Condensation nuclei enter the atmos-phere in a number of natural andman-made forms including volcanic dust,factory and forest-fire smoke, ocean-spraysalt, and sulfate particles emitted byoceanic phytoplankton. See also freezingnucleus.

cloud droplet A liquid water dropletthat constitutes clouds; cloud droplets aretypically 20 µm in diameter and so aresome 100 times smaller than a typical RAIN-DROP. Their fall speeds range from 0.01m s–1 to about 0.3 m s–1 depending on theirsize; this settling speed is usually more thanbalanced by the air’s upward motion thatforms the cloud in the first place.

cloud echo A radar signal that is re-flected by the water droplets within acloud.

cloud height The elevation of the baseof a cloud above the Earth’s surface.

cloud-height sensor See ceilometer.

cloud motion vectors (CMV) Esti-mates of wind speed and direction basedon the successive location of particulartypes of cloud on half-hourly (or someother time interval) thermal infrared im-ages from GEOSTATIONARY SATELLITES. Theclouds must be of the type that act as goodtracers of the wind, such as trade-wind cu-mulus.

cloud physics The scientific study of theformation and evolution of CLOUDS andPRECIPITATION. It includes, for example, themicrophysical processes of cloud forma-tion, nucleation, aerosol and cloud interac-tions, cloud electrification and lightning,and ice physics.

cloud searchlight (ceiling light) An in-strument that is used to measure the heightof a cloud base. The narrow beam from apowerful searchlight is projected verticallyonto the cloud base and an observer usingan alidade or clinometer, located at aknown distance from the searchlight, mea-sures the angle of inclination, from whichthe height is calculated. Compare ceilome-ter.

cloud seeding A method of distributingsubstances in clouds in an attempt to mod-ify the weather in some manner, for exam-ple to induce precipitation (see artificialrain), or suppress hail or wind speed. Anumber of techniques are used to induceprecipitation. Substances, such as silver io-dide, which has a similar structure to thatof ice, are dispersed in supercooled cloudto promote the growth of ice particles. Dryice is used to lower temperatures to below–40°C (–40°F) to induce glaciation inclouds. Hygroscopic particles of salts (e.g.ground salt particles), which take onwater, are dispersed to promote coales-cence. Ice-nucleating substances such as sil-ver iodide are released into convectiveclouds to suppress hail formation but re-sults are inconclusive.

cloud cluster

50

FREEDOM PALESTINE


cloud street Low cloud that forms, inan unstable layer of air which is usuallycapped by a temperature inversion, intodistinct lines or streets of CUMULUS alongwhich the air ascends. Between the streets,the air sinks to create intervening lines ofclear sky. They stream along with the windand are normally of limited depth.

cloud tag Small distinct fragments ofcloud on the outer surface of a tornado’sfunnel cloud that permit an observer to de-duce, by tracking the ‘tag’ visually, that thefunnel is rotating.

cloud-to-air discharge A LIGHTNING

discharge in which the lightning channelpropagates away from a charged region todissipate in apparently clear air.

cloud-to-cloud discharge An electricalLIGHTNING discharge between two differentclouds (inter-cloud discharge), in which thedischarge bridges the clear air between theclouds, or between two regions of thesame, extensive cloud (intra-cloud dis-charge); no charge exchange occurs withthe Earth’s surface. Cloud-to-cloud dis-charges are more common than cloud-to-ground discharges.

cloud-to-ground discharge (ground dis-charge) LIGHTNING that leads to the ex-change of charge between an electrifiedcloud and the Earth’s surface. Charge ismost commonly transferred from thelower, negative region of a thundercloud,although the properties of individual dis-charges vary considerably. The STEPPED

LEADER initially facilitates the breakdownof the air, after which peak currents of 20kA typically flow for 10 microseconds,when the main conduction channel hasbeen established. Positive cloud-to-ground(CG) strikes are less common than negativeCG strikes, but can arise from stronglysheared winter storms in which large cur-rents flow; they also occur more often insome areas, for example the North Sea inEurope.

cloud type See cloud classification.

CMV See cloud motion vectors.

coalescence A mechanism central to theCOLLISION–COALESCENCE PROCESS of rain-drop growth in which large drops, togetherwith ice crystals in frozen cloud tops, falland grow by coalescence with smallerdroplets. Factors favoring coalescence arestrong upward turbulence, a high moisturecontent, and water cloud of great verticalextent.

coalescence efficiency In cloud physicsand raindrop growth, the fraction of clouddroplets that collide and merge to formlarger droplets. See collection efficiency.

coastal desert See west coast desert.

col A feature on an isobaric map sepa-rating two ridges of high pressure or anti-cyclones (highs) that are opposite oneanother from two troughs of low pressureor lows that are also opposite one another.It is characterized by a weak pressure gra-dient and is similar in pattern to a col or asaddle on a land surface topography map.In the summer, cols can be associated withthundery weather.

51

col

low100810081008101210121012

hig

h

highCOL

102010201020

1012

1012

1012

100810081008

100410041004 100010001000

996996996992992992

100810081008

COL

low

low

high

102410241024

Fig. 1: Anticyclonic colFig. 1: Anticyclonic colFig. 1: Anticyclonic col

Fig. 2: Cyclonic colFig. 2: Cyclonic colFig. 2: Cyclonic col

1016101610161020 mb1020 mb1020 mb

101610161016

101210121012

100010001000 1004

10041004

Pressure patterns in a col

FREEDOM PALESTINE


cold advection The horizontal move-ment of an air mass characterized by lowtemperatures and increasingly low relativehumidity, to regions of greater warmth. Insynoptic meteorology, cold advection is as-sociated with the invading air followingthe passage of an active COLD FRONT (ANA-FRONT). See advection.

cold air funnel A FUNNEL CLOUD or arelatively weak TORNADO formed in a con-vective storm that has developed in associ-ation with a pool of cold air aloft.

cold anticyclone An ANTICYCLONE

(high) that forms during late fall or winter-time over middle- and high-latitude conti-nents. Such highs are produced by intenseRADIATIVE COOLING and are shallow sys-tems that are no more than 2–3 km (1–2miles) deep. See Siberian high.

cold boreal forest climate See Köppenclimate classification.

cold conveyor belt See conveyor belt.

cold front The leading edge of cold airof an advancing mass of relatively colderair. It is an integral feature of a middle-lat-itude FRONTAL CYCLONE. As a cold front

sweeps across a site, the air temperatureand humidity fall. The generally overcastthick cloud of the WARM SECTOR gives wayeither to scattered convective cloud (overmaritime areas) or cloud-free skies (partic-ularly across extensive continents in win-tertime) after a cold frontal passage. Anana-cold front is characterized by warm airthat rides up and over it, producing a sharpline of heavy showers and/or thunder-storms slightly ahead of the surface front,and extensive precipitation behind it. Incontrast, a kata-cold front is typified bydry air that overtakes the front in the mid-dle troposphere so that much of the precip-itation is suppressed, and falls mainly inthe warm sector.

cold-front clearance The improvementin visibility and clearing of cloud cover thatgenerally follows the passage of a COLD

FRONT. These conditions contrast stronglyto the pre-cold frontal weather in theWARM SECTOR in which extensive low cloudand precipitation will reduce visibility.

cold low (cold pool) 1. A low filled withair that is colder at all heights than the airsurrounding it. This means that the windcirculating around the low will strengthenwith height.2. See cut-off low.

cold pole (pole of cold) 1. The locationin either hemisphere of the lowest mean an-nual temperatures of the Earth’s surface. Inthe northern hemisphere this is in Siberia,and believed to be near Verkhoyansk witha mean annual temperature of –16.3°C(2.7°F). In the southern hemisphere a meanannual temperature of –58°C (–72.4°F) hasbeen recorded for the Pole of Inaccessibil-ity.2. The location in each hemisphere thatholds the record for the lowest recordedtemperature. The Vostok station inAntarctica holds the record for the coldestrecorded temperature in the world with–89.2°C (–128.6°F) recorded on July 21,1983. In the northern hemisphere, the low-est recorded temperature is –67.8°C(–90°F), which has been recorded at both

cold advection

52

directionof front’smovement

north

clear,cold

precipitationregion

cold,drierair

NWwind

mild,moistair

SW wind

NW wind

possiblesnow

SWwind

thunderrain,

rain

1008

1004

1000

0 km 100

and/ordrizzle

Cold front

FREEDOM PALESTINE


Verkhoyansk (February 7, 1892) andOymyakon (February 6, 1933) in Siberia.

cold pool See cold low.

cold sector The extensive region ofcolder air that follows a COLD FRONT in adeveloping FRONTAL CYCLONE.

cold-water desert See west coastdesert.

cold wave An outbreak of unusuallycold air bringing a rapid fall in temperatureacross a region. It is, for example, often as-sociated with relatively prolonged and se-vere conditions across a large proportionof the US. The National Weather Servicedefinition of a cold wave is based on thecriteria of the rate of temperature fall andthe minimum to which the temperaturefalls, depending on the location and time ofyear.

collar cloud A usually circular ring ofcloud associated with a TORNADO; it sur-rounds the upper part of a WALL CLOUD.Collar cloud is sometimes used as a syn-onym for the wall cloud itself.

collection efficiency (Ec) A measure ofthe efficiency of the raindrop generationprocess in clouds. Higher values of Ec de-note increased efficiency. It is defined asthe product of the COLLISION EFFICIENCY (E)and the COALESCENCE EFFICIENCY (E′), i.e. Ec= E.E′. In the example of a larger droplet Aand a smaller droplet B that is to be ab-sorbed by A, the collision efficiency is afunction of the radius (r) of droplet B (i.e.rB); E declines as rB decreases. The coales-cence efficiency (E′) is defined as: numberof droplets coalescing/number of colli-sions. If all colliding droplets coalesce, thenE′ = 1.

collision–coalescence process One ofthe currently accepted theories of theprocess of raindrop growth in warmclouds, based on the mechanisms of colli-sion, coalescence, and rear capture. Thesemechanisms are thought to be responsiblefor rainfall generated from tropical convec-

tional clouds where the BERGERON–FIND-EISEN PROCESS cannot operate; they are alsoimportant in mid-latitude cloud systems.

Cloud droplets, formed by condensa-tion of water vapor onto CLOUD CONDENSA-TION NUCLEI, vary in size from less than 1µm to over 20 µm. The larger drops, withterminal velocities increasing in proportionto their diameters, begin to fall faster thanthe smaller droplets. During their fall thelarger droplets collide with smallerdroplets and often the smaller droplets areabsorbed through coalescence. The largercloud droplets thus grow into raindrops atthe expense of the smaller ones. Once araindrop reaches 5 mm it becomes unstableand breaks up into smaller drops. The col-lision–coalescence process is affected bythe range of droplet sizes; the depth of thecloud, which determines the quantity ofcloud droplets available; the strength ofupdrafts within the cloud; and the electri-cal charge of the droplets.

collision efficiency In cloud physicsand raindrop growth, the fraction of waterdroplets that collide with a larger fallingdrop: the higher the collision efficiency, thehigher the rainfall. See collection effi-ciency.

Colorado low A cold season low-pres-sure system that forms to the lee (east) ofthe Rocky Mountains, usually in SE Col-orado, and commonly tracks northeast-ward, producing extensive snowfallsacross the region toward the Great Lakes.A Colorado low is an example of a LEE DE-PRESSION.

color display A means of portraying aremotely sensed image more clearly to theeye, by using a range of colors to stress par-ticular features. Color coding differentcloud-top temperatures for deep CONVEC-TIVE CLOUD can be useful to meteorologists,for example.

comfort index See temperature–humid-ity index.

comfort zone The range of tempera-ture, humidity, and air movement within

53

comfort zoneFREEDOM PALESTINE


which the majority of people are mostcomfortable and work most effectively. Seealso temperature–humidity index.

comma cloud On the mesoscale, an ex-tensive region of CUMULIFORM cloud orga-nized into a distinctive comma shape. Suchcloud features tend to be more commonover wintertime high-latitude oceans, andare somewhat smaller features than mid-dle-latitude FRONTAL CYCLONES. They arenormally associated with heavy showers.

On the synoptic scale, comma cloud isalso used more generally for the largerfrontal cyclone cloud patterns that lookvery much like a comma when seen onsatellite imagery.

comma echo The RADAR return or echofrom heavy precipitation in a SQUALL LINE

that takes on a comma-like pattern duringthe later stages of its life cycle, after it hasbeen through the BOW ECHO phase.

concrete minimum temperature Ameasure of temperature recorded from astandard MINIMUM THERMOMETER that isexposed at the center of, and in contactwith, a smooth slab of concrete of specifiedstandard dimensions. In this case the bulbof the thermometer measures the tempera-ture of the air in contact with the concrete.

condensation The process by which liq-uid water is formed from gaseous WATER

VAPOR; it is the opposite process to EVAPO-RATION. The capacity of a parcel of air tohold water vapor decreases with tempera-ture and hence, cooling of the air forcessaturation (see saturated air) and thenCONDENSATION. Condensation may occurunder adiabatic conditions in which risingair expands and cools without any ex-change of energy outside the air parcel (for-mation of clouds); by surface boundarylayer mixing with cold air, particularlyunder inversion conditions (formation offog); or by contact with surfaces of lowertemperature (formation of dew).

condensation funnel The rapidly ro-tating cloud feature of a TORNADO madevisible by the condensation of water

droplets. These are formed in associationwith the extreme fall of pressure experi-enced as the moist ambient air is drawninto the tornadic circulation.

condensation level The level above thesurface at which the dew-point is reachedand CLOUD DROPLETS condense out of as-cending moist air. The height of the con-densation level depends on the air’s surfacetemperature and dew-point temperature; alarge difference between them will mean ahigh condensation level while a small dif-ference will lead to a low condensationlevel or CLOUD BASE.

condensation nucleus See cloud con-densation nucleus.

condensation trail (contrail) An ini-tially thin trailing line of water droplets orice crystals that condenses or freezes out ofthe water vapor emitted in the exhaustfrom jet engines. Whether a condensationtrail occurs or not depends on a criticaltemperature at flight level. How persistentthey are is largely a function of the drynessof the air at the aircraft’s flight level; long-lasting condensation trails are expressionsof high relative humidity where they occurand they tend to spread out on the wind. Acondensation trail is also sometimes re-ferred to as a vapor trail, which is inaccu-rate because vapor is invisible.

condensation-type hygrometer Seedew-point hygrometer.

conditional instability The atmos-pheric state in which the magnitude of theENVIRONMENTAL LAPSE RATE (ELR) is higherthan the SATURATED ADIABATIC LAPSE RATE

(SALR) but lower than the DRY ADIABATIC

LAPSE RATE (DALR). A rising unsaturatedair parcel will initially cool at the dry adia-batic lapse rate; as the parcel is cooler thanits surroundings a condition of atmos-pheric STABILITY exists. With continuedvertical uplift (e.g. over a mountain bar-rier), the air parcel will reach DEW-POINT

and become saturated. Owing to the re-lease of latent heat of condensation, theparcel will now cool at the slower satu-

comma cloud

54

FREEDOM PALESTINE


rated adiabatic lapse rate. Eventually, theair parcel will become warmer than its sur-roundings and hence UNSTABLE, typicallyresulting in the development of CUMULUS

clouds and THUNDERSTORMS. The descrip-tion ‘conditional’ is applied in this case be-cause the instability is conditional on thesaturation of the air mass.

55

constancy of wind

temperatureofenvironment

temperature

hei

gh

t h

ELRDALR

rising saturatedair parcel coolingadiabatically

rising unsaturatedair parcel coolingadiabatically

SA

LR

unstable

A

B

A = level of free convectionB = lifting condensation level (LCL)

Conditional instability

conditional instability of the secondkind (CISK) The process in which CON-VERGENCE at low levels in a WIND FIELD

produces CONVECTION and CUMULUS for-mation. The consequent release of latentheat enhances the degree of convergenceand increases convection, so establishing apositive feedback loop that may lead ulti-mately to the formation of a large-scale dis-turbance, such as a HURRICANE.

conduction The transfer of heat by mol-ecular diffusion from one body (such as awarm ocean current) with a relatively highsurface temperature to another body (vol-ume of air) with a lower temperature withwhich it is in direct contact. As air is a poorconductor of heat it is only close to theEarth’s surface that conduction is signifi-cant as a mechanism of heat transfer withinthe atmosphere. See also convection; radi-ation.

confluence The process in which airstreams from different directions flowsmoothly together from either side of, andtoward, an asymptotic line. It is the oppo-site of DIFLUENCE. See also convergence; di-vergence.

congestus /kŏ-jess-tŭs/ (towering cumu-lus, convective cumulus) A species ofcloud, the type of CUMULUS cloud charac-terized by marked sprouting of cumuliformbulges with sharp outlines in its upperreaches, which make the cloud appear likea cauliflower. It is generally bright whitewith a base of light to dark gray. It cangrow very tall through CONVECTION, possi-bly extending up to the top of the tropos-phere, where it may spread out to formCUMULONIMBUS cloud, and can bring muchprecipitation in the form of showers.

Congo Air Boundary (Zaïre Air Bound-ary) A largely meridional boundary thatmay exist in the vicinity of 25°E across theCongo in central Africa. Although not uni-versally recognized, it represents a zone ofconfluence between moist air from theequatorial section of the Atlantic Oceanand drier air that may move west or north-west from eastern Africa, especially if ananticyclone persists over or to the east ofsouthern Africa. The movement eastwardof the moist air is erratic and is sometimesknown as the ‘southwest monsoon’ – de-spite the proximity of the Equator, whichlies across the northern part of the CongoBasin. Due to sluggish air currents, thezone of active convergence may remainquasi-stationary, leading to prolonged andheavy precipitation.

coning In pollution studies, emissionsfrom a chimney stack under atmosphericconditions of near neutral stability suchthat concentrations of a pollutant at agiven distance downwind from the stackmay be described by a normal or Gaussiandistribution, being the same for both verti-cal and horizontal cross-sections perpen-dicular to the flow.

constancy of wind A statistical term re-ferring to the variation over time in wind-

FREEDOM PALESTINE


flow parameters, such as velocity, direc-tion, and vertical profile at a particular lo-cation. Constancy is a state in which thesevariables do not change with time. Giventhat the magnitudes of the forces acting onhorizontal air motion are constantlychanging, it is rare for such parameters toshow collectively absolute consistency forlong periods of time.

constant-pressure chart See isobaricchart.

constant-pressure surface See isobaricsurface.

Contessa del vento /kon-tess-ă del ven-toh/ A type of cloud formed by an eddyto the lee of an isolated peak. It has arounded base and is topped by a protuber-ance that is inclined upwind; it occurs oc-casionally to the lee of Mount Etna, Sicily,with a west wind.

continental air mass A very extensiveregion of air with broadly uniform proper-ties of temperature and humidity, that hasits SOURCE REGION over a large landmass orcontinent and is associated with a CONTI-NENTAL ANTICYCLONE. Such AIR MASSES arevery cold and dry in the wintertime and hotand dry in the summertime. Examples arethe CONTINENTAL POLAR air masses ofhigher latitude North America and Eurasiain the winter, and the CONTINENTAL TROPI-CAL of the Saharan region of North Africain the summer. See also Siberian high.

continental anticyclone (continentalhigh) An ANTICYCLONE located over avery extensive landmass. Such highs occuracross the higher latitudes of North Amer-ica and Eurasia in the winter, for example,and over the subtropics of North Africa insummer.

continental climate A climatic type ofthe interiors of the extensive continentalland masses. It is characterized by low pre-cipitation and low humidity with a largediurnal temperature range.

continentality /kon-tă-nen-tal-ă-tee/ Inclimatology, a measure of the degree towhich a region of the Earth’s surface is in-fluenced by a land mass. A continentalityindex, of which there are a number of vari-ants, is a numerical method of expressingthe degree of continentality of a region; it isusually based on a range of temperatures.Examples include Berg’s index of continen-tality, (K) in which the frequency of conti-nental air masses (C) is related to that of allair masses (N): K = C/N (%). Compareoceanicity.

continental polar air mass (cP) A colddry AIR MASS that forms in the high-pres-sure regions (anticyclones) of the northernhemisphere, especially those over centralAsia (the SIBERIAN HIGH) and northernCanada, which are particularly pro-nounced in winter. Marked cooling occursin the lower layers of air overlying thesnow-covered land masses, producing sta-ble air and little vertical mixing. In combi-nation with the subsidence characteristic ofhigh-pressure regions, a prominent temper-ature INVERSION typically extends up toabout 850 hPa in continental polar air.During summer, continental warming overSiberia and Canada leads to considerableweakening of the source regions. Nosources for continental polar air are foundin the southern hemisphere as a result ofthe dominance of the oceans in the middlelatitudes.

continental tropical air mass (cT) Awarm dry AIR MASS that has its source re-gion over an extensive arid or desert landmass in the middle or low latitudes. In thenorthern hemisphere summer, continentaltropical air is associated with the areas ofgenerally light winds and upper tropos-pheric subsidence over the major conti-nents, such as central Asia. In the US,continental tropical air is most prevalent insummer and has its source in the desertsouthwest and over Northern Mexico.Such air masses are unstable at low levels,but usually stable aloft. In the southernhemisphere continental tropical air is moreprevalent in the winter in Australia andsouthern Africa. In summer a small source

constant-pressure chart

56

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of continental tropical air lies over Ar-gentina.

contour 1. A line on a map or chart join-ing areas of equal elevation above, or equaldepths below, sea level on topographicmaps.2. (height contour) In dynamic and synop-tic meteorology, a line on an ISOBARIC

CHART that connects all points at which theheights of an ISOBARIC SURFACE above sealevel are equal. Conventionally these aredrawn at intervals of 3, 6, or 12 decameters(1 decameter = 10 meters). The distance be-tween contours represents the steepness ofa gradient: tight contours, with little spacebetween each contour, represent a steepgradient; contours with large gaps betweenthem represent a slack gradient. Where thecontours are close together the horizontalpressure gradient is larger.

contour current In the ocean, a bottomcurrent that flows parallel to the slope of acontinental margin, along the western sideof the ocean basin.

contrail See condensation trail.

contrast In remote sensing, the differ-ence in PIXEL values in an image. High con-trast occurs when there is a large range ofradiances over a scene, from highly reflec-tive to dull surfaces in the visible, for ex-ample.

contrast stretching A technique used inremote sensing to enhance the CONTRAST

across all or part of an image in order tohighlight or exaggerate certain featuresthat do not show up well in the originalversion. To enhance the contrast the rangeof digital number values of pixels in theimage may be artificially increased.

convection /kŏn-vek-shŏn/ The transferand mixing of heat by mass movementthrough a fluid (e.g. air or water). It is oneof the major mechanisms for the transfer ofheat within the atmosphere, together withCONDUCTION and RADIATION. The convec-tion process is of major importance in thetroposphere, transferring sensible heat and

latent heat from the Earth’s surface into theBOUNDARY LAYER, and by promoting thevertical exchange of air-mass properties(e.g. heat, water vapor, and momentum)throughout the depth of the troposphere.Convection is generally accepted to be ver-tical circulation, whereas ADVECTION isusually horizontal.

convection current In the ocean, a cir-culation current that results from densitydifferences associated with variations intemperature between water masses; it oc-curs when a warmer less dense mass ofwater rises and a mass of water that iscooler and denser sinks.

convective available potential energy/kŏn-vek-tiv/ (CAPE) The quantity of en-ergy released through CONVECTION used bymeteorologists to estimate the likely inten-sity of THUNDERSTORMS. The magnitude ofCAPE can be determined from a THERMO-DYNAMIC DIAGRAM. In an environment con-ducive to storm development, the greaterthe area between the environment curveand the air parcel’s path curve between thelevel of free convection (LFC) and the limitof convection (LOC), then the greater is theCAPE. Numerically:

CAPE = ∫LLOF

CC g(Tp–Te)/Te.dz,

where Te is the temperature of the envi-ronment, Tp is the parcel temperature, andg is the acceleration of free fall (9.8 m s–2).To estimate the thunderstorm’s maximumupdraft velocity, the CAPE is equated tothe kinetic energy in order to obtain a mea-sure of the storm’s strength.

convective boundary layer The lowerlayer of the atmosphere in which dynamicsand thermodynamics are affected by thepresence of the underlying often aerody-namically rough land surface. This unsta-ble boundary layer forms at the surface andgrows in its depth throughout the day asthe ground is heated by incoming solar ra-diation. CONVECTION currents transfer theheat received upward into the atmosphere.

convective cell The circulation within afluid when warming causes some of thefluid to rise and cooling causes it to sink.

57

convective cellFREEDOM PALESTINE


For example, the distinct circulation of UP-DRAFT and DOWNDRAFT that characterizesthe basic cellular flow within a THUNDER-STORM. The cells can grow into much morecomplex patterns in an outbreak of severeCONVECTION. The HADLEY CELL is a verymuch larger scale example of convectionwithin the atmosphere, although the termis not normally used in this sense.

convective cloud A cloud formed as aresult of CONVECTION within the atmos-phere. These clouds are typically CUMULUS,and the species are MEDIOCRIS and CONGES-TUS as well as CUMULONIMBUS.

convective condensation level (CCL)The height in the atmosphere at which airthat has risen from the surface by CONVEC-TION becomes saturated. The convectivecondensation level increases as the surfacetemperature (T°C) increases, assuming theDEW-POINT temperature (Td) remains fairlyconstant. As a result, the base of CUMULUS

clouds tends to occur at a higher level inthe early afternoon than in the morning orearly evening. The convective condensa-tion level differs from the LIFTING CONDEN-SATION LEVEL (LCL) because it takesvertical mixing into account. As surface airis usually well mixed, the convective con-densation level and lifting condensationlevel are frequently nearly identical. Thelifting condensation level can be deter-mined from a THERMODYNAMIC DIAGRAM; anumerical approximation being:

LCL (meters) = 120 (T – Td)where T is the air temperature (°C), and Tdis the dew-point temperature (°C) at thesurface.

convective inhibition (CIN, CINH, neg-ative energy) The amount of energy nec-essary to overcome a capping INVERSION.The convective inhibition is typically muchsmaller than the CONVECTIVE AVAILABLE PO-TENTIAL ENERGY (CAPE); even a very lowCIN value can effectively prevent convec-tion. Forecasters must determine whetherthis cap can be broken (e.g. heating of theunderlying air is one possible mechanism)and its energy released into a severe THUN-DERSTORM.

convective instability (potential instabil-ity) The atmospheric condition in whicha deep layer of otherwise STABLE air wouldbecome unstable if forced to rise, for ex-ample, over a mountain barrier or at afront, thereby reaching its DEW-POINT. Up-lift has the effect of increasing the observedENVIRONMENTAL LAPSE RATE (ELR) of thetotal thickness of the raised layer. If thisnew environmental lapse rate is greaterthan the SATURATED ADIABATIC LAPSE RATE

(SALR), then this atmospheric layer be-comes UNSTABLE and may overturn. Quan-titatively, the EQUIVALENT POTENTIAL

TEMPERATURE must decrease with height inthe atmospheric layer for convective insta-bility to occur.

convective outlook In the US, a fore-cast for thunderstorm activity and severityduring the following 24-hour period acrossthe country issued by the Storm PredictionCenter (SPC) and National Weather Ser-vice (NWS).

convective precipitation (convectionalrainfall, convectional precipitation) PRE-CIPITATION that results from convectivemotion (CONVECTION) in the atmosphere.When moist air that lies above a heatedland surface is warmed by conduction itrises, expands, and then cools adiabaticallyuntil its temperature falls below the DEW-POINT. The water vapor within the air con-denses to form clouds, typically of theCUMULONIMBUS type, from which heavy in-tense rain may fall. Convective rainfall istypically associated with equatorial cli-mates, the cold front of an unstable polarair mass, and thundery rain that occurs intemperate regions on a hot summer after-noon.

convective temperature The tempera-ture to which air near the ground must beheated in order to generate surface-basedCONVECTION; one indicator of the like-lihood of THUNDERSTORM development. Its calculation rests on a number of as-sumptions: for example, storms can some-times develop long before or well after theconvective temperature has been reached.Nonetheless, the convective temperature

convective cloud

58

FREEDOM PALESTINE


is a useful parameter for forecasting theonset of convection in certain flow condi-tions.

convergence 1. In the atmosphere, theprocess in which there is an inflow of airinto an area in association with a piling upof mass, which is compensated by verticalmotion, and a decrease in speed along thelines of motion of the air streams. In thelower atmosphere (generally below 550hPa) the compensatory vertical air motionis upward. With altitude the rate of inflowof air gradually drops away to zero until itchanges to one of outflow, with DIVER-GENCE (the opposite of convergence) in thehigher levels above the lower-level conver-gence. When convergence is occurringabove about 550 hPa the air motion isdownward, with divergence taking placebelow at the surface. An example of amajor area of convergence is the IN-TERTROPICAL CONVERGENCE ZONE, wherethe northeast and southeast trade windsslow down quite dramatically as they flowinto the zone from either side. See also con-fluence.2. In the ocean, the meeting of oceanicwater masses or surface currents at a pointor along a line. The denser water mass onone side sinks below the lighter less densewater on the other side. See also AntarcticConvergence.

convergence zone A region withinwhich CONVERGENCE is marked. It can beelongated, as in the INTERTROPICAL CON-VERGENCE ZONE, or roughly circular as inthe air flow toward a mid-latitude CY-CLONE.

59

Coordinated Universal Time

upper airdivergenceconvergence

high pressuredivergence convergence

low pressure

surface

Convergence and divergence

widespreadrainbeneath

5 km

war

mco

nvey

orbe

lt

1 km

0 km 200

asce

ndin

gclo

udy

air

Conveyor belt

conveyor belt 1. In meteorology, an airflow characteristic of most middle-latitudeFRONTAL CYCLONES (frontal depressions).The warm conveyor belt (WCB) is a majorair current that runs relative to the movingfrontal cyclone at about 1 kilometer abovethe surface. It has its root in the WARM SEC-TOR and flows ahead of and parallel to theCOLD FRONT, gradually ascending to turnanticyclonically over the WARM FRONT. It isthe air stream that conveys the major pro-portion of a frontal cyclone’s heat andmoisture poleward. It can, in the case of anana-cold front, turn rearward to flow upand over such a front. The cold conveyorbelt (CCB) is a compensating cold air airflow in frontal cyclones.2. In oceanography, sometimes applied toa major circulation system in the oceans.See also Atlantic conveyor.

Coordinated Universal Time (UTC)The timescale in which weather observa-tions and forecasts are usually reported,based on the local time at the 0° meridianat Greenwich, England. The timescale

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known as International Atomic Time iscomputed by the Bureau International desPoids et Mesures (BIPM), near Paris,France, from a number of atomic clocks lo-cated internationally at various observato-ries; from this the Coordinated UniversalTime is derived. See also Greenwich MeanTime; Universal Time; Zulu Time.

coreless winter (kernlose winter) Aunique feature of the climate in Antarctica,the condition in which surface tempera-tures remain almost the same from the au-tumn equinox to the end of winter.Temperatures decrease to their minima to-ward winter but by the end of the seasonare little lower than at its commencement.It represents a balance between surface ra-diational cooling and atmospheric counter-radiation from above the intense surfaceinversion.

Coriolis effect (Coriolis force) The ap-parent deflection, due to the rotation of theEarth, experienced by an object or by air asit moves across the surface of the Earth. Itoccurs when this motion is referred to thecoordinate system that is fixed to the Earthand rotates with it. The size or magnitudeof the effect depends directly on the rota-tion rate of the planet (which is a constant),the latitude, and the speed of the air (i.e.the wind speed). The Coriolis effect is zeroat the Equator and increases to a maximumat the poles. It works in such a way that an

air parcel is deflected to the right in thenorthern hemisphere, and to the left in thesouthern hemisphere. It is responsible, forexample, for the eastward drift of the GulfStream. The Coriolis force is strictly a ficti-tious force that has to be taken into ac-count when air (or a projectile) movesacross the surface of the rotating Earth. Itis quantified as the acceleration per unitmass experienced by air and is equal to theproduct of twice the ANGULAR VELOCITY ofthe Earth, the sine of the air parcel’s lati-tude, and the speed at which the air is trav-eling (i.e. the wind speed). The geostrophicbalance is defined as the equality betweenthis Coriolis force and the horizontal PRES-SURE GRADIENT FORCE. The Coriolis forceacts to the right of the observed wind in thenorthern hemisphere, and to the left in thesouthern hemisphere.

corona 1. An optical phenomenon inwhich one or more colored circles of lightappear to surround the Sun or Moon. Thecolors range from blue on the inner circlethrough red on the outside and are pro-duced by the DIFFRACTION of light by waterdroplets in a thin layer of cloud, usually al-tostratus, in the atmosphere. In a fullcorona a series of rings surround the innerAUREOLE. Compare halo.2. The outermost layer of the Sun’s atmos-phere, which extends into space for overtwo million km from the visible surface ofthe Sun (the photosphere). During a solareclipse the corona is apparent as radiantlight forming a faint halo around the darkrim of the Sun.

[Latin: crown]

cosmic radiation Very high energy ra-diation, some of which may be of solar ori-gin, which may penetrate the Earth’satmosphere where most is absorbed ortransformed into secondary radiation.Monitoring is providing valuable informa-tion about meteorological phenomena, es-pecially lunar tidal motions in the upperatmosphere.

counterglow See gegenschein.

counterradiation (back radiation, incom-

coreless winter

60

path of projectileviewed from space

North Pole

path of projectileviewed from groundsurface

direction of Earth’s rotation

Coriolis effect

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ing longwave radiation) The emission oflongwave radiation by clouds and gases inthe troposphere back to the Earth’s sur-face. The amount depends upon the emis-sivity of dense clouds, which is high (90%)and of the GREENHOUSE GASES (variable, butabout 60%).

counter sun See anthelion.

coupled ocean–atmosphere model Seegeneral circulation model.

crachin /krash-ang/ In the region of thecoast of southern China and Gulf ofTonkin, low stratus cloud and persistentdrizzle that occurs during the early part ofthe year. It is associated with the replace-ment of dry continental air by moist mar-itime air.

crepuscular rays /kri-pus-kyŭ-ler/ Therays of light interspersed with darker raysof shadow that appear to diverge from theSun when it is partially obscured, byclouds, behind hills or mountains, or justbelow the horizon. The apparent diver-gence is a result of perspective as the raysare in reality parallel. Without the effect ofSCATTERING, produced by particles, such asdust and water vapor within the atmos-phere, the rays would not be visible. Whenthe Sun is relatively high the crepuscularrays shining downward are known by anumber of popular names, for example, Ja-cob’s ladder and the Sun drawing water.When the Sun is low the rays of light ap-pear to diverge upward and may be yel-lowish to reddish as a result of thedifferential scattering effect. Occasionallythe rays of light and shadow appear to con-verge at the antisolar point (the point onthe horizon opposite to that of the settingSun) and are known as anticrepuscularrays. The illusion that the rays are con-verging is also produced by perspective.

critical temperature 1. (critical point)That temperature above which a gas can-not be liquefied by an increase in pressurealone.2. A minimum temperature level belowwhich a particular plant cannot grow or

below which it cannot survive; for exampletemperatures below freezing point, 0°C(32°F), can kill or damage some plants.

Cromwell Current See Equatorial Un-dercurrent.

cross-valley wind system A thermallyinduced wind system that blows during theday across the axis of a valley toward theheated valley sideslope.

crown fire A fire that spreads across thetops of trees or bushes more or less inde-pendently of any surface fire. A crown firecan be classed as either running or depen-dent as a result of the degree of associationit has with a surface fire. Wind is importantin spreading the fire and once started it isdifficult to bring under control.

cryosphere /krÿ-ŏ-sfeer/ The parts ofthe Earth in which water is in its frozenstate. This includes snow cover, ice sheets,ice caps, and glaciers, floating ice, season-ally frozen ground, and perenially frozenground (permafrost). Satellite-borne in-struments have provided a method ofstudying the extensive and often inaccessi-ble cryosphere to observe and measurehow it is responding to changes in climate.Changes in the cryosphere have relevanceto sea levels.

cryotic /krÿ-ot-ik/ Denoting tempera-tures below 0°C.

cumuliform /kyoom-yŭ-lă-form/ Of orrelating to heaped clouds, such as CUMULUS

and CUMULONIMBUS, that form as a result ofconvection. Compare stratiform. See alsocloud classification.

cumulonimbus /kyoom-yŭ-loh-nim-bŭs/A large low cloud of very great vertical ex-tent, one of the cloud genera (see cloudclassification), consisting of huge towerswhose upper reaches appear fibrous, indi-cating streamers of ice crystals beingstretched out in the wind. Additionally, it isoften topped by smooth ice crystal cloudoften in the form of an anvil or extensivehorizontal plume where the glaciated top

61

cumulonimbusFREEDOM PALESTINE


spreads out at the top of the troposphere.Cumulonimbus contains deep and verypowerful updrafts than can sometimes ex-ceed 50 knots (25 m s–1). If convection andupdrafts are especially strong the momen-tum of the rising air may push upward intothe stable stratosphere and form an OVER-SHOOTING TOP. Beneath the cloud base,which is often very dark, there are raggedclouds, frequently VIRGA, and rain shafts.The cumulonimbus base may be as low as300 m (1000 ft) above the surface and itstop as high as 13,000 m (43,000 ft) ormore in low latitudes. The cloud is com-posed of water droplets in its lower levelsrising to ice crystals in the upper levels inthe glaciated top; strong updrafts anddowndrafts associated with the convectionlead to mixing of both water droplets andice crystals in the middle levels. Cumu-lonimbus clouds are often associated withheavy showers of hail, rain, or snow, aswell as lightning and thunder. Cumulonim-bus may appear as a THUNDERSTORM and agroup may merge into a MESOSCALE CON-VECTIVE COMPLEX; the SUPERCELL is an espe-cially turbulent long-lasting cumulonimbusthat can produce TORNADOES.

cumulus /kyoom-yŭ-lŭs/ A type of LOW

CLOUD, one of the cloud genera (see cloudclassification), with a flat base and ‘bub-bling’ upper surface. They are usually scat-tered as individual small ‘units’ that, whenilluminated, have brilliant tops and graybases. The edges of the cloud will some-times appear ragged. Cumulus clouds formthrough convection in ascending thermals.[Latin: ‘heap’]

cumulus congestus See congestus.

cup anemometer (rotating-cups anem-ometer) The most commonly usedANEMOMETER, which utilizes the kinetic en-ergy of the wind to measure wind speed. Itconsists of three, four, or sometimes sixhemispherical cups mounted symmetricallyon a vertical axis of rotation. The rotatingcups either operate a counter with each ro-tation or drive an electric generator operat-ing an electric meter calibrated in terms ofthe wind speed. Wind speeds suitable for

this type of anemometer range between 5and 100 knots. A propeller may be used in-stead of the cups (see propeller anemome-ter).

current See ocean current.

curvature A common feature of atmos-pheric flow, particularly in synoptic-scalecirculation, when the wind blows more-or-less parallel to the isobars that curvearound cyclones and anticyclones (highs),etc. The degree of curvature is expressed asthe radius of curvature, measured from thecenter of circulation to the point in ques-tion; it is used in the calculation of theCYCLOSTROPHIC WIND and the GRADIENT

WIND.

cut-off high A synoptic-scale area ofhigh pressure that remains in the high lati-tudes after a highly wavy long-wave pat-tern (with large north–south andsouth–north excursions) changes to one inwhich the flow is mainly zonal (most oftenwest to east). It often occurs as a result ofBLOCKING. Cut-off highs are part of thesame highly meridional wavy circulationpattern of which CUT-OFF LOWS are an inte-gral component. They lie poleward of thecut-off lows, are similarly slow moving,and are associated with anticyclonicweather.

cumulus

62

0 km 1000

low

high

upper wind flow

Cut-off high and low

FREEDOM PALESTINE


cut-off low (closed low) A synoptic-scale feature, usually found in middle andsubtropical latitudes, in which a normallyslow-moving cyclonic circulation remainsafter an atmospheric long wave TROUGH

has relaxed; i.e. the very wavy large-scalepattern of flow changes to one in which theflow is more west to east, parallel to linesof latitude. The result is that cut-off lowsare left in lower latitudes, extending verti-cally from the surface into the upper TRO-POSPHERE while the main west–eastcirculation is shifted toward the pole. Thetrapped cold air in the cut-off often sparksdeep convection over the subtropicaloceans.

cycle see hydrologic cycle.

cyclic storm A THUNDERSTORM that un-dergoes cycles or pulses of weakening andstrengthening. Cyclic SUPERCELL thunder-storms may produce multiple tornadoes.

cyclogenesis /sÿ-klŏ-jen-ĕ-sis/ The pro-cess of the generation of a cyclonic circula-tion or low-pressure system, or of thestrengthening and intensification of cy-clonic circulation around an existing low-pressure system. Compare cyclolysis.

cyclolysis /sÿ-klol-ă-sis/ The process as-sociated with the disappearance of a cy-clonic circulation or low-pressure system,or a weakening of the circulation aroundan existing system. Compare cyclogenesis.

cyclone 1. (low, low-pressure system,depression) A region of relatively low pres-sure, characterized by inward-spiralingflow that is counterclockwise in the north-ern hemisphere and clockwise in the south-ern hemispheres and leads to convergence.On a synoptic chart a cyclone appears as aset of closed isobars, circular or elliptical,around the central low pressure. Cyclonescan extend up through the TROPOSPHERE, asfor example in an EXTRATROPICAL CY-CLONE, or be relatively shallow features,such as a HEAT LOW. They tend to be mobilefeatures and are often associated withwidespread cloud, precipitation, and wind.The mid-latitude cyclone (mid-latitude de-

pression) is responsible for much of theweather that occurs in the middle-lati-tudes, especially during winter, and usuallytravels from west to east, crossing the US in3–4 days. It is also important on the globalscale in redistributing heat from subtropi-cal to higher latitudes. It often begins as asmall disturbance or wave (see frontalwave) on the POLAR FRONT; the wave devel-ops into the warm sector of the cyclone, airbegins to curve around in the cold sector,and cyclonic circulation develops anddeepens. As the COLD FRONT travels fasterthan the WARM FRONT it begins to overtakeit, forming an OCCLUDED FRONT. In the dis-solving stage the warm sector is forcedaloft and the surface fronts dissipate.

A cyclone in the tropics or subtropicsmay strengthen into a TROPICAL CYCLONE (aHURRICANE or TYPHOON).2. The regional name in countries of theSouth Pacific and the Indian Ocean for a TROPICAL CYCLONE (a HURRICANE orTYPHOON).

cyclone family A succession of frontalwaves (wave cyclones) that travel along thePOLAR FRONT. The first to develop is knownas the primary and each successive frontalwave is a secondary, forming a ‘family’ oftypically three, four, or five. Each frontalwave tends to originate to the southwest ofits predecessor (in the northern hemi-sphere) and travels northeastward at alower latitude, deepening and occludinguntil it dissolves.

cyclone wave See frontal wave.

cyclonic circulation (cyclonic rotation)The circulation or rotation of an air flowthat is in the same sense as the Earth’s ro-tation when viewed from above. In thenorthern hemisphere this is a counterclock-wise inward spiral, while in the southernhemisphere it occurs in the clockwisesense. Such circulation is characteristic ofCYCLONES and is associated with large-scaleascent and, if the air is moist enough, ex-tensive cloud and precipitation.

cyclonic precipitation See frontal pre-cipitation.

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cyclonic precipitationFREEDOM PALESTINE


cyclostrophic wind /sÿ-klŏ-stroff-ik/ Awind that blows around circular ISOBARS

when they are very tightly curved. It is usedto calculate the theoretical speed of the windunder conditions when the CENTRIPETAL

ACCELERATION is much larger than theCoriolis acceleration (see Coriolis effect).TROPICAL CYCLONES are the kind of systemfor which this wind is the best approxima-tion to their tightly curved circulation.

cyclostrophic wind

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daily mean The mean value of the max-imum and minimum temperaturesrecorded in one 24-hour period; i.e. the av-erage temperature for a day.

DALR See dry adiabatic lapse rate.

Dalton’s law /dawl-tŏnz/ A law that de-scribes the behavior of a mixture of gases.The law states that the total pressure of agas mixture is equal to the sum of the par-tial pressures that make up the mixture.Applied to the atmosphere: the total AT-MOSPHERIC PRESSURE is the sum of all thepartial pressures exerted by each individualgas in the atmosphere. Nitrogen, for exam-ple, with a concentration of 78%, con-tributes 780 hPa if the sea-level pressure is1000 hPa. The law is named for the Britishchemist John Dalton (1766–1844).

damp haze A HAZE in which some of thehygroscopic dry particles become damp-ened when the relative humidity risesabove about 75%, most often when the aircools in the evening and at night.

Dansgaard–Oeschger event A briefabrupt climate oscillation in the last glacialperiod during which temperatures roserapidly before returning to their formerlevels. From analysis, using the oxygen-iso-tope method, of the deep ice cores obtainedfrom the Greenland Ice Core Project(GRIP) and Greenland Ice Sheet Project 2(GISP 2), 23 such events were identified be-tween 110,000 and 15,000 years BP. Theevents are named for two ice-core paleo-climatologists: Willi Dansgaard (1922–00;Danish) and Hans Oeschger (1927–98;Swiss).

dart leader In a LIGHTNING discharge,

the LEADER initiating a subsequent light-ning channel after the main RETURN STROKE

in a multistroke discharge. It travels morerapidly than the first STEPPED LEADER as itfollows the remains of the original ionizedchannel. Dart leaders give lightning itsflickering appearance, but not every light-ning flash will produce one.

data buoy A moored buoy in the oceanwith an instrument package to record andtransmit data such as air temperature andpressure, wind speed and direction, sea-surface temperature, and wave data. In the US, the National Data Buoy Center(NDBC), part of the NATIONAL OCEANIC

AND ATMOSPHERIC ADMINISTRATION

(NOAA), administers and gathers datafrom a network of data buoys. The Tropi-cal Atmosphere Ocean (TAO) array (seeTAO/TRITON array) is a system ofmoored buoys to record data for the detec-tion and prediction of El Niño and LaNiña.

dayglow A faint radiant emission in themesosphere that is a form of AIRGLOW. Itcannot be detected by eye but is recordedby polar-orbiting satellites.

debris cloud In meteorology, the vol-ume of particulate matter thrown up fromthe surface by a tornado to form a rotatingcloud of debris; of soil and vegetation overagricultural land, for example.

deepening The decrease in value withtime of the central and surrounding sea-level pressure within the circulation of alow-pressure system. The result is thatmass is being exported from the total aircolumn overlying the disturbance fasterthan it is being supplied. The process of

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D

FREEDOM PALESTINE


deflation

66

deepening usually means that the winds areintensifying simultaneously. A large valuefor deepening would be sea-level pressurefalling 10 hPa in three hours. Compare fill-ing.

deflation Wind removal of fine particlesand materials from the land surface such assilt, clay, dust, and snow.

deforestation /dee-fô-rĕ-stay-shŏn / Theprocess of removing vegetation from landareas, often for the purposes of using theland for agriculture or using the vegeta-tion, often trees, for building materials,fuel, or commercial logging purposes. De-forestation is an important component inGLOBAL WARMING studies; forests are amajor CARBON SINK, so their loss poten-tially adds to the likelihood of increasedgreenhouse gases; moreover, when burnedor decomposed the vegetation releases thecarbon, which combines with oxygen toform carbon dioxide. The removal of land-surface vegetation is also a potential haz-ard to local ecosystems, with the loss ofanimal habitats and removal of topsoilfrom land-water runoff, together with af-fects on the microclimate of the area. It isestimated that currently over ten millionhectares of forest are lost ever year.

degree 1. A unit of measure on a tem-perature scale. See Celsius scale; Fahren-heit scale; Rankine scale.2. A unit of measure of latitude and longi-tude: each degree is divided into 60 min-utes.

degree-day A measure of the differencebetween the mean daily temperature and aspecified reference temperature for a givenday: Degree-day = Reference temperature –(maximum temperature + minimum tem-perature)/2. For a specified period, for ex-ample a month or a year, the number ofdegree-days is the sum of all degree-daysfor that period.

delta T The average rate of change oftemperature with height (LAPSE RATE)within an atmospheric layer. Delta T is cal-culated by simply dividing the temperature

difference between the top and bottom ofthe layer by the change in height. As deltaTs are indicators of possible THUNDER-STORM development, they are most fre-quently calculated for the layer betweenthe 700 hPa and 500 hPa pressure levels, inorder to provide a proxy indicator of theamount of instability evident in middle lev-els of the atmosphere (see absolute insta-bility). Values greater than about 18 areindicative of sufficient instability for severethunderstorm development.

dendrochronology /den-droh-krŏ-nol-ŏ-jee/ (tree-ring analysis) The science ofdating events or specimens by making useof the annual growth rings of trees. The an-nual width of the tree ring varies in relationto climate, such as the amount of precipi-tation and the temperature, among otherfactors. Dendrochronology has been usedin the study of past climatic conditions andclimatic trends, and also of past atmos-pheric pollution. Chronologies extendingback to almost 10,000 years BP have beenproduced for bristlecone pine, individualspecimens of which can live for over 4000years. These chronologies have been usedto recalibrate the CARBON-DATING process.

dendroclimatology /den-droh-klÿ-mă-tol-ŏ-jee/ The study of past CLIMATE usingtree growth rings as an indicator of localclimate conditions. The distance betweentree growth rings is a function of the cli-mate conditions experienced in that region.New wood grows between the old woodand the bark (cambium layer) so that whenwater is plentiful the tree devotes its energyto produce new large cells. During dryerperiods, such as summer, the tree producessmaller cells giving the appearance of aring.

density The mass of material per unitvolume, usually expressed in kg/m3. LikeATMOSPHERIC PRESSURE, the air density de-creases exponentially with height, fromaround 1.2 kg/m3 at the Earth’s surface, toabout 0.7 kg/m3 at 5000 m (16,500 ft). Thedensity of air also varies according to itstemperature, with colder masses of airbeing denser than warmer ones. In water

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bodies, such as the oceans, the density ofthe fluid is affected by its temperature andsalinity.

deposit gauge An instrument designedto collect and measure the products of at-mospheric pollution.

deposition 1. The process by whichwater vapor from the atmosphere changesto the solid phase of ice without passingthrough the liquid phase of water and, indoing so, releases latent heat. This occurs,for example, in the formation of ice crys-tals and snowflakes in the atmosphere andof frost on surfaces. It is the opposite ofSUBLIMATION.2. See acid deposition.

depression 1. See cyclone.2. See tropical depression.

depression tracks See storm tracks.

derecho /dĕ-rech-oh/ In the US, a wide-spread convectively induced windstorm as-sociated with a MESOSCALE CONVECTIVE

SYSTEM (MCS) that can produce strongSTRAIGHT-LINE WINDS across areas severalhundred kilometers long and over 150 km(100 miles) wide. The word derecho wasfirst used by Gustavus Hinrichs(1836–1923), an Iowa weather researcher,in 1888. In the warm season, the mainderecho corridor is located in the southernGreat Plains. During the cool season, dere-cho activity occurs in the southeast statesand along the Atlantic seaboard. Tempo-rally, derechos are primarily late eveningor overnight events during the warm sea-son, but are more evenly distributedthroughout the day during the cool season.Marginal instability and strong synoptic-scale forcing favor derecho formation.[Spanish: ‘straight’]

descriptive meteorology The descrip-tive study of the atmosphere and weatherphenomena, with little or no explanationof the processes or theory.

desert A region of the Earth’s surfacethat has an arid climate (see dry climate),

which can support little or no vegetation.Deserts occur in the high-pressure areas ofthe subtropics where air strongly subsides,warming adiabatically, and preventingcondensation. Examples of such areas arethe Sahara, Saudi Arabia, Iran, Iraq, Cali-fornia, and much of central Australia.Desert conditions are not restricted to thetropics and hot climates; cool deserts in-clude the Gobi Desert in Mongolia. Seealso tropical and subtropical desert cli-mate.

desertification /di-zer-tă-fă-kay-shŏn/The progressive destruction or degradationof existing vegetative cover to producedesert or desert-like conditions. This canoccur through either lack of rainfall orthrough deforestation, overgrazing, orburning, or combinations of these factors.Desertification can lead to local CLIMATE

CHANGE through an increase in albedo, adecrease in precipitation, and greater winderosion of topsoil. It was responsible forthe DUST-BOWL in the 1930s when droughtand poor farming practices led to parts ofthe US Great Plains suffering severe soilerosion and dust storms. During the early1970s, the Sahel region of Africa, border-ing the southern Sahara, underwent deser-tification as a result of severe droughts,which, in combination with overstocking,caused famine, heavy losses of livestock,and crop failures. Compare desiccation.

desert wind A dust-laden airflow capa-ble of entraining particles from the desertfloor and transporting them aloft to otherparts of the desert or sometimes vast dis-tances beyond; examples include the HAR-MATTAN and SCIROCCO.

desiccation /dess-ă-kay-shŏn/ In clima-tology, the long-term reduction or disap-pearance of water from a region, such asoccurs when precipitation levels decreasethrough climate change. See also desertifi-cation.

dew Moisture deposited in the form ofwater droplets on the ground surface or onobjects close to ground level, such as bladesof grass. Dew forms when nocturnal long-

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dewFREEDOM PALESTINE


wave radiation from the Earth’s surfacecools the lowest layer of the atmosphere tobelow the DEW-POINT, allowing condensa-tion to occur. Prerequisites for dew forma-tion include calm conditions or very lightwind speeds, less than 1 knot or 1 mph ata height of 2 m (6.5 ft), that allow the airto remain in contact with the ground forlong enough to be cooled to dew-point; ahigh water-vapor content near the surface;and a suitable radiating surface. Calm andclear nocturnal weather conditions thatfollow relatively warm days during springand autumn are the most favorable for dewformation. If the temperature drops belowthe freezing point, 0°C (32°F), followingdew formation the water droplets solidifyinto ice forming white dew.

dew gauge An instrument for measur-ing dew formation, usually through mea-surement of the weight of the deposition.

dew-point (Td, dew-point temperature)The temperature to which air must becooled, at constant pressure and moisturecontent, in order to saturate it. Continuedcooling below the dew-point will causecondensation of water droplets if the at-mospheric conditions are favorable. Dew-point provides a good indicator of themoisture content of the air. See also dew.

dew-point hygrometer (chilled-mirrorhygrometer, condensation-type hygrome-ter) A HYGROMETER that measures thetemperature at which dew forms (the dew-point). It works by artificially cooling ahighly polished metal mirror surface at aconstant pressure and constant vapor con-tent until dew or frost forms on the surface.Cooling methods include the use of com-pressed carbon dioxide, dry ice, liquid air,or mechanical refrigeration.

dew-point temperature See dew-point.

dew pond A shallow pond created arti-ficially to provide water for livestock infields without running or piped water, andparticularly common on chalk or limestonerock. Traditionally dew ponds were linedwith clay but artificial linings are now

used. The water in the ponds is derivedprincipally from natural precipitation andnot from dew.

diabatic /dÿ-ă-bat-ik/ (non-adiabatic) Athermodynamic process in which the tem-perature of an air parcel changes as a resultof the exchange of heat (energy) with thesurroundings. In a diabatic process:

∆Q ≠ O.Most thermodynamic processes near theEarth’s surface are diabatic, owing to thecontinual mixing of air and turbulence.Compare adiabatic.

Diablo wind /dee-ab-loh/ A wind thatoccurs below canyons in the East Bay hills(Diablo range) of northern California.Such a wind develops when high pressure islocated inland over Nevada and lowerpressure is situated along the central Cali-fornian coast. The Diablo wind is similar incharacter to the SANTA ANA wind experi-enced in southern California. In extremecases, wind velocities can exceed 50 knots(58 mph).

diagnostic model In pollution studies,a method used to model wind fields thatdetermines the transport and residencetimes of pollutants. It requires the solvingof the continuity equation (conservation ofmass) to determine wind fields, and a denseobservational network to achieve reason-able results.

diamond dust A type of precipitation,occurring especially in high latitudes, com-posed of minute unbranched crystals of icethat appear to be suspended in the air orfloat slowly to the ground surface fromhigh cloud or from an apparently cloudlesssky. Optical phenomena, such as SUN PIL-LARS and HALOES, may be seen in associa-tion with diamond dust; on cold nightswhen diamond dust occurs the reflectionsof street lights can produce pillars of light.

differential motion The commonplaceobservation that clouds at different levelsnormally move with different speeds anddirections.

dew gauge

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diffraction A bend in the direction of awave when it passes through a small aper-ture or around an obstacle or barrier. In-terference patterns following theobstruction result when the diffractedwaves either reinforce each other or canceleach other out. In meteorology, the effectsof diffraction are observed in optical phe-nomena, such as the BISHOP’S RING,CORONA, GLORY, FOGBOW, and IRIDESCENCE.

diffuse radiation (sky radiation) In me-teorology, the component of incomingsolar radiation that has been scattered bymolecules of air, aerosols, particulate mat-ter, clouds, etc., which falls on the surfacein nearly equal amounts from nearly allparts of the sky during daylight.

diffuse reflection The reflection of elec-tromagnetic radiation evenly in all direc-tions.

diffusion The process by which proper-ties of the atmosphere, such as HUMIDITY

and HEAT, are mixed within the fluid of theair. Molecular diffusion is not significantcompared to the role of turbulent diffusionthat is carried out by EDDY motions in theair.

difluence /dÿ-floo-ĕns/ (diffluence) Theflow of a fluid, for example air, in whichthe STREAMLINES spread apart along the lineof the motion. It is the opposite of CONFLU-ENCE. See also convergence; divergence.

digital image In remote sensing, animage produced by the conversion of thevariations in radiation received from theremotely sensed scene into PIXELS (pictureelements), which form the basic buildingblocks for the image. The radiance is ex-pressed as a numerical value (the digitalnumber) within a wide range of possiblevalues. Compare analog image.

dimethyl sulfide (DMS) An importantsulfur-bearing gas produced from naturalsources. Large amounts of the gas aregenerated by the metabolic processes expe-rienced by certain oceanic algae (phyto-plankton) and thence transferred into the

TROPOSPHERE where the gas ultimately con-tributes significantly to the presence of sul-furic acid.

direct circulation A circulation systemin which warm lighter air ascends and colddenser air descends, associated with theconversion of potential energy – related tothe juxtaposition of air of different densi-ties – into kinetic energy.

directional shear The rate of change ofwind direction with height or along a hor-izontal plane. In the vertical plane, the sur-face wind is usually backed (see backing) indirection from the GRADIENT WIND in thefree atmosphere. The degree of shear is afunction of the roughness of the surfaceand the stability of the air. See also windshear.

direct radiation (beam radiation) Theshort-wave radiation arriving directly onthe surface of the Earth from the Sun; i.e. ithas passed through the atmosphere with-out being absorbed, reflected, or scattered.Globally, it represents about 25% of theradiation received at the top of the atmos-phere.

discontinuity /diss-kon-tă-new-ă-tee/ Inmeteorology, a horizontal zone acrosswhich temperature, humidity, or wind ve-locity change very rapidly, for example aFRONT.

dishpan experiment A laboratory ex-periment to simulate convection and at-mospheric movements in which a rotatingdishpan (essentially a shallow cylinder)that has a small cylindrical center, is filledwith water, between the central core cylin-der and its circumference. The ring ofwater represents the atmosphere in a sim-ple way, while the core cylinder is the poleand the outer rim the Equator. The experi-ment then looks at the role of different ro-tation rates of the cylinder and differentEquator-to-pole temperature gradients onthe way in which the fluid transports heatfrom the outer to the inner region.

dispersion diagram A diagram upon

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dispersion diagramFREEDOM PALESTINE


which the dispersion of points is plotted,for example rainfall against time. This en-ables the visualization of the behavior ofrainfall for a station or region over time.

dissipation trail (distrail) A line formedin the wake of an aircraft as it flies througha thin cloud layer. It may form as a resultof evaporation of the cloud droplets byheat from the exhaust emitted by the air-craft’s engines. Compare condensationtrail.

distrail See dissipation trail.

disturbance A general weather-produc-ing system (with precipitation and strongwinds for example) associated with a dete-rioration in conditions. Examples of such afeature on a mean-sea-level pressure chartwould be a traveling FRONTAL CYCLONE or,on a much smaller scale, a THUNDERSTORM.

disturbance line A line of storms trav-eling west across West Africa usually asso-ciated with EASTERLY WAVES. They may beaccompanied by high winds forming gustfronts – similar to microbursts from mid-dle-latitude thunderstorms. Disturbancelines are most frequent during the AFRICAN

MONSOON season. The storms are associ-ated with uplift of very moist tropical air,possibly of Equatorial origin, and descentof drier Saharan air from above. Lines,which can be up to several hundred kilo-meters in length, may suddenly form anddisappear within 2 hours or can sometimesbe tracked for up to 24 hours.

diurnal variation (diurnal range) Therange of a meteorological element, for ex-ample temperature, pressure, or relativehumidity, between the maximum and min-imum values observed during the course ofa solar day.

divergence In general, the decrease overtime in the flux of an atmospheric propertyin a unit volume. This might be, for exam-ple, the rate of gain of water vapor from acubic meter box into which, with flow inone direction only, a lower concentrationis entering through one vertical face than is

leaving through the opposite one. The dif-ference, or the divergence, must be relatedto the evaporation of water into the airwithin the box.

In the circulation of the atmosphere, itis the process in which there is an outflowor decrease in air mass from an area andconsequent spreading of the field of flow(the opposite of CONVERGENCE); air streamsincrease in speed along their line of motion.Such flow in the lower TROPOSPHERE occursin the spiraling outflow from ANTICY-CLONES (highs), for example. This horizon-tal divergence is calculated by consideringthe horizontal rates of change of the com-ponents of the wind. Divergence is on av-erage most marked in the lower and uppertroposphere; it is at a minimum at around600 hPa, the level of non-divergence. Di-vergence is compensated by vertical mo-tion. See also difluence.

D-layer (D region) The lowest layer orregion of the IONOSPHERE at a height of50–90 km (30–55 miles) above the Earth’ssurface. The free electron density is at itslowest in this layer and the ionization dis-appears during the night.

DMS See dimethyl sulfide.

Dobson spectrophotometer A ground-based instrument used for measuringozone within the atmosphere. The instru-ment monitors the ozone content in thevertical column above it by comparing theultraviolet light intensity at two wave-lengths: one of which is strongly absorbedby ozone and the other that is weakly ab-sorbed. The instrument was originally de-veloped during the 1920s by the Britishmeteorologist Gordon Miller Bourne Dob-son (1889–1976), who was among the firstscientists to study atmospheric ozone.

Dobson unit (DU) The unit of measurefor atmospheric ozone. One unit is definedas equal to 0.01 mm of ozone at 1 atmos-phere pressure and 0°C (32°F). It is namedfor the British meteorologist GordonMiller Bourne Dobson (see Dobson spec-trophotometer).

dissipation trail

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doctor A cooling refreshing wind thatmoderates the often unpleasant and some-times unhealthy conditions of many tropi-cal environments. It is most commonlyapplied to the HARMATTAN wind of WestAfrica; the regular summer SEA BREEZE

along the coast of Western Australia is alsooften known as the doctor. The Cape doc-tor is a strong southeasterly wind thatblows onto the South African coast.

doldrums The region of light and vari-able surface winds in the vicinity of the In-tertropical Convergence Zone (ITCZ). Inthis region, in the age of sail, vessels wouldsometimes be becalmed.

dominant wind The wind that predom-inantly affects a particular location. Thewind climate of a location may be con-trolled more by locally induced wind circu-lations as opposed to the PREVAILING WIND,which is generated as a result of the loca-tion of high- and low-pressure areas at thesynoptic scale. For example, a town posi-tioned in a deep valley is often affected bylocal valley winds that may blow in a to-tally different direction to the prevailingwind.

Doppler effect (Doppler shift) The ef-fect in which the frequency of waves emit-ted by an object appears to change as theobject moves relative to a stationary ob-server. This occurs, for example, withsound waves emitted by a whistle, say, onan approaching and then receding train;the apparent frequency of the waves in-creases as the train comes toward the ob-server, then decreases as it recedes. Thesame effect occurs in radiation backscat-tered to a RADAR by falling RAINDROPS orCLOUD DROPLETS. As a result of the Dopplereffect raindrops moving away from theradar have a return frequency shifteddown, while those moving toward theradar have a frequency shifted up. The ef-fect is named for the Austrian physicistChristian Doppler (1803–53), who discov-ered it in 1842. See also Doppler radar.

Doppler radar A type of RADAR thattakes advantage of the DOPPLER EFFECT by

measuring the frequency of the signal scat-tered back to it from an object or sourceand detecting whether this is moving to-ward or away from the radar receiver. Inmeteorology it is used, for example, in thetracking of precipitation to enable deduc-tions to be made regarding wind move-ments. Since raindrops are borne on thewind, the frequency shift observed at theradar will enable the mapping of the com-ponent of the wind flow toward or awayfrom the radar antenna. Therefore, notonly is precipitation mapped, but so tooare the low- and middle-level zones of CON-VERGENCE and DIVERGENCE from the de-duced wind observations. Low-levelconvergence along a line may relate to apotential for THUNDERSTORM development,for example. In the US, the NationalWeather Service (NWS) operates a net-work of Doppler radar (NEXRAD), usingWeather Surveillance Radar (WSR–88D),which provides a powerful system capableof measuring reflectivity echoes.

Doppler shift See Doppler effect.

downburst A strong localized DOWN-DRAFT resulting in an outburst of verystrong damaging winds at or near theground level that can be experienced be-neath THUNDERSTORMS or showers orsometimes in clear air. The wind surges outin the direction the storm is moving andcan cause damage that is similar to thatproduced by a tornado; it may last for5–30 minutes with gusts up to 60 m s–1

(130 mph). A large downburst extendingover 4 km (2.5 miles) in the horizontal di-mension is also sometimes known as amacroburst. A downburst less than 4 kmacross is called a MICROBURST.

downdraft An anomalously strong ver-tical current of descending air, usuallywithin a storm, but sometimes in clear air.In the deep convective clouds of storms it isformed by the dual action of rain fallingwithin the cloud, by it evaporating andtherefore cooling the surrounding air andby the weight of the rain. When the down-draft meets the ground it fans out awayfrom the parent cloud as a noticeably cool

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downdraftFREEDOM PALESTINE


current of air with a gust front at its lead-ing edge. Its temporal duration is usuallyshort, with a spatial scale of 10–100 m(30–300 ft) in a storm. Downdrafts associ-ated with clear air turbulence (CAT), how-ever, are hazardous to aircraft and maylead to a plane falling thousands of feet inas little as one minute.

downslope flow A thermally induceddownslope movement of cold air, which isoften evident in mountainous terrain andusually occurs at night. Unlike such shal-low downslope KATABATIC flows are thedownslope winds (e.g. the warm FÖHN

WIND), which are produced by the large-scale flow that extends far above themountain tops.

down-valley wind A LOCAL WIND thatblows down a valley in a mountainous en-vironment. It is usually observed duringthe late nighttime and early morning pe-riod before sunrise when DOWNSLOPE

FLOWS have ceased and a MOUNTAIN WIND

fills the valley.

downwelling (sinking) In the oceans,the sinking of surface water, which is usu-ally caused through density differences insea water, as a result of cooling and/or in-creased salinity, or through convergence ofwater masses with different properties. Thedenser colder water tends to sink.

drainage wind See katabatic wind.

drizzle Light slowly falling PRECIPITA-TION in which the water droplets are tinyand resemble a fine spray. The diameters ofdrizzle droplets fall typically within therange of 0.2–0.5 mm (0.008–0.02 in).Given their small size, drizzle droplets areunable to fall to the ground if there isstrong vertical air motion. A high relativehumidity (with a DEW-POINT depression ofless than 2°C) between the cloud and theEarth’s surface is required to prevent thedroplets evaporating before they reach theground. Drizzle usually falls from low stra-tus clouds in which the base is lower than300 m (1000 ft), and is often associatedwith the stratiform cloud of a WARM

FRONT. Heavy drizzle mostly occurs fromclouds formed through OROGRAPHIC upliftover upland areas. See also freezing drizzle.

dropsonde /drop-sond/ An instrumentpackage that is released from an aircraftand borne by parachute to obtain sound-ings of the atmosphere below. The mea-surements recorded are temperature,atmospheric pressure, and humidity. Someversions carry GPS systems enabling addi-tional wind measurements to be recorded(sometimes known as a dropwindsonde).Dropsondes are dropped into TROPICAL CY-CLONES (HURRICANES), for example, to ob-tain information of the cyclone’s intensity.

dropwindsonde See dropsonde.

drought A period of dry weather of suf-ficient length for the lack of water to causea hydrological imbalance in an area. De-pending on the discipline, several defini-tions exist that attempt to quantifydrought severity and magnitude. In theUK, an absolute drought was defined in1919 as 15 consecutive days without mea-surable rain. In the US, drought is morecommonly defined as an extended periodof anomalous moisture deficit. This placesmore emphasis on the summer season withhigher evapotranspiration rates from soiland vegetation in which drought is definedas a temporary negative deviation in envi-ronmental moisture status. Other defini-tions include, ABSOLUTE DROUGHT, PARTIAL

DROUGHT, PHYSIOLOGICAL DROUGHT, andhydrological and agricultural droughts.The most versatile method of definingdrought intensity is in terms of a deviationfrom the annual mean rainfall, water bal-ance, or riverflow. Drought severity canthen be measured by multiplying thedrought intensity by the duration of thedrought. See also Palmer Drought SeverityIndex.

drought cycle A pattern of periodicdroughts caused by prolonged periods ofinsufficient rainfall.

dry adiabat A line drawn on an THER-MODYNAMIC DIAGRAM that traces the path

downslope flow

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of a dry air parcel moving through the at-mosphere adiabatically. A dry adiabat isalso a line of constant POTENTIAL TEMPERA-TURE (θ), this being the temperature of anair parcel after it has been brought dry adi-abatically to the 1000 hPa pressure level.Theta can be read directly off a thermody-namic diagram or calculated by means of aformula.

dry adiabatic lapse rate (DALR) Therate at which temperature changes in anunsaturated parcel of air that rises or sinksadiabatically (i.e. there is no exchange ofheat with the surrounding air). As the airparcel rises, if condensation does notoccur, the energy used by expansion causesthe temperature of the parcel to drop at thedry adiabatic lapse rate, which is approxi-mately 9.8°C km–1. The dry adiabatic lapserate is sometimes called a process lapserate, as it refers to the thermal behavior ofan air parcel as opposed to that of the sur-rounding environmental air.

See adiabatic lapse rate; lapse rate; sat-urated adiabatic lapse rate.

dry air 1. In climatology, air of low REL-ATIVE HUMIDITY, below 60%.2. In meteorology, air that contains noWATER VAPOR.

dry-bulb temperature The tempera-ture recorded by a DRY-BULB THERMOMETER

exposed to air and shaded from direct solarradiation.

dry-bulb thermometer A THERMOME-TER with a dry bulb used to determine theordinary temperature of the air. It is usedin conjunction with a WET-BULB THER-MOMETER in a PSYCHROMETER to measurerelative humidity.

dry climate One of the major climategroups in the KÖPPEN CLIMATE CLASSIFICA-TION, designated as B. In this climategroup, potential evaporation and transpi-ration exceeds precipitation on averagethroughout the year, and there is a con-stant deficiency in water. The group is sub-divided into the arid (desert) climates,designated by BW, and the semiarid

(steppe) climate, designated by BS. The dryclimate group occupies over 26% of thetotal land area of the globe. See also desert.

dry deposition The settling out undergravity of pollutants, largely in the BOUND-ARY LAYER. The pollutants are deposited onsoil, vegetation, or water bodies at theEarth’s surface and may account for thelarger part of deposition in dry climates. Inhumid climates, the amounts removed bydry and wet deposition are approximatelyequal.

dry ice Solid carbon dioxide used incloud-seeding operations. Dry ice particlesare most often dropped from an aircraftinto supercooled clouds, where they vapor-ize at a temperature of –78.5°C (–109°F).The cooling associated with this change ofphase produces very large numbers of icecrystals that may then grow large enoughto fall through the cloud – to finish ulti-mately at the surface as precipitation.

dry intrusion A feature that is typicallya few hundred kilometers long and perhapsa hundred kilometers wide that marks thedescent of very dry air from the lowerSTRATOSPHERE or upper TROPOSPHERE. It isimportant because it is now known that itplays a critical role in the rapid deepeningof FRONTAL CYCLONES, mainly over themiddle-latitude oceans. The downward-plunging dry air both spins up the pre-ex-isting VORTICITY of the system andenhances the horizontal density gradients –both of which mean stronger winds. Thedry intrusion becomes a component part ofthe cloud and water vapor pattern pre-sented by a rapid deepener (see explosivecyclogenesis). The feature was first recog-nized during the latter part of the 20th cen-tury, on water vapor images from weathersatellites.

dry lightning See heat lightning.

dry line (dew-point front) A boundarythat separates a moist air mass from a dryair mass. It refers especially to the generallynorth–south orientated boundary thatoften extends across the central and south-

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ern High Plains of the US in spring andsummer, separating warm and moist airmoving north out of the Gulf of Mexicofrom the usually warmer and drier air fromthe west. The dry line often moves to theeast in the afternoon and moves back west-ward at night. It is usually a narrow zoneacross which the DEW-POINT temperaturecan fall by as much as 9°C per kilometer,and in which the moister air rides up andover the drier air. These conditions, cou-pled with low-level CONVERGENCE, can leadto the development of severe storms thatmay become tornadic along the dry line orin the moist air to the east of it.

dry-line storm A storm that developsalong a DRY LINE, which indicates a bound-ary between warm dry air and warm moistair, marking a region of instability. Dry-line storm is sometimes used synony-mously with a LOW-PRECIPITATION STORM

that characteristically develops along drylines.

dry season A period exceeding a monththat occurs at about the same time everyyear when no or very little precipitation isrecorded. It is usually applied to tropical

climates that experience a wet-and-dry an-nual rainfall regime (see tropical wet-dryclimate), classified as Aw in the KÖPPEN CLI-MATE CLASSIFICATION scheme. In the trop-ics, winter is the dry season and isassociated with the withdrawal of thezenithal Sun into the opposite hemisphere.Simultaneously, the regional subtropicalhigh-pressure cell develops under the influ-ence of a dominant Hadley Cell meridionalcirculation. The length of drought, in gen-eral, increases toward the respective tropic,until it stretches through the entire yearand ceases to be a seasonal feature.

dry slot An elongated filament-like in-trusion of very dry stratospheric air thatplunges down into the TROPOSPHERE fol-lowing COLD FRONTS or even overrunningsuch features. It can also refer to the regionof tropospheric subsidence, involving adrying out of the air, that very often occursafter the passage of a cold front.

Stratospheric dry slots are known to oc-casionally be associated with EXPLOSIVE CY-CLOGENESIS in middle latitudes.

dry snow A type of powdery SNOW witha density less than 0.1 kg m–3. It contains

dry-line storm

74

cold dryair LOW

warmmoist air

hotdryair

dry

line

Dry line

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very small ice crystals that do not bond to-gether and is common in continental interi-ors away from maritime influences.

DU See Dobson unit.

duplicatus /dew-plă-kah-tŭs/ A cloudvariety consisting of superposed sheets orpatches of cloud at slightly different levels;it can occur with some low, middle, or highclouds including STRATOCUMULUS, ALTOCU-MULUS, ALTOSTRATUS, CIRROSTRATUS, andCIRRUS. See also cloud classification.

dust Microscopic dry particulate matterheld in suspension within the atmosphere.The dust may originate, for example, fromvolcanic eruptions, dust storms raisingquantities of dust into the atmospherefrom the surface, and smoke particles. Thesmallest particles can act as AITKEN NUCLEI.Dust can produce SCATTERING and haze.

dust-bowl A region affected by droughtand subsequent dust storms. The expres-sion was first used, in 1935, to describe theregion in the Great Plains of the south-cen-tral US in which a combination of droughtand poor farming practices led to crop fail-ures, loss of topsoil, and severe duststorms. See also desertification.

dust devil (dust whirl, sand pillar) A lo-calized whirl of dust in a desert area, inwhich the particles are swept around a cen-tral vortex, reaching heights of up to 900 m(3000 ft) above the Earth’s surface. Thedust devil is caused through intense localheating of the ground surface by the Sun,which results in a strong convection cur-rent being formed. Dust devils typicallytravel across the desert floor at speeds of4–13 knots (5–15 mph), although veloci-ties in excess of 30 knots (35 mph) havebeen observed. In general, dust devils donot pose a serious hazard, even to aviation,because they are only a few meters in di-ameter and gradually die away. Terminol-ogy varies with location: dancing devil(southwestern US); sand auger (Death Val-ley, California); devil (India); desert devil(South Africa).

dust plume An elongated plume of pol-luted air that extends downwind of a majorcity. Due to the warmth of the city relativeto its surroundings, warm air rises convec-tively over the city center, creating a dustdome over the urban area in which aerosolconcentrations may be more than onethousand times higher than in a rural envi-ronment. If the regional wind speed isgreater than 8 knots (9 mph), this dustdome extends downwind in the form of adust plume. As a result, the city’s pollu-tants are spread over the surroundingcountryside: for example, Chicago’s dustplume is sometimes visible 240 km (150miles) downwind of the city.

dust storm An event in which solid par-ticles of dust and debris are suspended bystrong winds, with an associated dramaticreduction in visibility. The US NationalWeather Service defines a dust storm ashaving wind speeds equal to or greaterthan 26 knots (30 mph) and visibility of0.8 km (0.5 miles) or less. Dust storms areusually restricted to arid middle-latituderegions in which there are sudden onsets ofstrong winds; poor agricultural practicescan exacerbate their occurrence. Duringthe 1930s frequent dust storms were a fea-ture of the Dust Bowl of the central south-ern US, which resulted from the removal oftopsoil by agriculture and lack of rainfallleaving barren dusty areas. The Caucasus(southern Russia) and central Australia areparticularly prone to dust storms. See alsosandstorm.

dust veil index (DVI, Lamb’s dust veilindex) A numerical index, devised in1963 by Hubert Horace Lamb (1913–97),a British climatologist, that ranks volcanicdust veils from volcanic explosions interms of the mass of material initially in-jected together with the duration and max-imum spread of the veil. Single volcanicexplosions such as Krakatau and MtAgung, rank as 1000 (the reference dustveil index). The index is an indication ofthe effects of volcanic dust on climate.

dust whirl See dust devil.

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DVI See dust veil index.

dynamic climatology The study of CLI-MATE and its relation to dynamical expla-nations associated with climate patterns ordeviations from long-term climate trends.This branch of climatology has developedsince the 1960s and uses models of atmos-pheric processes to simulate climate andclimate change.

dynamic meteorology (dynamical me-teorology) One of the main branches ofMETEOROLOGY in which the laws of physicsand mathematics are applied to studies of processes and motion within the at-mosphere. Computer models are used astools in the study of the complex processes,for example, in modeling the generalglobal circulation. See also synoptic meteo-rology.

DVI

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Earth observation satellite A SATEL-LITE focused on mapping the detail of theEarth’s surface, including natural re-sources, rather than clouds and watervapor, for instance. Earth observationsatellites tend to be in polar orbits and havevery much better RESOLUTION (down to afew tens of meters for the smallest PIXEL di-mension) than the meteorological satel-lites. See also European Remote SensingSatellite.

Earth Observing System (EOS) Acomponent of NASA’s Earth Science En-terprise (ESE), a program that waslaunched in 1991 to study the Earth as anintegrated system. Earth Science Enterpriseincludes a series of Earth-observing satel-lites, data system, and teams of scientists.The second part of the program began in1999 when the first Earth Observing Sys-tem satellite, Terra, was launched; thispolar-orbiting satellite circles the Earth 16times a day at a height of 705 km (438miles). Its instruments gather a wide vari-ety of data that is contributing to the un-derstanding of climate change (e.g. onaerosols, cloud cover, vegetation and snowand ice cover, and sea-surface tempera-tures and phytoplankton levels in theoceans). In 2002 the second satellite, anear-polar low-inclination satellite, Aqua,was launched. This focuses on collectingdata on the water cycle (e.g. evaporationfrom oceans, water vapor, precipitation,clouds, and snow and ice cover). A furthernear-polar low-inclination satellite waslaunched in 2004: Aura, which collects in-formation on the composition and chem-istry of the atmosphere, including ozoneconcentrations. The Earth Observing Sys-tem is intended to make ‘long-term global

observations of the land surface, solidEarth, biosphere, atmosphere, and oceans’.

Earth Radiation Budget Experiment(ERBE) A program launched in 1984 byNASA to study the radiation budget (i.e.the incoming solar radiation and the long-wave radiation leaving the Earth) using ra-diometers carried on three Earth-orbitingsatellites (the NASA ERB satellite (ERBS)and two NOAA satellites).

Earth rotation See rotation of theEarth.

earth science Any one of the sciencesconcerned with the study of the Earth, in-cluding CLIMATOLOGY, METEOROLOGY, ge-ology, geomorphology, geophysics,geochemistry, hydrology, and oceanogra-phy.

Earth Science Enterprise See EarthObserving System.

Earth Summit See United NationsFramework Convention on ClimateChange.

East Australian Current A warm cur-rent in the Pacific Ocean that flows south-east along the east coast of Australia. It isan example of an eastern BOUNDARY CUR-RENT.

easterlies The belts of generally persis-tent winds that blow from the east towardthe west; this includes the easterly TRADE

WINDS and the POLAR EASTERLIES.

easterly wave (tropical wave) A synop-tic-scale wave found on either side of theEquator between about 5° and 15° of lati-

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tude. The waves move westward within adeep easterly flow (flowing from the east)at about 3–4 km (2–2.5 miles) above thesurface, and may show an inverted ‘V’structure when viewed either by satellite oron isobaric charts. They travel west at anaverage speed of 5–10 m s–1 and take35–40 days to circumnavigate the Earth. InAfrica, easterly waves are very importantweather-producing systems, bringing fairlywidespread rain over West Africa, wherethey are associated with DISTURBANCE

LINES, in the northern summer. They formalong the very strong temperature gradientbetween the hot Saharan air to the northand the cooler Gulf of Guinea air to thesouth, and occur typically every four-to-five days. Usually they travel fromCameroon right across the region and outacross the Atlantic Ocean. Some developultimately into HURRICANES that trackacross the Atlantic Ocean and may hit theCaribbean, Central America, and the US.

Other easterly waves of note are thosethat run right across the tropical North Pa-cific, sometimes evolving into TYPHOONS.

eastern boundary current See bound-ary current.

East Wind Drift See Antarctic CoastalCurrent.

EBM See energy balance model.

eclipse The partial (partial eclipse) ortotal (total eclipse) obscuration of oneheavenly body by another. In a solareclipse, the Moon obscures the light fromthe Sun, producing a shadow on the Earth.In a lunar eclipse, the Earth is aligned be-tween the Sun and the Moon, and theshadow of the Earth falls on the Moon.

ECMWF See European Centre forMedium-Range Weather Forecasts.

ecoclimate /ee-koh-klÿ-mit/ The climatein relation to the flora and fauna of anarea.

ecological climatology (ecoclimatology)The branch of BIOCLIMATOLOGY that stud-ies the relationship of living organisms toclimate, such as the geographical distribu-tion of plants and animals in relation to cli-mate.

eddy In the atmosphere, a distinct masswithin a turbulent fluid that retains itsidentity and behaves differently for a shortperiod within the general larger volumeflow. An eddy thus ranges in size from mi-croscale TURBULENCE (1 cm for example) tomany hundreds of kilometers in the formof FRONTAL CYCLONES and ANTICYCLONES.The smallest scale eddies are critical in theprocess of, for example, heat and watervapor transfer from the Earth’s surfaceinto the air, while frontal cyclones trans-port heat toward the poles.

eddy diffusion (turbulent diffusion) The mixing of a property of the atmos-phere by EDDY motions.

eddy viscosity A means by which tur-bulent motion within the atmosphere over-comes the resistance to deformation of thefluid; i.e. how viscous the fluid is. Thismeans that, for example, the effective mix-ing of momentum by the air’s turbulentflow depends directly on the decrease ofthe wind speed with height and a measureof how easily mass is transported upwardby the fluid motion.

eastern boundary current

78

low

ridge

direction ofwave’s movement

region of heaviest rain

trough

wind direction

Easterly wave

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edge enhancement 1. In remote sens-ing, an image-processing technique to in-crease contrast between adjacent tones andemphasize edge features or lines on animage.2. An automatic technique used to findedge features on an image. It is based oncomparing an area of PIXELS to find gradi-ents or sudden changes in value from oneto its neighbor, sometimes along a line ofcontrasting pairs of pixels. An examplemight be the sharp change across a coast.This change would be distinct from themore even values over land and over sea,either side of the coastal edge.

effective precipitation (EP) That partof the total PRECIPITATION (P) input remain-ing after EVAPORATION (E) losses and thusavailable for the growth of vegetation. It isexpressed by the formula:

EP = P – E.

effective temperature 1. The tempera-ture of a planet, such as the Earth, that itwould have if it behaved like a black bodyand absorbed all incoming radiation andre-radiated this back to space.2. In the US, a thermal comfort index thatattempts to measure the comfort or dis-comfort of temperature. The effective tem-perature relates to the sensation of warmthor cold experienced by the human body asbeing dependent on the temperature, hu-midity, movement of air, and amount ofclothing worn. See also temperature–hu-midity index.

EISCAT (European Incoherent Scatter)Three incoherent scatter radar systems thatare based in Scandinavia, with the twomain transmitter sites located nearTromsø, Norway and near Longyearbyenon the Norwegian island of Spitsbergen.Receiver stations are located in Sodankylä,Finland, and Kiruna, Sweden. EISCAT isused for the study of the Earth’s ionosphereand interactions with the upper atmos-phere, magnetosphere, and solar wind. It isfunded by Norway, Sweden, Finland,Japan, France, and the UK.

Ekman depth /ek-măn/ See Ekman ef-fect.

Ekman effect 1. In the oceans, thechange in direction of current and thelower velocity that occurs in water layerswith increased depth. Wind stress sets inmotion the water movement in the surfacelayer of the ocean. The surface water is di-rected theoretically at an angle of 45° (inreality usually 20–40°) to the wind, to theright in the northern hemisphere, as a re-sult of the CORIOLIS EFFECT. With depth theflow is increasingly deflected to the rightand velocity decreases away from the ef-fects of the wind. In the southern hemi-sphere the converse occurs where the flowis deflected to the left while the velocity de-creases with depth. The Ekman effect isnegligible at the Equator. The Ekmandepth (Ekman layer) is the depth of waterwithin which the current change occurs; itis around 100 m (330 ft) depending on lat-itude and the wind speed. The net trans-port direction within the Ekman layer is atan angle of 90° to the wind; this is theEkman transport. The effect was devel-oped in 1902 as a mathematical model –the EKMAN SPIRAL.2. In the atmosphere, the way in which thewinds within the friction layer in the lowerTROPOSPHERE gradually change speed anddirection moving from the surface up to thefree atmosphere, where friction does notplay any role in the air’s motion. The suc-cessive end points of the wind vectors forma spiral shape up through the friction layer(see Ekman spiral). The wind speed gradu-ally increases with height as the influenceof friction weakens. In the northern hemi-sphere, the wind gradually turns to theright of its surface direction (in the south-ern hemisphere the effect is to the left), spi-raling in that sense up to where it is inbalance as the GEOSTROPHIC WIND. Theangle that the wind makes across the ISO-BARS near the surface varies with its rough-ness, so that it is around 25° to 30° forcontinental surfaces and 10° to 20° overthe sea.

Ekman layer In the atmosphere, thelayer of transition between the surface

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Ekman layerFREEDOM PALESTINE


boundary layer, where shearing stress isconstant, and the free atmosphere. The di-rection of the wind alters as it approachesthe Earth’s surface due to the increasing in-fluence of friction. At the top of this layeris the GEOSTROPHIC WIND, which is locatedapproximately 500–1000 m (1600–3300ft) above the Earth’s surface in the middlelatitudes of the northern hemisphere.Below this level develops the EKMAN SPIRAL,which describes the backing (changes inthe counterclockwise direction) of thewind from the direction of the geostrophicwind. Friction acts to decrease wind speedand thus reduces the effect of the Coriolisforce (see Coriolis effect); this causes thewind to blow across isobars in the direc-tion of the pressure gradient (i.e. towardlow pressure). The angle between thegeostrophic wind direction and the surfacewind increases with an increase in fric-tional drag; for example, over land theangle of difference is much greater thanover sea.

Ekman spiral The theoretical mathe-matical model that was originally formu-lated in 1902 by the Swedish physicist,Vagn Walfrid Ekman (1874–1954), to ex-

plain the currents resulting from windblowing over the ocean (see Ekman effect),which was first observed in the motion ofice floes by the Norwegian explorer,Fridtjof Nansen (1861–1930). The modelhas been subsequently applied to the varia-tions in wind direction and speed betweenthe surface and level of geostrophic flow inthe atmosphere. The spiral refers to theshape defined by the successive end pointsof the vectors of the oceanic current flowwith increased depth, or of the wind vec-tors with increased altitude.

Ekman transport See Ekman effect.

E-layer (E region, Heaviside layer, Heavi-side–Kennelly layer) The layer or regionof the IONOSPHERE, at heights of 90–140km (55–90 miles) above the Earth’s sur-face, that marks the lower region of a dis-tinct maxima of electron concentrationand that is capable of reflecting radiowaves back to Earth. The upper level ismarked by the F-LAYER.

electromagnetic radiation (EMR) En-ergy propagated through space or amedium in the form of waves associated

Ekman spiral

80

decrease invelocitywith depth

surface current

45°wind

Ekman effect

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with electric and magnetic fields oscillatingat the same frequency. The transverse wavemotion acts at right angles to the directionof propagation of the energy, the speed ofwhich is about 3 × 108 meters per second

(known as the speed of light). The electro-magnetic spectrum is the range of electro-magnetic radiation having wavelengthsmeasured in kilometers down to microme-ters. The three sections that are of impor-tance in radiation climatology and theEarth’s atmosphere are the two short-wavebands of ULTRAVIOLET (UVA, UVB, andUVC) and the visible band for SOLAR RADI-ATION, and the infrared (IR) or LONGWAVE

RADIATION predominantly emitted from theEarth’s surface.

electromagnetic spectrum See electro-magnetic radiation.

element In climatology, one of the mainatmospheric conditions, i.e. precipitation,humidity, temperature, atmospheric pres-sure, and wind.

elevated convection CONVECTION thatoccurs in an elevated layer that is not incontact with the Earth’s surface. Elevatedconvection most frequently occurs whenthe air near the ground is relatively cooland stable, but that aloft is UNSTABLE. Insuch situations, stability indices based onsurface data, such as the LIFTED INDEX, usu-ally underestimate the degree of instability.Elevated convection is less likely to lead tosevere weather conditions than surface-based convection.

El Niño /el neen-yoh/ The unusuallywarm surface water conditions that occurin the central and eastern Pacific at peri-odic intervals. Originally, the name wasapplied to the warm ocean current thatflows south along the coasts of Ecuadorand Peru in South America, occurring sea-sonally around Christmas or near the be-ginning of the year. Generally, this currentpersists until the end of March but on atimescale of 2 to 7 years it becomes unusu-ally warm and persists for up to 18months. It is to this major event that thename now applies. During an El Niñoevent the normally cold water to the westof South America and in the central Pacificbecomes much warmer, while the waters inthe western Pacific are cooler.

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El Niño

10–15 –

10–14 –

10–13 –

10–12 –

10–11 –

10–10 –

10–9 –

10–8 –

10–7 –

10–6 –

10–5 –

10–4 –

10–3 –

10–2 –

10–1 –

1 –

10 –

102 –

103 –

104 –

105 –

– 1020

– 1019

– 1018

– 1017

– 1016

– 1015

– 1014

– 1013

– 1012

– 1011

– 1010

– 109

– 108

– 107

– 106

– 105

– 104

– 103

– 102

– 10

– 1

gamma rays

X-rays

ultravioletradiation

visible light

infrared(heat)

radiation

EHF

SHF

UHF

VHF

HF

MF

LF

VLF

radiofrequencies

freq

uen

cy/k

Hz

wav

elen

gth

/m

Electromagnetic radiation

FREEDOM PALESTINE


El Niño now often applies morebroadly to the whole of the disruption tothe ocean–atmosphere system that takesplace, i.e. the EL NIÑO SOUTHERN OSCILLA-TION (ENSO) phenomenon, with the reper-cussions it has for climate and weatheraround the world. The Southern Oscilla-tion represents the atmospheric reaction tothe changes in sea surface temperature andis manifest by a seesawing of pressure overthe tropical Indo-Pacific region. See also La

Niña. [Spanish: ‘Christ child’ or ‘little boy’,because of its original association withChristmas]

El Niño Southern Oscillation (ENSO)The large-scale atmosphere–ocean interac-tion event that occurs irregularly across thetropical Pacific Ocean every 2 to 7 years,linking the SOUTHERN OSCILLATION and EL

NIÑO. The Southern Oscillation is one facetof the phenomenon that in the atmosphere

El Niño Southern Oscillation

82

P A C I F I CP A C I F I CP A C I F I C

O C E A NO C E A NO C E A N

P A C I F I CP A C I F I CP A C I F I C

O C E A NO C E A NO C E A N

Under normal conditions the trade winds push water from east to west and warm wateraccumulates around Indonesia

INDONESIAINDONESIAINDONESIA

AUSTRALIAAUSTRALIAAUSTRALIA

warmwarmwarm

seaseasea

normal ocean currentsnormal ocean currentsnormal ocean currents

UNITED UNITED UNITED STATESSTATESSTATES

PERUPERUPERU

normal trade windsnormal trade windsnormal trade winds

El Niño currentEl Niño currentEl Niño current

increase inincrease inincrease insurface surface surface temperaturestemperaturestemperatures

weakerweakerweakertrade windstrade windstrade winds

INDONESIAINDONESIAINDONESIA

AUSTRALIAAUSTRALIAAUSTRALIA

UNITED UNITED UNITED STATESSTATESSTATES

PERUPERUPERU

In El Niño years, the trade winds weaken and the warm water flows back toward the coast ofIn El Niño years, the trade winds weaken and the warm water flows back toward the coast ofIn El Niño years, the trade winds weaken and the warm water flows back toward the coast ofLatin AmericaLatin AmericaLatin America

El Niño

FREEDOM PALESTINE


is marked typically by the gradual east-ward migration of a significantly warmocean water pool from its normal locationin the western Equatorial Pacific. Thisevent is stimulated by the relaxation of thePacific TRADE WINDS that may even reverseinto westerlies. This leads to the genesis of,and eastward migration of, what is knownas a Kelvin wave – a large patch of slowlymoving very warm water that is a veryshallow dome or wave shape some 15 cm(6 in) high but a few hundred kilometers ei-ther side of the Equator. The anomalouslywarm water has dire local consequences asit migrates to affect regions that are nor-mally dry. It sparks off very active THUN-DERSTORMS that can culminate, after theirjourney across the Pacific, in disastrouslytorrential downpours in Peru. In addition,the unusually located deep CONVECTIVE

CLOUDS transport large amounts of heatinto the upper TROPOSPHERE into regions inwhich the JET STREAMS are consequentlystrengthened and sometimes shifted. Thiscan and does have an influence on anom-alous weather patterns in regions of theworld that are very far from the tropicalPacific.

ELR See environmental lapse rate.

emagram /em-ă-gram/ See skew-T log-Pdiagram.

emissions Those substances (particu-late, gas, or aerosol) that are dischargedinto the atmosphere as a waste product orpollution, either from an anthropogenicsource (e.g., exhaust emissions from auto-mobiles and aircraft, and emissions fromindustrial and agricultural processes) orfrom a natural source (e.g., volcanic activ-ity and forest or bush fires). See also airpollution.

emissions trading A regulatory schemein which credits or permits are allocated toparties emitting pollutants (e.g. a firmemitting sulfur dioxide) by a regulatory or-ganization. Those parties that reduce theiremissions are able to sell or trade theircredits to those parties that are unable toreduce their emissions. The concept was

proposed in the KYOTO PROTOCOL to enableparties to trade the assigned amounts oftheir GREENHOUSE GAS emissions al-lowances in order to fulfill their emissionscommitments.

emissivity /em-ă-siv-ă-tee/ The ratio ofthe actual power per unit area emitted by asurface or body to that of a BLACK BODY

(perfect radiator) at any given temperature.The Earth’s surface, oceans, and cloudshave an emissivity value close to 1 (blackbody), while high cirrus clouds have avalue of 0.2.

EMR See electromagnetic radiation.

energy balance See energy budget;global energy balance; radiation budget.

energy balance model (energy budgetmodel, EBM) A simple MODEL used toanalyze the solar radiation incident on theEarth and to predict variation of surfacetemperature with latitude. For this pur-pose, atmospheric motions are omitted, al-though surface air temperature and surfacealbedo are used. The model can be used toconsider the Earth as a whole to give theglobal average temperature. Energy bal-ance models are used, for example, in stud-ies of climate change.

energy budget The surface energy in-puts and outputs that form a part of the en-ergy balance of the Earth–atmospheresystem. Most of the exchanges of energytake place on the Earth’s surface, which istherefore called the ‘active surface’. In itssimplest form the energy budget may be ex-pressed as:

Q* + H + LE + G = 0,where Q* is net radiation, H is sensibleheat, LE is latent heat (L is the latent heatof vaporization of water and E is theamount of water evaporated), and G is theheat flow into the ground. See also globalenergy balance; radiation budget.

enhanced greenhouse effect Seegreenhouse effect.

enhanced V A V-shaped region of

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enhanced VFREEDOM PALESTINE


colder cloud tops seen on infrared satelliteimages of a thunderstorm ANVIL. This re-gion signifies an area of strong updraft andtherefore potentially severe weather; theevidence on the satellite data provides auseful tool that can be used to detect andmonitor thunderstorms.

ensemble method (ensemble forecasting)In weather forecasting, a relatively recentinnovation in NUMERICAL WEATHER PREDIC-TION in which the initial state of the atmos-phere, upon which the model prediction isrun, is not taken as fixed. This means that,at the EUROPEAN CENTRE FOR MEDIUM-RANGE WEATHER FORECASTS (ECMWF) forexample, 51 different forecasts are run si-multaneously, each one beginning withsubtly different initial conditions – it is thislarge collection that constitutes the ‘ensem-ble’ in the method. The reason for themethod is to attempt to incorporate therole of ‘chaos’ in the quality of the weatherforecast, because a small perturbation thatcannot possibly be observed may play asignificant role in the evolution of theweather globally or regionally.

The ensemble method produces 51forecasts for each daily operational run,then automatically groups them into pre-dictions (for day three, for example), thatall point broadly in the same direction re-garding the future weather conditions. Onsome occasions a large grouping will indi-cate the same trends, no matter what thewide range of changed initial conditionswere. At other times there will be a numberof clusters, each of which suggest quite dif-ferent forecast outcomes. The method isemployed to give forecasters an indicationabout the confidence they can have in howthe weather will evolve over a period ofdays. This degree of confidence will varyfrom day-to-day and geographically onany particular day.

ENSO See El Niño Southern Oscilla-tion.

entrainment /en-trayn-mĕnt/ The processin which air from the environment sur-rounding a developing cloud is draggedinto the ascending CONVECTION current

within the cloud. This has the effect of re-ducing the current’s buoyancy; sometimesthe cloud system may dissipate completelyif very dry air is introduced, leading toevaporation of the cloud droplets. The areain which this takes place is the entrainmentzone.

entrance region That part of an uppertropospheric JET STREAM in which the hori-zontal flow of air accelerates markedly intothe elongated core of a jet. The right en-trance of a jet stream is commonly associ-ated with cyclonic features in the lowertroposphere, while the left entrance islinked to anticyclonic features.

entropy /en-trŏ-pee/ (S) A measure ofthe energy within a closed thermodynamicsystem that is unavailable to do work. It isa measure of the disorder of the system –the greater the disorder, the higher the en-tropy. Thus, the entropy of a gas is higherthan that of a liquid because its constituentatoms and molecules are in a less orderedstate.

environmental hazard A process,caused by human activity or NATURAL HAZ-ARD, that has a detrimental effect on the en-vironment and/or human life. Examples ofhuman-induced environmental hazards in-clude the Exxon Valdez oil spillage ofMarch 1989, when 11 million gallons ofoil poured into the Prince William Sound,Alaska, affecting 750 km (470 miles) ofcoastline. Many wildlife and fish speciesstill have not recovered.

environmental lapse rate (ELR) Theactual rate at which temperature changeswith height throughout the TROPOSPHERE asrecorded, for example, by a radiosonde. Apositive LAPSE RATE is usually observed inthe atmosphere indicating that tempera-ture decreases with height. On average thisis approximately 6.5°C km–1 but the ratevaries according to local factors, for exam-ple height, season, time of day, surfacecharacteristics, and is a function of the pre-vailing weather conditions. The environ-mental lapse rate is lower nearer to theground surface, during the winter season,

ensemble method

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and over continental areas. Lapse rates arerecorded, for example, during radiosondeascents and plotted on thermodynamic di-agrams.

environmental temperature soundingThe determination of the changes in tem-perature with height. A vertical profile oftemperature is usually obtained using bal-loon-borne instruments or measured usingremote-sensing equipment. See also bal-loon sounding.

Environment Canada A major federalorganization that includes the METEORO-LOGICAL SERVICE OF CANADA, the country’ssource for meteorological information. Inaddition it undertakes a legally enforceableenvironmental protection role, includingproblems related to pollution.

ENVISAT (Environmental Satellite) Alarge and sophisticated polar-orbitingsatellite launched by the European SpaceAgency (ESA) in 2002 to provide data andmonitoring of the atmosphere, ocean, land,and ice. It carries a payload of 10 instru-ments including the Advanced SyntheticAperture Radar (ASAR), GLOBAL OZONE

MONITORING BY OCCULTATION OF STARS

(GOMOS) instrument, and the MediumResolution Imaging Spectrometer Instru-ment (MERIS). See also European RemoteSensing Satellite.

eolian /ee-oh-lee-ăn/ (aeolian) Pertainingto the wind; for example, materials eroded,transported, and deposited on the Earth’ssurface by the wind are referred to as eo-lian deposits. Eolian processes are mostcommon in arid environments and on ex-posed beaches.

EOS See Earth Observing System.

EP See effective precipitation.

epilimnion /ep-ă-lim-nee-ŏn/ The upperwarmer layer that lies above a THERMO-CLINE and below the surface of a body ofwater, such as a lake or sea.

equable In climatology, denoting a cli-mate type in which there is little variation.

equation of motion In meteorology, aset of equations based on Newton’s secondlaw of motion that relates the total forceacting on a unit mass of air to the acceler-ation induced. The accelerations are of thewesterly, southerly, and upward compo-nents of atmospheric motion. The acceler-ation with regard to the two horizontalcomponents is related to the horizontalPRESSURE GRADIENT FORCE, the Coriolisforce (see Coriolis effect), and the frictionalforce.

Equator The great circle around theEarth, along the 0° parallel of latitude. Seealso thermal equator.

equatorial climate The climate of thelow latitudes between about 10°N and S.The climate is dominated by the Intertrop-ical Convergence Zone (ITCZ). Tempera-tures are consistently high with little range.Rainfall and humidity are also high.

Equatorial Countercurrent An oceancurrent that flows east between the west-ward-flowing North and South EQUATOR-IAL CURRENTS. Westward flow of theEquatorial Currents results in a pressuregradient produced by the higher sea sur-face, to the west of the tropical oceanbasins. To balance this gradient, the low-velocity Equatorial Countercurrent trans-ports excess water eastward.

Equatorial Current Broad ocean cur-rents in the equatorial regions, which aredriven by the trade winds and generally di-rected westward, flowing through themajor oceans: the North Equatorial Cur-rent and the South Equatorial Current.Separating the two currents is the narrowEQUATORIAL COUNTERCURRENT, whichflows eastward at the warmest part of theEquator. At the Equator, an EQUATORIAL

UNDERCURRENT is also produced but at adepth of about 100 m (330 ft). In the In-dian Ocean the North Equatorial Currentonly flows during the northeast monsoon(November to March); it is in summer re-

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placed by the Southwest Monsoon Cur-rent, which flows eastward, when thesouthwest monsoon is occurring.

Equatorial orbit A satellite orbit thatoccurs in the Earth’s Equatorial plane.

Equatorial trough An approximatelyeast–west orientated and elongated pres-sure minimum that extends around theEarth at very low latitudes and is associ-ated with low-level CONVERGENCE and ex-tensive cloudiness. It is located in thevicinity of the Equator, lying between theSUBTROPICAL ANTICYCLONES of the northernand southern hemispheres and is indistin-guishable from the INTERTROPICAL CONVER-GENCE ZONE (ITCZ) when the latter is close to the Equator. It exhibits somenorth–south seasonal migration so thatover tropical continents it moves a signifi-cant distance away from the Equator. TheEquatorial trough may be detected up to aheight of 5.8 km (3.6 miles) but is mostpronounced near the surface. It coincideswith the DOLDRUMS over the oceans.

Equatorial Undercurrent (CromwellCurrent) The undercurrent that flows ata depth of approximately 100 m (330 ft )below the surface and transports watereastward and toward the Equator.

Equatorial westerlies The westerlywinds that occur when the NORTHEAST andSOUTHEAST TRADE WINDS have to flowacross the Equator into the INTERTROPICAL

CONVERGENCE ZONE. If this occurs moredeeply into the opposite hemisphere bymore than about 5° of latitude, they are de-flected into Equatorial westerlies. During astrong El Niño Southern Oscillation event,there can be extensive westerlies along theEquator across part of the Pacific Ocean.

equatorial zone See tropics.

equinoctial gale A strong wind that oc-curs at or around the vernal (March 21) orautumnal (September 22) equinoxes. Theview that strong winds occur with a greaterfrequency than the long-term average atthe equinoxes is frequently expressed, but

this opinion is not generally supported byanalyses of past data in such countries asthe US and the UK; it therefore still lacks aplausible physical explanation.

equinoctial rains Seasonal rains thatoccur in areas close to the Equator at thetime of the equinoxes, or shortly after.

equinox 1. Either of the two points atwhich the ecliptic intersects the celestialequator.2. Either of two dates at which the Sun ap-pears to cross the celestial equator. Thevernal (or spring) equinox occurs on 20 or21 March, and the autumnal equinox on22 or 23 September in the northern hemi-sphere; this is reversed in the southernhemisphere. Day and night are of equal du-ration at the equinoxes.

equivalent potential temperature(theta-e) The EQUIVALENT TEMPERATURE

transformed dry adiabatically to the 1000hPa pressure level. It can be calculatedusing a THERMODYNAMIC DIAGRAM by fol-lowing the DRY ADIABAT from Te at a givenpressure level down to the 1000 hPa level,this being approximately equal to mean-sea-level pressure.

equivalent temperature The tempera-ture (Te) obtained when an air parcel is ex-panded adiabatically, at constant pressure,until the parcel’s water vapor content hasbeen condensed out and the latent heat ofcondensation used to raise the air tempera-ture.

ERBE See Earth Radiation Budget Ex-periment.

ERS See European Remote SensingSatellite.

ESA See European Space Agency.

etesian wind /i-tee-zhăn/ (meltemi) Amoderate to strong northwesterly to north-easterly wind that blows in the easternMediterranean, especially in the AegeanSea area, between about mid May and midOctober, occurring most frequently in July

Equatorial orbit

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and August. It is caused by the steep pres-sure gradient that exists between an area ofhigh pressure over the Western Mediter-ranean and an elongated trough of lowpressure that extends westward from NWIndia. The etesian wind typically attains itshighest velocity of around 30 knots (35mph) during the late afternoon, when con-vection over the hot land temporarily in-creases the pressure gradient; it graduallydeclines, and dies away during the eveningand night. While the etesian wind can berefreshing, its strength and dryness meanthat windbreaks (e.g. a row of cypresstrees) have to be planted on the northernside of fields to prevent damage to agricul-tural production.

EUMETSAT /yoo-met-sat/ The Euro-pean weather satellite organization, basedin Darmstadt, Germany. It is sponsored bythe weather services of 19 member statesand, in addition, has agreements with 11cooperating states within Europe. Its primeobjective is to establish, maintain, and ex-ploit European systems of operationalweather satellites. The first METEOSAT satel-lite was launched by the EUROPEAN SPACE

AGENCY (ESA), in 1977; EUMETSAT tookover the responsibility for the Meteosatsystem from ESA in 1987. The MeteosatSecond Generation (MSG) satellite system,which came into operation in the firstdecade of the 21st century, is to providemore frequent and comprehensive data inparticular for NOWCASTING and NUMERICAL

WEATHER PREDICTION, and for climate mon-itoring and research. In addition, the devel-opment of the EUMETSTAT Polar System(EPS) will introduce the MeteorologicalOperational (MetOp) polar-orbiting sys-tem of satellites.

European Centre for Medium-RangeWeather Forecasts (ECMWF) An in-ternational medium-range weather fore-casting organization, located in Reading,UK. It is sponsored by its 18 Europeanmember states and has cooperation agree-ments with eight other European states. Es-tablished by a Convention in 1973, it hasproduced operational MEDIUM-RANGE

WEATHER FORECASTS since 1979. Its princi-

pal objectives are to develop numericalmethods for medium-range weather fore-casting; to prepare medium-range weatherforecasts for distribution to the meteoro-logical services of the member states; tocarry out scientific and technical researchto improve the forecasts; and the collectionand storage of appropriate meteorologicaldata.

European Incoherent Scatter See EIS-CAT.

European Remote Sensing Satellite(ERS) One of the two satellites operatedby the European Space Agency (ESA) innear-polar orbit to perform remote sensingof the details of the Earth’s surface, espe-cially oceans, ice-caps, and coastal regions,for environmental monitoring purposes.ERS-1 was launched in 1991 and retiredfrom service in 2000; ERS-2 was launchedin 1995. Advanced microwave techniquesare used to collect measurements and im-ages; the data collected includes measure-ments of wind speed and direction, surfacetemperatures, cloud cover, atmosphericwater vapor levels, and ozone levels. Itsscatterometer data have been used by oper-ational weather forecast centers to deducesea-level winds over vast data-void areas.

European Space Agency (ESA) An or-ganization that operates Europe’s spaceprogram. It was formed in 1975 and has 17member states, with its headquarters inParis, France. The organization is com-posed of four establishments and has, over-all, the role of developing and promotingEuropean activities in space. The EuropeanSpace Operations Center (ESOC) is lo-cated in Darmstadt, Germany, while theResearch and Technology Center (ESTEC)is in Noordwijk, The Netherlands. ESAlaunched the first Meteosat satellite in1977 and is developing, with EUMETSAT,the Meteosat Second Generation (MSG)satellite system.

eustasy /yoo-stă-see/ A global variationin sea level brought about by a change inthe amount of water in the oceans. The fac-tors involved include the melting or growth

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of glaciers, ocean temperature fluctuationsat depth resulting in density changes andan expansion or contraction of the world’soceans, and alterations in basin shapecaused by tectonic movements.

evaporation The process by which aliquid (in meteorology and climatology thisis invariably WATER) is transformed to a gas(WATER VAPOR); it is the opposite process toCONDENSATION. In meteorology, evapora-tion may also encompass SUBLIMATION, bywhich solid water (ice) is converted towater vapor with no intermediate liquidwater stage. For climatological purposesevaporation refers to the loss of water fromthe Earth’s surface to the atmosphere, usu-ally expressed in millimeters per day,month, or year. Evaporation is a majorcomponent in the HYDROLOGIC CYCLE.

evaporation fog (mixing fog) A fogformed by the mixing of two unsaturatedvolumes of air at different temperatures.The mixing process leads to saturation andcondensation of fog droplets under perti-nent conditions, for example, when warmrain falls into a cooler layer of air near thesurface, as occurs in warm fronts.

evaporation pan A pan used to holdwater during observations to determine therate of evaporation at a given location.Evaporation pans are usually set above theground on a small plinth, while evapora-tion tanks are buried in the ground. Seealso evaporimeter.

evaporimeter /i-vap-ŏ-rim-ĕĕ-ter/ (atmom-eter) An instrument used for measuringthe rate of evaporation of water into the at-mosphere. There are two main types. Thefirst type measures the evaporation ratefrom a free water surface. In the open panevaporimeter a tank is sunk into theground so the water surface is at groundlevel and evaporation is measured using amicrometer gauge. Allowances for precipi-tation and drainage are also taken intoconsideration. The second type of evapor-imeter, of which there are several designs,measures the evaporation rate from a con-tinuously wet porous surface. The Piché

evaporimeter uses an inverted graduatedcylinder of water, the mouth of which iscovered with a filter paper disk. As waterevaporates off the filter paper, the level ofthe water reduces and the amount of evap-oration can be measured directly. See alsolysimeter.

evapotranspiration /i-vap-oh-tran-spă-ray-shŏn/ The combined processes ofEVAPORATION from the Earth’s surface andTRANSPIRATION from plant vegetation thattransfer water in the form of WATER VAPOR

to the atmosphere. It is a fraction of PO-TENTIAL EVAPOTRANSPIRATION.

evapotranspirometer /i-vap-oh-tran-spă-rom-ĕ-ter/ An instrument used for mea-suring the rate of EVAPOTRANSPIRATION. Itconsists of a tank containing soil, in whichvegetation is planted, and designed so thatall water added to the tank and water leftfollowing evapotranspiration can be mea-sured.

exit region That region of a JET STREAM

in which the air is leaving the elongatednarrow core of maximum winds and slow-ing down markedly. It is commonly associ-ated with cyclonic features in the lowertroposphere below the left-exit and anticy-clonic features below the right-exit region.

exosphere /eks-ŏ-sfeer/ The outermostregion of the Earth’s atmosphere, extend-ing from approximately 700 km (430miles) above the Earth’s surface. Gas den-sities are very low, mean free paths arelarge, and collision frequencies are lowenough for particles to achieve sufficientvelocity from a collision to escape from theatmosphere.

explosive cyclogenesis A very quicklydeepening FRONTAL CYCLONE, usually overmiddle-latitude oceans, in which the rate ofdeepening (or the fall of central pressure) isat least 24 hPa in one day. This rate ofdeepening defines the system as a bomb.Such disturbances are associated princi-pally with very damaging winds.

exposure The exposure of a site to

evaporation

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SOLAR RADIATION; this depends upon its lat-itude, elevation, aspect, slope, and lack ofshielding by obstacles (for example, build-ings and vegetation).

extended-range weather forecastingSee medium-range weather forecasting.

extraterrestrial radiation Shortwaveradiation, almost exclusively from the Sun,reaching the outer limits of the atmosphere(the top of atmosphere or TOA). This flowof radiation, the SOLAR CONSTANT, has avalue that varies with the radiant output ofthe Sun but is around 1367 W m–2.

extratropical cyclone A traveling CY-CLONE (low-pressure system) in middle andhigher latitudes, outside the tropics. It nor-mally refers to a frontal system but is alsoapplicable to a non-frontal depression.

extremes of climate The highest orlowest values for a meteorological parame-ter, or an exceptionally large range be-tween the highest and lowest values for ameteorological parameter, over a specifictime period, such as a month, season, oryear.

eye (eye of the storm) The generally cir-cular central region of an intense TROPICAL

CYCLONE, usually about 15–20 km (9–12miles) in diameter, with a small horizontalpressure gradient. This region is character-ized by light winds and no rain, and a clearsky or broken clouds. As the eye passesover a location the violent winds associ-ated with the storm reverse their direction.See also hurricane; storm surge.

eye of the wind A nautical term used todescribe the direction from which the windis blowing.

eye wall cloud The narrow zone ofcloud within the band of convection thatsurrounds the eye of a TROPICAL CYCLONE

(HURRICANE or TYPHOON). It is essentiallyan upright cyclinder that is some 12–15 km(7–9 miles) deep, consisting of a ring ofvery vigorous CUMULONIMBUS clouds. It isthe narrow zone across which the hurri-cane’s horizontal pressure gradient is at itssteepest; as a result the strongest windsoccur underneath the eye wall cloud, as dothe highest rainfall totals and severestthunderstorms.

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Fahrenheit scale A standard tempera-ture scale widely used in the US; it is basedon the freezing point of water at 32°F andthe boiling point of water at 212°F with180 equal divisions between these twofixed points. The scale was developed orig-inally in 1714 by Gabriel Daniel Fahren-heit (1683–1736), a German physicist,basing the zero point on the temperature ofan equal ice–salt mixture. Although theFahrenheit scale was widely used it hasbeen largely replaced internationally, andfor scientific purposes in the US, by theCELSIUS SCALE and by thermodynamic tem-peratures measured in KELVINS. The con-version formula from Celsius to Fahrenheitis:

°F = (°C × 1.8) + 32.

fair-weather cumulus LOW CLOUDS

with flat white or gray bases and ‘bubbling’white tops that are not very developed ver-tically. Fair-weather cumulus clouds aregenerally widely spaced with blue sky ap-parent between them. They are quite com-mon on pleasant summer afternoons whensurface heating on a sunny warm daysparks off shallow CUMULUS clouds thatdrift innocuously across the sky. Fair-weather cumulus may be used to describeboth HUMILIS and MEDIOCRIS species of cu-mulus.

fall See autumn.

fallout The process by which dry partic-ulates descend from the atmosphere andreturn to the surface under gravity. See alsowashout.

fallstreak /fawl-streek/ See virga.

false cirrus The CIRRUS cloud that was

originally the anvil topping a CUMULONIM-BUS cloud but has become detached as theoriginal cloud dissipates. Such false cirrusoccurs often in the afternoon and eveningof a thundery day, vestiges of the stormsthat have died out.

false color In remote sensing, the repre-sentation on an image of data collected inwavelengths that are not normally visible.The non-visible electromagnetic spectrumis shown as one or more of the red, green,or blue components; for example, data inthe infrared wavelength is frequently dis-played as visible red. The presence of a par-ticular range of sea-surface temperatures ina remotely sensed scene may be repre-sented by using different colors not naturalto such a scene. The warmest sea may, forexample, be false-colored in red to empha-size its location.

fanning In pollution studies, emissionsfrom a chimney stack under predominantlystable atmospheric conditions, usually inthe presence of a weak to moderate tem-perature inversion, where horizontal dis-persal of pollutants is several times greaterthan vertical dispersal.

fata morgana /fah-tă mor-gah-nă/ Anoptical phenomenon, a complex type of su-perior MIRAGE, in which distant objectsmay appear as distorted images resemblingwalls, castles, or turrets, as a result of tem-perature inversions within the lowest lay-ers of the atmosphere. The phenomenon,which occurs sometimes in the Strait ofMessina, Italy, was there named for Mor-gan le Fay (from Arthurian legend), whowas believed to use sorcery to produce suchillusions. [Italian: Morgan the fairy]

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fathom A nautical measurement ofdepth equivalent to 6 ft (1.8 m).

feeder band 1. (outer convective band)In tropical meteorology, one of severallines or bands of thunderstorms that spiralaround the center of a TROPICAL STORM orTROPICAL CYCLONE.2. (inflow band) In a thunderstorm, a bandof low cloud, one of several lines of suchclouds ahead of, and moving into, the coreregion of the storm. The feeder bands movewith the low-level flow and may be an in-dication of the strength of the warm moistair flowing toward the main updraft.

Ferrel cell /fe-rĕl/ (middle-latitude cell)The large-scale wind circulation cell thatexists in the vertical north–south plane inmiddle latitudes. It is characterized by flowtoward the poles in the lower TROPOS-PHERE, often as southwesterly winds in thenorthern hemisphere (northwesterlies inthe southern hemisphere) that have theircore in the SUBTROPICAL ANTICYCLONES. Theair in the Ferrel cell then ascends withinFRONTAL ZONES. In the upper tropospherethere is a return component that flowsback toward the subtropics, where it meetsthe upper outflow from the HADLEY CELL ina zone of CONVERGENCE. It descends belowthis to replenish the air that flows out fromthe surface high. The cell is named for theUS meteorologist William Ferrel (1817–91), who originally described a middle-lat-itude circulation cell.

fetch The length of the surface area ofsea or ocean over which a wind blows inone direction unimpeded by land; this is animportant factor that affects the height ofthe waves.

fibratus /fÿ-brah-tŭs/ A species of CIRRUS

and CIRROSTRATUS cloud that appears as athin veil, detached straight filaments, or ir-regularly curved filaments, none of whichend in hooks or tufts. See also cloud classi-fication.

fiducial point /fi-dew-shăl/ An acceptedfixed point of reference for an instrumentor scale. For example, the point to which

an instrument must be reset before it can beused.

field capacity (field moisture capacity,soil moisture storage capacity) The waterthat remains in a soil after it has been satu-rated and then left to drain by gravity.

filling The phenomenon of increasingbarometric pressure within a low-pressureregion. Thus, a FRONTAL CYCLONE (frontaldepression) can ‘fill’ but an ANTICYCLONE

(high) cannot. A rapid rate of filling wouldbe 10 hPa in three hours; it is more typi-cally 1 or 2 hPa every three hours.

fire A hazard, caused either by naturaldisaster or human activity, in which firespreads over wide areas destroying vegeta-tion, wildlife, and property, releasingquantities of smoke and pollutants into theatmosphere. Fire can have positive in addi-tion to negative impacts. It has been usedby humans to modify the landscape, clearforests for agriculture, improve the qualityof grazing land, and encourage the rapidregeneration of plant species. A wildfire ina forest or bush area may be caused, for ex-ample, by human carelessness or lightningstrikes, often preceded by drought. It canbe worsened by strong winds that help tospread the fire over large areas. Regions es-pecially susceptible to wildfire are thosethat have a marked dry season with hottemperatures.

The incidence of EL NIÑO SOUTHERN OS-CILLATION events has also been related toincreased fire risk in those parts of thewestern Pacific that have reduced convec-tion and precipitation when it occurs. Dur-ing 1997 extensive forest fires in Indonesia,many of which were started to clear forestbut subsequently grew out of control, hadan impact on many parts of SE Asia, in-cluding Malaysia, Singapore, and Thai-land, as a result of smoke spreading overthe region creating severe pollution and af-fecting weather. In late 1997 and early1998 Australia suffered widespread bushfires. These events were partly influencedby the 1997–98 ENSO. See also crown fire;National Fire Danger Rating System.

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firn /fern/ (névé) Granular snow that hassurvived at least one summer melt season.It is intermediate in the conversion of snowinto pure glacial ice by the process of firni-fication, its density being in the range of0.4 to 0.89 kg m–3.

firn line (firn limit, annual snowline) Aline on a glacier that marks the upper limitof the region in which winter snowfallmelts during the summer ablation season.It is often clearly marked on many glaciers,separating hard blue ice below from snowabove.

firn wind See glacier wind.

flanking line A component of a tor-nadic supercell that occurs on the rear ortrailing side of such a system. It is formedof increasingly tall CUMULUS and cumuluscongestus cloud (see congestus) that feedinto the massive SUPERCELL. The flankingline is defined by the rear-flank downdraftthat is part of the supercell’s circulation.The WALL CLOUD is usually found under-neath the area in which the flanking line in-tersects the tallest growing cloud in thesystem. See also tornado.

flash flood A sudden and destructiveFLOOD occurring with little or no priorwarning caused usually by excessive rain-fall and/or rapid snow and ice melt. Otherfactors that can increase the likelihood of aflash flood include changes in land usewithin a watershed, which can acceleraterunoff rates, and changes in basin shapesdownstream. For example, basins that be-come narrower downstream reduce the ca-pacity of the channel. Flash floods are mostcommon in mountainous regions but canalso occur over flat terrain. The suddenfailure of a dam can also produce a flashflood. In the US the National Weather Ser-vice issues a Flash Flood Watch when it ispossible that rainfall will cause flash flood-ing in a particular area; a Flash FloodWarning is issued if a flash flood is occur-ring or is imminent in the area.

F-layer (F region) A layer or region ofthe IONOSPHERE in which there are high

concentrations of ions and electrons; it ex-tends from about 150 km (90 miles) abovethe Earth’s surface to about 1500 km (900miles). Two F-layers are identified. The F1-layer tends to have stable ion and electronconcentrations; it is found at around 160km (100 miles) but its height varies sys-tematically depending on the latitude, thetime of day, the season; it also varies withsunspot activity. The F2-layer (also knownas the Appleton layer), found at around300 km (200 miles), is less predictable andis influenced by the solar and lunar tidesand the Earth’s magnetic field.

floccus /flok-ŭs/ A species of cloud inwhich the constituent CUMULIFORM unitsappear like small tufts that have an overallragged look, often with VIRGA. It is appliedto CIRRUS, CIRROCUMULUS, and CIRROSTRA-TUS cloud. See also cloud classification.

Flohn climate classification /flohn/ Agenetic CLIMATE CLASSIFICATION proposedin 1950 by the German climatologist Her-mann Flohn (1912–97). The classificationis based on the global wind belts and pre-cipitation characteristics and recognizedseven major categories of regional cli-mates. See also Köppen climate classifica-tion; Strahler climate classification;Thornthwaite climate classification.

flood An overflowing of a body of waterfrom its natural confines produced by ex-cessive water input into a river, or a tem-porary incursion of ocean or sea systemonto areas not normally submerged.Floods can be caused, for example, by veryintense precipitation on to saturatedground, the melting of snow packs, the col-lapse of dams (natural and man-made),high tides, TSUNAMIS, and STORM SURGES.

flood forecasting (flood prediction) The process of predicting river or coastalflooding, defined as inundation of landareas that are not normally submerged.Fluvial flood prediction is part of the rou-tine operations undertaken by hydrolo-gists. They require meteorological input inthe form of observed and predicted PRECIP-ITATION patterns, including their speed and

firn

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direction of movement, and the intensity ofthe rain or snowfall. Predicted precipita-tion patterns may be obtained from NU-MERICAL WEATHER PREDICTION. The actualpatterns are mapped by precipitationRADAR networks, and the digital data arefed automatically into hydrological floodprediction models. In addition, the forecastmodels require quantification, for exam-ple, of the soil moisture levels within a par-ticular catchment, and the evaporationlosses during the prediction period. Theymust also be set up to incorporate the melt-ing of snowpack including its extent anddepth, its location, and how rapidly it oc-curs. Flash flooding (see flash flood) is avery dangerous phenomenon that occurson a relatively small-scale; it is difficult topredict since it is most commonly related tovigorous THUNDERSTORMS that can breakout virtually anywhere – over a hilly ormountainous area, for example – on a hotspring or summer’s day. The forecast canpoint to the high risk of such floods, butcannot easily pinpoint the location or thetime at which they will occur. The stormsthat produce them can often develop in aslittle as tens of minutes.

The forecasting of coastal flooding re-lies on numerical weather prediction. Low-lying coasts can be flooded by swollenrivers that issue into the sea across them orby STORM SURGES that move onshore in as-sociation with HURRICANES (TROPICAL CY-CLONES) or TROPICAL STORMS. The location,height, and timing of such surges are wellhandled by routine weather forecast mod-els, and in the US have historically beenmost likely to affect the coastline fromsouthern Texas as far as the mid-Atlanticstates. However, major storm surges alsoaffect coasts bordering the Gulf of Mexico(e.g. Hurricane Katrina in August 2005).Storm surges can also occur with deepFRONTAL CYCLONES that track with a par-ticular speed and route across relatively en-closed ocean basins. This is the case aroundthe shores of the North Sea, for example,where low-lying areas of the eastern UK,northern Germany, and The Netherlandsare susceptible.

flood peak estimation The process of

predicting the timing and height of themaximum levels in stream discharge at var-ious points down the main channel of aflooding river. These factors will dependpartly on how the channel’s shape changesalong its course, and how large the contri-bution from groundwater storage is as theflood peak moves downstream.

flood prediction See flood forecasting.

Florida Current A northward-flowingsection of the GULF STREAM that extendsfrom the Straits of Florida to Cape Hat-teras, North Carolina. It is characterizedby low salinity and temperatures above6.5°C (44°F).

fluorocarbon /floo-ŏ-rŏ-kar-bŏn/ Asynthetic compound of carbon and fluo-rine that also often contains other ele-ments, such as hydrogen, chlorine, orbromine. The fluorocarbons include theCHLOROFLUOROCARBONS (CFCs), HY-DROCHLOROFLUOROCARBONS (HCFCs), HY-DROFLUOROCARBONS (HFCs), andPERFLUOROCARBONS (PFCs).

flux The rate of flow of particles, fluid,or radiation crossing a unit area per unittime (e.g. heat flux).

foehn pause /fayn, føekig;n/ See föhnpause.

foehn wall See föhn wall.

foehn wind See föhn wind.

fog A dense mass of minute waterdroplets suspended in the atmosphere thatobscures the lower layers of the atmos-phere and reduces horizontal visibility toless than 1000 m (3300 ft). It is defined asthick fog if visibility is 100 m (330 ft) orless. Fog is normally caused by the conden-sation of water vapor that results when airis cooled to below its DEW-POINT and is gen-erally associated with calm weather condi-tions. Formation is aided when there is aconcentration of suspended smoke and/ordust particles in the lower atmosphere;such particles act as condensation nuclei

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and may cause fog to form before the satu-ration point is reached. There are severaltypes of fog, most being defined accordingto the mechanism of cooling. See advectionfog; evaporation fog; frontal fog; hill fog;ice fog; radiation fog; sea fog; steam fog;upslope fog. See also smog.

fog bank Fog that has the appearance ofa well-delineated mass when observedfrom a distance, especially at sea.

fogbow /fog-boh/ (white rainbow, mist-bow, cloudbow) A faintly colored orwhitish arch, the inner edge of which isbluish and the outer reddish, seen oppositethe Sun in fog or mist. It is similar in for-mation to the rainbow and is due to refrac-tion and reflection, together with somelimited diffraction, of light from the Sun orMoon by small water droplets, the diame-ters of which are less than 100 µm (0.004in).

fog-drip (fog precipitation) Precipita-tion in the form of moisture from a FOG

bank that has been intercepted by vegeta-tion. It is common along the coast of aridregions where humidity is constantly highand there is a cold ocean current offshore(e.g. S California and Peru). Other forms ofprecipitation in these regions are scarceand fog-drip may be sufficient to sustainvegetation in such arid environments.

föhn pause /fayn, føekig;n/ (foehnpause) A temporary cessation of a FÖHN

event at the ground due to the formation orintrusion of a cold air layer, which lifts theföhn off the Earth’s surface.

föhn wall (foehn wall) A stationary lineor bank of clouds that is an indicator of theexistence of a FÖHN WIND over a mountainrange. The name was first applied to thecloud observed in the European Alps. Theair ascending the south side of the Alps asa southerly wind will lead to condensationupslope as far as, and overlapping, the lineof the Alpine watershed. This elongatedcloud that runs along the crest line of themountains can be seen from the downslopeside, appearing like a wall of cloud that

does not move as the wind flows throughit. See also chinook arch; helm wind.

föhn wind (foehn wind, fon wind) Awarm and dry wind that blows down theLEEWARD side of a mountain range. Theword föhn was originally used in the Euro-pean Alps for a southerly wind, which can result in a sudden increase in tempera-ture (sometimes by 8° to 11°C) in thenorth–south orientated valleys of thenorthern Alps; this can lead to rapid snowmelt and avalanches. In the Reuss Valley ofSwitzerland, a föhn wind typically blowson 48 days per year, occurring most fre-quently during the spring when a depres-sion moves to the north of the Alps.Consequently, warm air is drawn in fromthe south and forced to ascend the south-ern slopes of the Alps at the saturated adi-abatic lapse rate (∼0.5°C/100 m), resultingin condensation and precipitation, with anassociated release of latent heat. The massof cloud enveloping the mountain is calledthe FÖHN WALL. This airflow is thenwarmed through compression at the dryadiabatic lapse rate (1.0°C/100 m) on theLEEWARD side of the range. Temperaturesat a given altitude are thus higher on theleeward side of the range when comparedto their WINDWARD-side equivalent. Thissame föhn effect is given different names inother parts of the world: BERG WIND (SouthAfrica); CHINOOK (Rocky Mountains,North America); NOR’WESTER (e.g. NewZealand); SAMOON (Iran); SANTA ANA (Cal-ifornia, US); and ZONDA (Argentina).

forced convection CONVECTION that isa product of mechanical turbulence, whichoccurs due to the development of eddies asan airflow crosses an uneven surface. Thisis distinct from free (thermal) convectionoriginating from the heating of the surface.

forcing (forcing mechanism) A mecha-nism or process that affects the balance ofa system and forces a response or feedback.For example, a forcing mechanism, such asa volcanic eruption or anthropogenic-in-duced enhanced GREENHOUSE EFFECTS, mayalter the energy balance between the

fog bank

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Earth’s incoming solar radiation and out-going infrared radiation.

forecast A statement (in verbal or mapform) of the anticipated state of theweather at a particular place or over a re-gion, up to the global scale, over a specificperiod. The word was introduced byRobert FitzRoy (1805–65), first director ofthe Meteorological Department of theBoard of Trade in the UK. Forecasts are de-fined according to the periods they cover.See long-range weather forecasting;medium-range weather forecasting; now-casting; short-range weather forecasting.

forest fire See fire.

forked lightning Visible LIGHTNING inwhich individual branches of lightingchannels can be seen, often as the lightningpasses from a cloud to ground (see cloud-to-ground discharge). The forked tortuouspath of a lightning discharge may also beevident in inter-cloud events (see cloud-to-cloud discharge).

Fortin barometer /for-tin/ An instru-ment used for measuring atmospheric pres-sure; the most common type of MERCURY

BAROMETER. It consists of a long glass tube(sealed at one end) that is evacuated, filledwith mercury, and inverted with the openend in a reservoir or cistern of mercury.The height of the mercury is a measure ofthe atmospheric pressure acting upon it.The scale within the cistern has to be resetto zero before use of the barometer.

fossil fuel A fuel derived from organicmatter that has been deposited over longperiods of geologic time; it includes coal,oil, natural gas, oil shales, and tar sands.All fossil fuels on combustion produce car-bon dioxide (CO2), which inevitably con-tributes to the enhanced GREENHOUSE

EFFECT.

fractocumulus cloud /frak-toh-kyoom-yŭ-lŭs/ A cloud type or species that ap-pears as ragged irregular shreds of cloud ata low level. They are shallow CUMULUS

clouds that do not have a smooth appear-

ance and occur when the cloud is beginningto evaporate and dissipate into the airaround it, or when strong winds are frag-menting it.

fractostratus cloud /frak-toh-stray-tŭs/ A cloud type or species that consists ofragged fragments of low gray or dark-graycloud that forms in strong winds belowNIMBOSTRATUS rain clouds, when it is alsoknown as SCUD. It may be located close tothe ground surface. It can also form withthe break up of a sheet of STRATOCUMULUS.

fractus /frak-tŭs/ A cloud type or speciesconsisting of ragged irregularly shapedpatches or shreds of CUMULUS or STRATUS.See fractocumulus cloud; fractostratuscloud. See also cloud classification. [Latin:‘broken’]

Framework Convention on ClimateChange (FCCC) See United NationsFramework Convention on ClimateChange.

frazil ice /fră-zil/ Tiny platelets or nee-dles of ice that form in rapidly flowing orturbulent rivers and lakes in which the tem-perature is below freezing point. Frazil par-ticles adhere to each other in thesupercooled water, rising to form spongyaccumulations or floc on the surface. Themovement of water prevents the ice crys-tals from forming a continuous sheet.Frazil ice is common in Canadian riverswhere the term was first applied.

free atmosphere The part of the atmos-phere lying above the PLANETARY BOUND-ARY LAYER in which the influence of surfacefriction and heat fluxes are negligible.

freezing and thawing indices Indicesused to measure the severity of climate.They are generally defined as the cumula-tive number of degree-days during whichair temperatures are below and above 0°C(32°F). The total annual freezing index isused, for example, to predict permafrost(perennially frozen ground) distribution, toproject maximum depth of frost penetra-tion, and to estimate thicknesses of ice on

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sea, lakes, and rivers. The total annualthawing index is also used in the predictionof permafrost, in addition to estimating thedepth of thaw of frozen ground.

freezing drizzle Precipitation compris-ing supercooled water droplets with diam-eters of less than 0.5 mm (0.02 in) thatdescends through relatively warm air in thelower layers of the atmosphere in which atemperature inversion is established but,on impact with the ground surface or ob-jects with a temperature at or below 0°C(32°F), freezes to form GLAZE. Comparefreezing rain.

freezing fog Fog in which the mass ofminute water droplets are supercooled (i.e.the water remains liquid below freezingpoint). When the fog comes into contactwith exposed surfaces at or below freezingpoint the droplets freeze to form RIME orGLAZE.

freezing level The height above meansea level at which the air temperature is0°C (32°F), or the pressure altitude abovea specific location of the 0°C (32°F)isotherm. This is not necessarily the level atwhich liquid water droplets becomes solidice; as a result of SUPERCOOLING liquidwater may remain unfrozen to tempera-tures as low as –40°C (–40°F).

freezing nucleus A tiny atmosphericparticle on which, at temperatures below0°C (32°F), the freezing of supercooledwater droplets occurs generating an accu-mulation of ice. The nuclei, which have asimilar crystalline structure to ice, may be,for example, very fine wind-blown soilparticles, volcanic dust, certain bacteria, oractual ice crystals. The temperature atwhich freezing occurs is variable depend-ing on the nuclei but is typically between–25°C (–13°F) and –35°C (–31°F); withoutthe presence of freezing nuclei the super-cooled water droplets in a cloud would notfreeze above –40°C (–40°F). The presenceof freezing nuclei is important if the BERG-ERON–FINDEISEN PROCESS of precipitationgeneration is to operate. See also cloudcondensation nucleus.

freezing point The temperature atwhich a liquid changes phase to become asolid. For pure water at standard atmos-pheric pressure this is at 0°C (32°F) or273.15 K. However, in reality a liquid maynot freeze at this temperature if the pres-sure varies from the standard atmosphericpressure, if impurities are present, orthrough SUPERCOOLING.

freezing rain Precipitation comprisingsupercooled water droplets with diametersin excess of 0.5 mm (0.02 in) that descendsthrough relatively warm air in the lowerlayers of the atmosphere where a tempera-ture inversion is established but, on impactwith the ground surface or exposed objectswhere the temperature is at or below 0°C(32°F), freezes to form GLAZE. Comparefreezing drizzle.

fret A regional name in the UK for anADVECTION FOG that drifts inland from theNorth Sea across northeast England. It oc-curs with the cooling of a moist inshoreflow and is associated with particularlymiserable weather with suppressed max-ima and often persistent drizzle. See alsohaar.

friagem A period of cold weather in theupper parts of the Amazon basin in Braziland eastern Bolivia during the southernhemisphere winter. It occurs when coldsouth polar air surges northward, east ofthe Andes and into the Amazon region,sometimes as far as the equator. In an ex-ceptionally cold friagem temperatures canfall to 11°C (52°F) and bring considerabledamage to Brazil’s coffee crops, such as oc-curred during June to July, 1994.

friction The mechanical force of resis-tance associated with the relative motionof the boundaries of two adjacent sub-stances (solids or fluids). In the atmos-phere, friction is important in the flow ofair across the Earth’s surface and has theeffect of decreasing the wind speed relativeto that ideally associated with a given pres-sure gradient (GEOSTROPHIC WIND). It alsoleads to the deflection of the air across iso-bars so that there is frictional inflow into a

freezing drizzle

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low and frictional outflow from a highwithin the PLANETARY BOUNDARY LAYER

(friction layer). This layer is typically about1 km (0.6 mile) deep, but can vary consid-erably in time and space. The influence offriction is partly related to the EKMAN SPI-RAL.

friction layer See planetary boundinglayer.

frigid zone (polar zone) One of thethree historic climatic zones, based on tem-perature and sunshine characteristics, intowhich the world was divided by the classi-cal Greeks; it consisted of the high-latitudeareas bounded by the Arctic or AntarcticCircle. Temperatures in the zone are coldall year round as a result of extended peri-ods of winter darkness and the low sum-mer Sun. See temperate zone; torrid zone.See also climate classification.

front The moving leading edge of an ex-tensive region of warm or cold air in theextratropics, associated with other weatherchanges, such as barometric pressure, hu-midity, and wind speed and direction.Fronts are shallow sloping zones of transi-tion that separate two different AIR MASSES

– MARITIME POLAR from MARITIME TROPI-CAL, for example. They are also zones ofdeep CLOUD and often widespread PRECIPI-TATION.

Fronts were defined originally soonafter World War I by workers led by Vil-helm Bjerknes (1862–1951) in Norway atthe BERGEN SCHOOL. The word ‘front’ wasused as an analogy to the front line in therecent war. See ana-front; cold front; kata-front; occluded front; stationary front;warm front. See also Antarctic front; Arc-tic front; Atlantic polar front; polar front.

frontal cyclone (frontal depression) Alow-pressure system or CYCLONE in extrat-ropical regions that has associated warm,cold, and occluded fronts. Frontal cyclonesare characterized by cyclonic inflow, ex-tensive layer cloud, and widespread precip-itation.

frontal fog Fog formed in association

with frontal precipitation that falls fromthe warmer air above the FRONT into thecooler unsaturated air below it. When thishappens very near to the surface, the chill-ing of the cooler air by the evaporation ofraindrops falling through it can lead to sat-uration and, sometimes, fog. It occurs mostcommonly in the shallow layer of cold airjust ahead of the WARM FRONT in whichwarm air lifts over the cold front.

frontal inversion A layer within theTROPOSPHERE within which both DRY-BULB

TEMPERATURE and DEW-POINT TEMPERATURE

increase with height. These increases areassociated with a change, moving upward,from cooler drier air in the polar air belowa FRONT to the relatively warmer andmoister tropical air above it. It is a stablelayer that can act as a lid to convectivecloud. The inversion is usually some fewhundreds of meters deep.

frontal lifting The forced uplifting of

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dry-bulb anddew-pointtemperature

warmer,moister

frontal inversionlayer, increase indry-bulb anddew-pointtemperature

dew-point dry bulb

colder, drier air

warmer,moisterair

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colder,drier

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warmer less dense air when two air massesof differing temperatures converge along afront. Along a cold front, the advancingcold air wedges under the warmer air forc-ing it up; along a warm front, the advanc-ing warm air rides up over the coolerdenser air.

frontal precipitation (frontal rainfall, cy-clonic precipitation) PRECIPITATION thatis generated as a result of large-scale upliftand cooling of warm moist air along thefrontal zones of a low-pressure system,usually in mid- and high-latitude regions.Its characteristics are dependent on thetype of front: rainfall from a WARM FRONT

is typically more prolonged but rather driz-zly; heavier bursts of rain are usually asso-ciated with a COLD FRONT.

frontal system See frontal cyclone.

frontal wave An undulation on a FRONT

that travels along the POLAR FRONT as awave-like perturbation, rather like a waverunning along a length of string when‘flicked’ at one end. The young frontalwave consists of a WARM FRONT and COLD

FRONT but gradually starts to occlude (OC-CLUDED FRONT) as time progresses.

frontal zone The zone of transition be-tween two adjacent AIR MASSES. See front.

frontogenesis /frun-tŏ-jen-e-sis/ Theprocess by which a FRONT is formed, oftenby the juxtaposition of two different AIR

MASSES that flow toward one another insuch a way that the horizontal temperaturegradient is strengthened. Compare frontol-ysis.

frontolysis /frun-tol-ă-sis/ The processby which a FRONT is suppressed or dissi-pates and disappears, usually in associa-tion with marked DIVERGENCE andSUBSIDENCE. Compare frontogenesis.

frost 1. A state in which the air temper-ature at Stevenson screen level (1.25 m; 4ft) falls to at or below the freezing point ofwater, i.e. 0°C (32°F).

2. Deposits of ice crystals that form on ex-posed surfaces and objects, such as grassand trees, as a result of direct sublimationof water vapor from the air when the frost-point temperature is reached. See also airfrost; glaze; ground frost; hoar frost; rime.

frost heaving (frost heave) Movementand swelling in the ground surface that oc-curs when freezing of water within the soiltakes place. The processes of frost heavingand thawing are responsible for the forma-tion of a number of the features ofperiglacial environments such as hum-mocks and patterned ground. Frost heav-ing can also displace structures such asbuildings, pipelines, and roads.

frost hollow See frost pocket.

frost pocket (frost hollow) A generallylow-lying location in which frosts, or moresevere frosts, tend to occur more frequentlythan in the surrounding areas. These loca-tions are commonly associated with a hol-low or narrow valley in which cold airaccumulates at night due to katabaticflows of cold air down the slopes and is ei-ther trapped or flows out slowly. See alsokatabatic wind.

frost-point (frost-point temperature)The temperature below 0°C (32°F) atwhich moisture in the air condenses as alayer of frost on any exposed surface. It isthe temperature to which a sample of air(containing water vapor) must be cooled,at constant pressure and humidity, toachieve saturation with respect to an icesurface. It is measured with a frost-pointHYGROMETER. See also dew-point.

frost-point temperature See frost-point.

frost smoke See Arctic sea smoke.

frozen ground See permafrost.

F scale See Fujita Tornado IntensityScale.

frontal precipitation

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Fujita–Pearson Scale /foo-jee-tă/ SeeFujita Tornado Intensity Scale.

Fujita Tornado Intensity Scale (F scale,Fujita–Pearson Scale) A reference scalerunning from F0 to F5 to classify thestrength of tornadoes, based on maximumwind speed and damage caused. It was de-vised in 1971 by the Japanese–Americanmeteorologist Tetsuya Fujita (1920–98)and Allen Pearson, based on wind speedsmeasured in miles per hour.

The wind speed within a tornado ishighly variable and difficult to measure,consequently a tornado’s intensity oftenhas to be inferred from the damage itcauses. See Table.

full-physics numerical model A three-dimensional computer model used in mete-orology that simulates the physicalcharacteristics of the meteorological fields,especially mesoscale motion present withinthe BOUNDARY LAYER. The model is anadaptation of the GENERAL CIRCULATION

MODEL (GCM) and is used in conjunctionwith air-quality models.

fumigation In pollution studies, the

emission of pollutants from a chimneystack under turbulent atmospheric condi-tions dominated by gusty winds. Relativelyhigh concentrations of pollutants may beperiodically brought down to the surface,interspersed by periods of low concentra-tion.

funnel cloud The visible rotating, gen-erally long and thin, tube-like cloud associ-ated with tornadic circulation that extendsdown from the cloud base but does notcome into contact with the ground surface.Funnel clouds can occur in different shapesand sizes, including a twisted contortedrope-like shape, a vertical wide, symmetricfeature, and like an elephant’s trunk hang-ing from the base of a towering CUMU-LONIMBUS cloud. The condensation ofwater droplets in these clouds is linked tothe very low pressures within their circula-tion. See also tornado.

funneling The process by which an air-flow is constrained by the presence of a val-ley, leading to convergence, higher windspeeds, and uplift. A similar effect can beseen in the air between an advancing frontand a mountain barrier.

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strength wind speed damage experiencedmph km h–1

F0 40–72 65–116 light damageF1 73–112 117–180 moderateF2 113–157 181–255 severeF3 158–206 256–334 considerableF4 207–260 335–420 devastatingF5 261–318 421–515 incredible

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Gaia hypothesis /gÿ-ă/ A theory, de-rived from a concept proposed by theBritish scientist James Ephraim Lovelock(1919– ) and the American biologistLynn Margulis (1938– ) in the 1970s,that the entire Earth is a self-regulatingecosystem with feedback mechanisms be-tween the abiotic (nonliving) and biotic(living) components. Thus the climate andbiogeochemical cycles are regulated by theliving organisms on Earth. The theory wasnamed for the Earth goddess of ancientGreece.

gale 1. A strong wind, the velocity ofwhich when recorded at a height of 10 m(33 ft) and averaged over a 10-minute pe-riod, is classified on the BEAUFORT SCALE asforce 7 (28 knots; near gale) or more. TheWorld Meteorological Organization de-fines the various categories of a gale as:near gale (28–33 knots), gale (34–40knots), strong gale (41–47 knots), andstorm (48–55 knots).2. A commonly used name for any strongwind.

gap wind (wind gap) A wind that blowsalong a valley or col in a ridge. In the US,for instance, a gap wind is experienced inthe town of Harrisburg, Pennsylvania.

GARP See Global Atmospheric Re-search Program.

garua The MIST or DRIZZLE that often en-velops the Peruvian Costa like a thick blan-ket of cloud. Given that extreme ariditycharacterizes this coastline, the garua pro-vides nearly all of the region’s rainfall,which in many locations (e.g. Peru’s capitalLima) is frequently less than 25 mm (10 in)per annum.

Gaussian plume model A continuousdispersion computer model of a pollutionplume from a chimney stack under nearneutral atmospheric conditions in whichpollutants are dispersed equally both verti-cally and horizontally. Cross-sectionsshow the typical ‘bell-shaped’ characteris-tics of a Gaussian or normal distribution.See also Gaussian puff model.

Gaussian puff model A computermodel that seeks to simulate pollution dis-persal from a stack under near neutral at-mospheric conditions in which DRY

DEPOSITION occurs at the surface; this is incontrast to the GAUSSIAN PLUME MODEL. Thenumber of ‘puffs’ impacting downstreammay be described by a Gaussian distribu-tion under CONING conditions.

GAW See Global Atmosphere Watch.

GCM See general circulation model.

GDPFS See Global Data-Processing andForecasting System.

gegenschein /gay-gen-shÿne/ (counter-glow) An optical phenomenon appearingas a faint glow in a dark night sky on theecliptic in the opposite direction to the po-sition of the Sun. It occurs as a result of thebackscattering of sunlight off small mete-oric dust particles. [German: ‘counter-glow’]

genera /jen-ĕ-ră/ See cloud classification.

general circulation (global circulation)The very large-scale averaged atmosphericcirculation pattern that results from thethermal and pressure gradients that occuron the same large scale. These are forced by

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the inequalities of incoming solar and out-going terrestrial radiation between theEquator and poles. The general circulationevolves significantly over the seasons sothat it is more intense in the middle andhigher latitudes during the winter than thesummer, for example, in response to thestronger imposed tropics-to-pole thermalcontrast in the cold season. One criticalrole of the general circulation is to mix airfrom the lower and higher latitudes inorder to offset the radiation imbalance.

The THREE-CELL MODEL characterizesthe principal features of the general circu-lation in the vertical plane. There are sig-nificant seasonal shifts of the cellularpattern, particularly of the HADLEY CELL,which migrates toward higher latitudes inthe summer in unison with the middle-lati-tude FERREL CELL, and toward lower lati-tudes in the winter. The middle latitudesare characterized by traveling FRONTAL CY-CLONES (frontal depressions) and ANTICY-CLONES (highs), notably across the oceans,while the wintertime continents are typi-fied by extensive areas of high pressure.The global circulation also involves thesubcontinental scale changes that are asso-ciated with the seasonal evolution of the

MONSOON. The best known occurs oversouthern Asia (see Asian monsoon), al-though a separate significant example alsoaffects extensive areas of West Africa (seeAfrican monsoon). In the middle andupper TROPOSPHERE, the large-scale meancirculation features include the LONG

WAVES in middle latitudes, the POLAR

FRONT, the SUBTROPICAL JET STREAM, andthe TROPICAL EASTERLY JET STREAMS, the lastof which is seasonal. The long waves ex-hibit an INDEX CYCLE that is related to fluc-tuations in the pattern of weather at thesurface in middle latitudes in particular –these mark changes of the general circula-tion over periods of several weeks. Theglobal oceans play a major role in the gen-eral circulation; the warm currents trans-port warm water to higher latitudes toameliorate sea temperatures within thetropics. At the same time, this warm wateris imported into higher latitudes to elevateotherwise cooler sea temperatures. Coldcurrents export cooler water into lower lat-itudes and suppress temperatures therebelow a higher possibility.

general circulation model (GCM) Aglobal computer MODEL that represents the

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horselatitudesHadley

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polar cellpolar high

doldrums

equatoriallows

L

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Earth’s climate in three dimensions (fourincluding time), with the most sophisti-cated models simulating physical, thermo-dynamic, and dynamic activity. Mostgeneral circulation models use equations todescribe processes over land, ocean, seaice, and atmosphere while coupledocean–atmosphere models resolve thefluxes between the ocean and atmosphere.General circulation models can be used toanalyze potential effects of climate changeusing changes in greenhouse gases or otherboundary conditions, or they may be usedto simulate past climatic conditions for pa-leoclimatology studies. See also Atmos-pheric Model Intercomparison Project;climate model.

generator cell The shallow patch thatproduces or generates ice crystals in thetropospheric region in which CIRRUS cloudform. The cells are the sources for suchcrystals that fall through a layer that expe-riences WIND SHEAR so that they tend to bestretched out as they fall into a zone inwhich the wind speed increases. The gener-ator cells are the upper fibrous ends to thehooks of cirrus that are part of the elon-gated filaments of the same cloud.

genitus /jen-ă-tŭs/ A word added to acloud type used in CLOUD CLASSIFICATION tosignify that a new kind of cloud genus hasbeen formed from the original cloud genus.For example, stratocumulus cumulogeni-tus is STRATOCUMULUS formed by thespreading of CUMULUS.

geographical information system (GIS)A computer system that includes the hard-ware, software, and data for capturing,storing, updating, manipulating, analyz-ing, and displaying geographical data.Within climatology and meteorology GIShas been used, for example, to producemaps on a global scale showing the inci-dence of natural hazards, includingtsunamis and storm surges, tropical stormsand cyclones, extratropical storms, andwinter storms.

geophysics The science that studies thephysical properties and processes of the

Earth. It encompasses a wide variety offields, including geomagnetism, tempera-ture distributions of the Earth’s interior,geology, seismology, meteorology, andphenomena of the outer atmosphere.

geopotential height The altitude of agiven point or layer in the atmosphere inrelation to the POTENTIAL ENERGY of a unitmass at that altitude above sea level.

Geostationary Meteorological Satel-lites (GMS) A series of satellites oper-ated by the Japanese MeteorologicalAgency. The last GMS (GMS - 5) waslaunched in 1995. It is one of the opera-tional meteorological GEOSTATIONARY

SATELLITES (currently five or six) positionedin orbit around the Earth, and is locatedabove the Equator at 140°E. Observationsfrom the satellites have been used in theWORLD WEATHER WATCH program.

Geostationary Operational Eviron-mental Satellite See GOES.

geostationary orbit A satellite orbit inwhich the satellite keeps pace with theEarth’s rotation about its polar axis. Thisoccurs in the Equatorial plane; the orbitmust be eastbound, in the same direction asthe Earth’s spin, and have a period that isthe same as the Earth’s, i.e. one day. Inorder to achieve this, the satellite has toorbit at the geostationary elevation ofabout 35,800 km (22,230 miles) above thesurface. It is moving very rapidly in space,but just keeping pace with the Earth so thatit appears to hover above the same SUB-SATELLITE POINT. See also geostationarysatellite.

geostationary satellite A satellite posi-tioned at about 35,800 km (22,230 miles)above the Earth’s surface in a GEOSTATION-ARY ORBIT. From this high altitude, geosta-tionary satellites can view the whole Earthdisk and make frequent scans (about every30–60 minutes). The spatial resolution ofimages from the geostationary satellites isabout 2.5 km (1.5 miles) per pixel. Themain meteorological geostationary satel-lites are positioned around the Equator to

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give full coverage of the Earth. Usually fiveor six geostationary satellites are in opera-tion at any one time and these include thetwo GOES-series satellites, operated by theUS NATIONAL OCEANIC AND ATMOSPHERIC

ADMINISTRATION (GOES-EAST over the USand South America at 75°W, and GOES-WEST over the Pacific at 135°W); ME-TEOSAT operated by EUMETSAT (overEurope/Africa at 0°); GMS operated by theJapanese Space Agency (over Japan/Aus-tralia at 140°E), INSAT, operated by theIndian space agency (over the IndianOcean), and GOMS operated by Russia(76°E). China also has a geostationarysatellite, Fengyun–2 (over China and theIndian Ocean).

geostrophic balance /jee-ŏ-stroff-ik/ SeeCoriolis effect.

geostrophic current In the oceans, alateral rotary current that results from thebalance between the horizontal pressuregradient force and the Coriolis force (seeCoriolis effect). The current that resultsflows parallel to the pressure gradient. Themajor surface currents, for example, theGulf Stream and West Wind Drift, are ap-proximate geostrophic currents.

geostrophic wind (geostrophic flow) Atheoretical steady non-accelerating windthat blows parallel to the isobars and re-sults from the balance between the realhorizontal PRESSURE GRADIENT FORCE andthe fictitious but important CORIOLIS EF-FECT. It is valid strictly for frictionlessstraight parallel flow. A geostrophic windscale is sometimes printed on weathermaps to aid the quick estimation of thewind speed from a mean-sea-level pressureanalysis. The geostrophic wind (Vg) is de-fined as:

Vg = (2ωsinθ)–1.∂p/∂n,where ω = angular velocity of the Earth(7.27 x 10–5 radians per second), θ is thelatitude, and ∂p/∂n is the horizontal pres-sure gradient in Pa/m (1 mb = 100 Pa).

geosynchronous orbit /jee-oh-sink-rŏ-nŭs/ An orbit in which a satellite orbits

the Earth once in 24 hours and follows thesame path.

GEWEX See Global Energy and WaterCycle Experiment.

ghibli /gib-lee/ (gibli) A hot and drySCIROCCO-type wind that blows from southto north across the North African countryof Libya. It is often experienced in thecountry’s capital, Tripoli.

GIS See geographical information sys-tem.

GISP 2 See Greenland Ice Sheet Project2.

glacial period See ice age.

glaciation 1. In meteorology, the form-ing of ice crystals from supercooled waterdroplets. When this occurs in the upperlayer of a cumulonimbus cloud, the cloudis said to have a glaciated upper layer.2. In climatology, the covering of land byglacial ice. See also ice age.3. The process of glacier growth and mod-ification of the land surface by glaciers.

glacier wind (glacier breeze, firn wind) A cold KATABATIC-type wind that blowsdown a valley glacier in the daytime, par-ticularly during the summer. Velocities aregreatest within 2 m (6 ft) of the surface.The glacier wind is a result of the coolingof air in contact with the ice; the higher airdensity over the glacier causing this air tosink. It is also observed on a much largerscale as gravity winds descending from thehigh-elevation ice sheets of Antarctica andGreenland.

glaze (glazed frost, clear ice) A coatingof clear smooth ice, sometimes of consider-able thickness, that forms as a result of rainor drizzle hitting a ground surface wherethe temperature, and that of the lower at-mosphere, is below 0°C (32°F), the freez-ing-point of water. In North America thisphenomenon is also referred to as silverthaw, so called because of the period ofrapid warming that often follows a severe

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frost. The term BLACK ICE is used for glazeon a road or sidewalk. A period of heavyrainfall that gives rise to a deposit of glazeis frequently referred to as an ICE STORM inNorth America. See also freezing drizzle;freezing fog; freezing rain.

glazed frost See glaze.

Global Atmosphere Watch (GAW) AWORLD METEOROLOGICAL ORGANIZATION

(WMO) program that uses over 300 obser-vation stations worldwide to provide data,assessments, and other information on theatmosphere’s chemical composition andphysical characteristics. The program wasestablished in 1989 and provides data ongreenhouse gases, ozone, radiation, opticaldepth, precipitation chemistry, chemicaland physical properties of aerosols, andother meteorological parameters.

Global Atmospheric Research Pro-gram (GARP) Formerly, a very largecomprehensive program of research, spon-sored by the World Meteorological Orga-nization (WMO) that aimed to stimulatean improved understanding of mainlylarge-scale features of the atmospheric cir-culation. In the summer of 1974 it con-ducted the GARP Atlantic TropicalExperiment (GATE), an international ex-periment to further understanding of thetropical atmosphere and its role in the GEN-ERAL CIRCULATION of the atmosphere. Theresearch covered a vast area from tropicalAtlantic Africa to South America.

global circulation See general circula-tion.

Global Data-Processing and Forecast-ing System (GDPFS) A system operatedby WORLD WEATHER WATCH (WWW) toprocess meteorological data obtained fromthe GLOBAL OBSERVATION SYSTEM (GOS),and to make such data available, throughthe GLOBAL TELECOMMUNICATIONS SYSTEM

(GTS), to the member countries of theWORLD METEOROLOGICAL ORGANIZATION

(WMO). The GDPFS provides meteorolog-ical analyses and a range of forecast prod-ucts from nowcasts, through short-,

medium-, long-, and extended range fore-casts, to climate forecasts. The three-tierstructure of the system includes global (seeWorld Meteorological Center), regional,and national meteorological centers.

Global Energy and Water Cycle Ex-periment (GEWEX) A program that ob-serves and models the hydrologic cycle andenergy fluxes in the atmosphere, land sur-face, and upper oceans. It was establishedin 1988 by the WORLD CLIMATE RESEARCH

PROGRAM (WCRP) and is primarily con-cerned with the effects of climate change.

global energy balance The annual en-ergy balance of the Earth–atmosphere sys-tem; the result of all the linkages betweenthe constituent energy components (radia-tion, sensible heat, latent heat, etc.) at allscales, including all feedback mechanisms.The annual energy inputs and outputsmust approximate to zero over time if thereis to be no net heating or cooling in the sys-tem. Averaged over a year for the whole ofthe Earth, the incoming and outgoing radi-ation is in balance. Averaged over latitude,incoming radiation is at a surplus betweenthe latitudes of 40°N and 40°S (i.e. moreenergy is stored than re-radiated), whilebetween the 40° latitudes and the polesthere is a deficit, in which outgoing terres-trial radiation is larger than the incomingsolar radiation. The redistribution of thisenergy through the atmospheric andoceanic circulation systems from low lati-tudes to high latitudes maintains the globalenergy balance. See also general circula-tion.

Global Observing System (GOS) Asystem operated by WORLD WEATHER

WATCH (WWW) to provide high-qualityand standardized meteorological observa-tions from the facilities of the membercountries of the WORLD METEOROLOGICAL

ORGANIZATION (WMO). Observations areprovided, for example, by geostationaryand polar-orbiting stations (manned andautomatic) on land and at sea, from air-craft, and from geostationary and polar-orbiting satellites, See also GlobalData-Processing and Forecasting System;

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Global Telecommunications System;World Meteorological Center.

Global Ozone Monitoring by Occulta-tion of Stars (GOMOS) An instrumentfor accurately monitoring and mapping thevertical distribution of ozone and othertrace gases (including nitrogen dioxide(NO2), nitrogen trioxide (NO3), and watervapor) in the stratosphere, between thetropopause and 100 km (60 miles). The in-strument uses measurements of the absorp-tion by the gases of starlight that travelsthrough the atmosphere to calculate thedata. It is carried on board the polar-orbit-ing satellite ENVISAT, which was launchedin 2002.

Global Ozone Monitoring Experiment(GOME) An instrument carried on theEUROPEAN REMOTE SENSING SATELLITE,ERS–2 (which was launched in 1995), tomeasure a range of atmospheric trace con-stituents, especially the levels and distribu-tion of ozone globally. It consists of anadir-viewing spectrometer that measuresthe solar radiation scattered in the ultravi-olet and visible spectral range in the Earth’satmosphere, to derive measurements ofsuch constituents as ozone, nitrogen diox-ide, water vapor, bromine oxide, and othertrace gases.

global positioning system (GPS) Asatellite-based coordinate positioning tooland navigation system that can rapidly andaccurately determine the latitude, longi-tude, and altitude of a point on or abovethe Earth’s surface. It is based on a constel-lation of 24 satellites orbiting the Earth ata very high altitude and uses a form of tri-angulation based on the known positionsand distances of three satellites relative tothe surface of the Earth. First developed bythe US Department of Defense to providethe military with a state-of-the-art posi-tioning system, GPS receivers are nowsmall enough and economical enough to beused by the general public. In meteorologyand climatology, GPS receivers are increas-ingly used, for example, in RADIOSONDES,and have experimentally been used in the

measurement of integrated (total column)precipitable water vapor.

Global Sea-Level Observing System(GLOSS) An international program, co-ordinated by the Joint Technical Commis-sion for Oceanography and MarineMeteorology (JCOMM) of the World Me-teorological Organization (WMO) and theIntergovernmental Oceanographic Com-mission (IOC), to establish sea-level net-works to provide high-quality data toclimate, oceanographic, and coastal sea-level research. A network of nearly 300sea-level stations has been establishedaround the world for long-term climatechange and sea-level monitoring.

Global Telecommunications System(GTS) The international system for thedissemination of meteorological informa-tion, which is part of the WORLD WEATHER

WATCH (WWW) of the WORLD METEORO-LOGICAL ORGANIZATION (WMO). The sys-tem links together the three WORLD

METEOROLOGICAL CENTERS (Melbourne,Moscow, and Washington), 15 RegionalTelecommunications Hubs, and numerousnational meteorological centers in a net-work of integrated meteorologicaltelecommunications centers. The use ofsatellites is central to the system of datacollection and data distribution at global,regional, and national level. See alsoGlobal Data-Processing and ForecastingSystem; Global Observing System.

global warming A warming of theEarth’s surface temperature, which is re-sponsible for changes in global climate pat-terns. While the term may be used broadlyto encompass past warming events, such asinterglacials, it is more commonly used torefer to recent global warming due to theenhanced GREENHOUSE EFFECT and anthro-pogenic influences. The quantities of manyGREENHOUSE GASES within the atmosphereare rising, especially of CARBON DIOXIDE.Since 1750 it has been estimated that car-bon dioxide levels in the atmosphere haverisen by 31% as a result primarily ofchanges in land use (e.g. deforestation) andthe burning of fossil fuels (e.g. coal, oil,

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and natural gas). In addition, levels ofmethane, another greenhouse gas, doubledin the 100 years up to the year 2000. Sincethe late 19th century the globally averagedtemperature of air at the Earth’s surfacehas risen by 0.3–0.6°C (0.5–1°F). Analysisof data for the northern hemisphere sug-gests that the increase in temperature forthe 20th century may be the largest of anycentury in the last 1000 years.

Complex computer models have beendeveloped to try to predict the changes toclimate that may occur as a result of in-creased greenhouse gas emissions and an-thropogenic influences. One prediction isthat if greenhouse gas emissions continueon a business-as-usual basis, the estimatedaverage global temperature rise will be be-tween 1.4 and 5.8°C for the period 1990 to2100. Some of the consequences envisagedby such warming include a rise of globalsea levels of about 0.50 m (1.6 ft) duringthe 21st century, boundary shifts in theworld’s vegetation zones, extension of de-sertification, and a reduction in extent ofpolar ice and glaciers.

global warming potential (GWP) Anindex that measures the relative potentialof various gases to contribute to green-house warming, and avoids the necessity todirectly calculate the changes in atmos-pheric concentrations. The reference gas iscarbon dioxide, and the global warmingpotential of other gases is given as a ratioof the radiative forcing that would resultfrom the emission of that gas over a certaintime period. It is calculated as the ratio ofthe radiative forcing that would resultfrom the emission of 1 kilogram of a green-house gas to that from emission of 1 kilo-gram of carbon dioxide over a fixed timeperiod (usually 100 years).

gloom The atmospheric condition whenlight is poor during daytime, such as occurswhen thick cloud or smoke obscures theSun, but horizontal visibility is unaffected.

glory An optical phenomenon consistingof one or more colored concentric rings,which are red on the outer edge and violeton the inner edge; they surround the

shadow of the observer’s head, appearingon cloud, fog, or mist. The rings are seen atthe antisolar point against the white back-ground. The phenomenon is frequentlyseen by pilots around the shadow, whichappears on clouds below, of the aircraft.Glories occur as a result of diffraction andreflection.

GLOSS See Global Sea Level ObservingSystem.

GMS See Geostationary MeteorologicalSatellite.

GMT See Greenwich Mean Time.

GOES (Geostationary Operational Envi-ronmental Satellite) A series of opera-tional meteorological GEOSTATIONARY

SATELLITES positioned in orbit around theEarth and operated by the NATIONAL

OCEANIC AND ATMOSPHERIC ADMINISTRA-TION (NOAA). Two are usually in positionat any one time above the Equator: GOES-EAST at 75°W over the US and SouthAmerica, and GOES-WEST at 135°Wover the eastern Pacific Ocean. The firstsatellite in the series to be operated byNOAA (GOES–1) was launched in 1975.

GOME See Global Ozone MonitoringExperiment.

GOMOS See Global Ozone Monitoringby Occultation of Stars.

GOS See Global Observing System.

GPS See global positioning system.

gradient In meteorology, the rate ofchange of an atmospheric property (e.g.temperature, humidity, or pressure), usu-ally in a horizontal or vertical direction. AFRONT is a zone across which there is a rel-atively steep horizontal gradient of temper-ature and humidity, for example. Incontrast, within an AIR MASS such gradientsare weak. See also pressure gradient.

gradient wind The theoretical windthat, unlike the GEOSTROPHIC WIND, is per-

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tinent to curved motion around a high or alow. It represents the balance betweenthree forces acting upon the movement ofair in a horizontal plane: the PRESSURE GRA-DIENT FORCE, the Coriolis force (see Corio-lis effect), and the CENTRIFUGAL FORCE, thelatter reflected by the curvature of the iso-bars around a pressure system. The gradi-ent wind refers to frictionless motion.

The gradient wind speed (V m s–1) is de-fined as:

V2/r = (2ωsinθ)(Vg –V),where r is the radius of curvature of theair’s path (m) and Vg is the geostrophicwind speed. The V2/r term is positive (neg-ative) when the motion is around a low(high) pressure area. Insertion of the requi-site numbers will lead to a quadratic equa-tion in V.

grass minimum temperature A mea-sure of the lowest temperature recordedovernight from a standard MINIMUM THER-MOMETER, the bulb of which is in contactwith the tips of blades of short grass,25–50 mm (1–2 in) above the ground.

grass minimum thermometer A MINI-MUM THERMOMETER in which the bulb is incontact with short grass, in an open area.In this case the minimum thermometerrecords the night-time minimum grass tem-perature.

graupel /gr’ow-pĕl/ (snow pellet, softhail) Precipitation composed of smallwhite opaque ice grains 2–5 mm (0.08–0.2in) and approximately spherical or conicalin shape, that resemble tiny snowballs.Graupel form by accretion when super-cooled water droplets in the atmospherecome into contact with falling ICE CRYSTALsor SNOWFLAKEs; the droplets freeze on con-tact, covering the crystals or snowflakes.The term graupel is also applied to pelletsof partially melted and refrozen snow, eachwith a soft core. Snow pellets often fall inshowers of snow; unlike SNOW GRAINS, theyoften bounce and sometimes break upwhen they hit the ground. Small hail refersto grains having a similar diameter butglazed with clear ice, which is more likelyto be associated with rain than snow. The

word graupel is derived from German, inwhich it means soft hail. See also hail.

gravity wave A type of atmosphericwave in which the pertinent forces arethose of gravity and buoyancy. They aremanifested as wave motion on the interfacebetween adjacent layers within the atmos-phere, for example, and can be set off bythe air flowing over a developing CUMULUS

cloud – in much the same way as a hill ormountain can stimulate downstream undu-lations in the flow.

gravity wind See katabatic wind.

green flash A glimmer of bright greenlight that occasionally is seen to fleetinglycolor the edge of the Sun as it disappearsfrom view on the horizon at sunset or justas it appears at sunrise. It is most likely tobe seen at locations with a low distant hori-zon, for example, over the ocean. Thegreen flash is caused by differential refrac-tion of light waves. One view is that thegreen flash is illusory as a result of lookingat the Sun.

greenhouse effect The warming effectthat results from short wave (infrared, vis-ible, and ultraviolet) solar radiation beinglargely able to pass unhindered to the sur-face of the Earth, where it is re-radiated aslonger wave (infrared) radiation (as hap-pens in a greenhouse). This outgoing long-wave radiation is partially absorbed byGREENHOUSE GASES (especially water vaporand carbon dioxide) in the atmosphere.The failure of most of the radiant energy toescape means that it is ‘recycled’ and re-tained as heat in the lowest part of the at-mosphere, an important component inmaintaining the Earth’s surface tempera-ture. Without greenhouse warming theEarth’s average surface temperature wouldbe around –18°C (0°F) and unable to sup-port life.

The natural effect of the recycling of ra-diant energy is, however, being enhancedby increases in the concentration of green-house gases particularly carbon dioxide(CO2), owing to human activity, notablysince the beginning of the Industrial Revo-

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lution (around 1700 AD). The burning ofFOSSIL FUELS (oil, coal, and natural gas) andthe clearing of land and burning of vegeta-tion, in particular, contribute to the rise ofcarbon dioxide concentrations. It is pre-dicted that the enhanced greenhouse effectresulting from the increased concentrationsof greenhouse gases will generate increasedGLOBAL WARMING and contribute to globalclimate change. Computer models havebeen used to attempt to predict the amountof warming that may result from furtherincreases in greenhouse gas levels. Onecomplication is the uncertainty as to howglobal warming will affect levels of watervapor, which have so far remained rela-tively constant, in the atmosphere.

greenhouse gas A gas that is mainlytransparent to solar radiation but absorbsstrongly in the infrared part of the electro-magnetic spectrum and re-emits such long-wave radiation in all directions. Theabsorption of this infrared radiation leadsto warming of the air. A number of gasescontribute to the natural GREENHOUSE EF-FECT, the most important of which is watervapor. The other greenhouse gases includeCARBON DIOXIDE (CO2), METHANE (CH4),NITROUS OXIDE (N2O), CHLOROFLUOROCAR-BONS (CFCs), PERFLUOROCARBONS (PFCs),HYDROFLUOROCARBONS (HFCs), andOZONE. Increases in the concentrations ofcarbon dioxide, in particular, andmethane, together with the presence of the

anthropogenic greenhouse gases is con-tributing to an intensification of the green-house effect. Under the KYOTO PROTOCOL,restrictions were to be applied to emissionsof six of the greenhouse gases. See alsoglobal warming.

greenhouse period A period during thegeologic history of Earth during whichthere were no glaciers, and sea levels werehigh. Compare icehouse period.

Greenland Ice Core Project (GRIP) AEuropean research project organizedthrough the European Science Foundationaimed at understanding past climatic con-ditions, based on ice cores obtained fromthe Greenland ice sheet. Its duration wasfrom January 1989 to December 1995 andits main aim was to retrieve and analyze a3000-m (9800-ft) long ice core drilledthrough the Greenland ice sheet at the sum-mit, 28 km (17 miles) east of the GREEN-LAND ICE SHEET PROJECT 2 site.

Greenland ice sheet An ice mass, thelargest in the northern hemisphere, thatcovers some 80% of Greenland. Fromnorth to south it extends for some 2400 km(1490 miles), and is 1094 km (680 miles)across at its maximum width; the averagethickness of the ice is about 1500 m (5000ft). It has been estimated that if all of the icesheet were to melt, sea levels would rise byabout 7 m (23 ft).

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energyproduction

5% NO2

5% O3 15%CFCs 20%

CH4

15%

40%

55%CO2

deforestation

Greenhouse effect: the relative effects of various greenhouse gases taking into account their abundance and effectiveness as absorbers.

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Greenland Ice Sheet Project 2 (GISP2) A US research project, conductedfrom 1989 to 1994, to extract ice coresfrom the Greenland ice sheet for studies ofatmospheric and past climatic conditions.Located 28 km (17 miles) west from theGREENLAND ICE CORE PROJECT site, near theGreenland summit, GISP 2 recovered a3045-m (9990-ft) long core, with the po-tential to yield over 110,000 years ofglobal change history.

Greenwich Mean Time /grin-ij/ (GMT)The timescale based on the apparent mo-tion of the mean Sun with respect to themeridian of 0° longitude at Greenwich,England. The standard times for differentareas of the world are calculated from this;15° longitude representing 1 hour in time.In 1928 the term UNIVERSAL TIME (UT) wasadopted for Greenwich Mean Time whenused for scientific purposes. The timescalehas been replaced by COORDINATED UNIVER-SAL TIME (UTC) to which it is approxi-mately equivalent.

gregale A strong northeast wind thatblows, mainly in winter, across the centraland western Mediterranean, includingMalta. It arises when there is high pressureover central Europe or the Balkans and lowpressure over N Africa, or when a low-pressure system moves south over theMediterranean.

GRIP See Greenland Ice Core Project.

ground clutter RADAR echoes receivedat the radar antenna that have been re-flected from the Earth’s surface or build-ings within some 30 km (20 miles) of theantenna. They are fixed features, and areoften removed automatically from a radardisplay in order not to confuse the opera-tional image of, for example, precipitation.

ground discharge See cloud-to-grounddischarge.

ground fog See radiation fog.

ground frost A condition in which thetemperature of the ground surface mea-

sured at short-grass level is at or below 0°C(32°F). Ground frost is frequently used inweather forecasts to indicate that a frostmay occur at ground level although the airtemperature remains above 0°C (32°F); insuch circumstances, an AIR FROST does notoccur at the same time.

ground heat flux The vertical move-ment of energy across the surface of theEarth. The flux is positive (downward)during the day and negative (Earth to at-mosphere) at night in the absence of warmor cold ADVECTION. It is an important com-ponent (G) of the ENERGY BUDGET.

groundhog day In North Americanweather folklore, the day (February 2) onwhich, if a groundhog emerging from itsburrow sees its shadow, winter will last an-other six weeks; if not there will be an earlyspring. The rationale behind this is that ifthe day is clear and sunny it is probablycold and anticyclonic, conditions that arelikely to persist, whereas no shadow im-plies a cloudy milder day.

ground ice 1. (anchor ice) Minute nee-dles of frazil ice that adhere as spongy ac-cumulations attached or anchored to thebed of a body of moving water, which itselfremains unfrozen.2. A body of clear ice found in the groundsurface of the permafrost zone in whichtemperatures have remained continuouslybelow 0°C (32°F); for more than twoyears.

ground mist A MIST formed by radiativecooling of damp air at night under clearcalm conditions in low-lying areas and val-ley floors.

ground stroke See cloud-to-ground dis-charge.

growing season The period of the yearduring which air temperatures remain suf-ficiently high to permit plant growth. In theUS, it is defined as the period between thelast killing frost in spring and the firstkilling frost in the autumn, i.e. between thelast day in spring with a minimum temper-

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ature of 0°C (32°F) or lower, and the firstday in the autumn with a minimum tem-perature of 0°C (32°F). In the UK a tem-perature of 6°C (43°F) is preferred to 0°C(32°F) due to the different temperatureregime on the eastern seaboard of theNorth Atlantic Ocean. The average lengthof the growing season may be obtainedfrom the curve of annual variation in themean temperature and may be expressed ingrowing degree units.

GTS See Global TelecommunicationsSystem.

Guiana Current (Guyana Current) Asurface ocean current that flows northwestalong the north-eastern coast of SouthAmerica. It is a branch of the AtlanticSouth Equatorial Current. As it flowsalong the coast it is fed by the Amazonriver, which reduces the salinity of thewater.

Guinea Current /g’in-ee/ A warm cur-rent in the Atlantic Ocean, an extension ofthe EQUATORIAL COUNTERCURRENT, thatflows east along the Guinea coast of westAfrica, into the Gulf of Guinea.

Gulf Stream A warm ocean current thatflows in the North Atlantic along the coastof east North America between Cape Hat-teras, North Carolina, and the GrandBanks of Newfoundland, Canada. Thename is frequently extended in commonusage to describe the system of connectedwarm Atlantic Ocean currents, includingthe FLORIDA CURRENT, the Gulf Stream, andthe NORTH ATLANTIC DRIFT. The GulfStream system is fed by the westward-flow-ing North EQUATORIAL CURRENT and flowsnorth from the Gulf of Mexico, throughthe Straits of Florida, and along the eastcoast of the US, where it is approximately80 km (50 miles) wide and has an averagespeed of 3.5 knots (4 mph). It leaves thecoast at around 40°N and becomes ataround 45°W the North Atlantic Drift,which crosses the Atlantic Ocean towardnorthwestern Europe.

gust A rapid but short-lived increase inwind speed experienced, in general, closeto the ground. It is caused primarily by me-chanical disturbance of an airflow andgustiness increases when the surface isaerodynamically rough, for example, asoccurs in towns and cities.

It can also be generated by wind shear(e.g. in CLEAR-AIR TURBULENCE) and byrapid changes in temperature with height.

gust front The leading edge of a suddenand often dramatic increase in wind speed(and direction) linked to, for example, theoutflow from a THUNDERSTORM in the formof a MICROBURST that has reached the sur-face and fanned out sideways. A SQUALL ismore prolonged than the increased windassociated with a gust front. A HABOOB isan example of a sand and dust-laden cur-rent that follows a gust front in desert re-gions.

gustnado A colloquial term in NorthAmerica for a short-lived vortex that de-velops on the gust front of a thunderstorm.It may be visible as rotating dust and debrisnear the ground. Gustnadoes are not con-sidered to be tornadoes, although windspeeds and damage caused may be up to

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SouthEquatorial Current

North Equatorial

Current

Gulf Stream

NorthAtla

nticDrift

CanariesCurrent

Gulf Stream

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the level of F1 on the Fujita Tornado In-tensity Scale.

GWP See global warming potential.

gyre /jÿr/ In oceanography, the large circu-lar flows of water within the ocean basinsthat generally rotate clockwise in thenorthern hemisphere and counterclockwise

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haar /har/ A regional term in Scotlandand northeast England for ADVECTION FOG

that has drifted inland from the North Seaacross southeast and eastern Scotland andnortheast England. It occurs most often inthe spring and early to mid summer, and isgenerally associated with miserableweather. See also fret.

haboob /hă-boob/ A type of dust stormthat occurs in northern Sudan, sendingclouds of sand and dust into the atmos-phere and resulting in poor visibility. It ismost common in the afternoon andevening between May and September, butcan occur at any time of the year. The duststorm is accompanied by a noticeable dropin temperature together with a sudden in-crease in wind speed and change in direc-tion. The haboob is mostly a dry squall,and is thought to result from downdrafts inlarge cumulonimbus clouds.

Hadley cell (tropical cell) A major fea-ture of the GENERAL CIRCULATION of the at-mosphere, confined mainly to the tropics.There are two Hadley cells that occur asTROPOSPHERE-deep features in thenorth–south vertical plane. Their low-levelsignature is the NORTHEAST and SOUTHEAST

TRADE WINDS that are best marked over thetropical oceans; it is these two currents thatconverge into the Intertropical Conver-gence Zone (ITCZ). This elongated zone ofCONVERGENCE is associated with very deepCUMULONIMBUS clouds within which thereis a net upward transport of very warmmoist air. Aloft, above the low-level con-vergence, the air flows away northwardand southward from an east–west elon-gated zone of DIVERGENCE in the upperTROPOSPHERE. The air in the two upper cur-rents gradually cools and sinks to the lower

troposphere at around 30° latitude, on av-erage, in either hemisphere. The deep sub-sidence in this region is associated with theSUBTROPICAL ANTICYCLONES, from whichone low-level current flows Equatorward,as the trades, to complete the cell. Hadleycells migrate seasonally, so are not oftensymmetrically positioned about the Equa-tor. They are furthest poleward during therespective hemisphere’s summer.

A simple thermal cell was originallyproposed in 1735 by George Hadley(1685–1765) to explain the movement oftropical warm air toward the poles andcold polar air toward the Equator, and alsothe origin of the trade winds.

Hadley Centre for Climate Predictionand Research A major national centerin the UK, funded by the Department of theEnvironment, that provides the UK withup-to-date assessments of changes in cli-mate. Opened in 1990, it is part of the MET

OFFICE, the national meteorological serviceof the UK. Its role is to understand theprocesses responsible for, and to simulate,the present-day climate through the devel-opment of state-of-the-art computer mod-els. In addition, it is a major internationalcenter for the simulation of past and futureglobal climate over a century or so, formonitoring actual climatic fluctuations,and to explain such features.

hail A form of PRECIPITATION comprisedof hard pellets of ice (hailstones) that havea concentric structure. A hailstone nor-mally has a diameter of between 5 and 50mm (0.2–2 in) but some have beenrecorded weighing in excess of 1 kg (2.2lb). The largest hailstone recorded in theUS fell in Coffeyville, Kansas, on Septem-ber 3, 1970; it had a diameter of some 14

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cm (5.6 in) and weighed 757 g (27 oz). Hailcauses considerable damage to crops andproperty.

Associated with the rapid ascent ofmoist air, a hailstone usually developsaround an ice nucleus in a cumulonimbuscloud in which strong updrafts and down-drafts are characteristic; it is sometimes as-sociated with thunderstorms. The icenucleus acquires fresh layers of ice by colli-sion and coalescence with supercooledwater droplets as it is borne upward, creat-ing a concentric structure – clear layerswhen the humidity is low and opaque lay-ers when the humidity is higher alternatingwithin the hailstone. Several such layersmay form before the hailstone acquires theterminal velocity to overcome the pro-nounced updrafts in the cloud and fall tothe ground. See also graupel.

hair hygrometer A mechanical HY-GROMETER that measures changes in RELA-TIVE HUMIDITY by the changes in the lengthof a strand of human hair, which expandsin length as relative humidity increases.

halo One of a number of optical phe-nomena that appear as colored or whitishrings and arcs around the Sun or Moonwhen seen through an ice-crystal cloud,typically cirrostratus, or through ice crys-tals suspended within the atmosphere, suchas diamond dust. When colored, the ringsor arcs are red on the innermost edge andviolet or blue on the outside edge (the op-posite to the color sequence in a CORONA).Haloes occur as a result of the REFRACTION

of light rays by the ice crystals. Points onthe halo are an angular distance of 22° (themost common halo) or 46° from the Sun orMoon.

halocarbon /ha-loh-kar-bŏn/ A chemi-cal compound consisting of carbon, possi-bly hydrogen, and any of the halogens, i.e.chlorine, fluorine, bromine, or iodine. Seealso chlorofluorocarbon; halon; hydro-chlorofluorocarbon.

halon /hal-on, hay-lon/ (bromofluorocar-bon) A group of chemical compounds:CHLOROFLUOROCARBONS (CFCs) that have

been compounded with bromine (to beused as fire retardants, for example). Theyare associated with changes in the concen-tration of stratospheric OZONE and manywere phased out under the MONTREAL PRO-TOCOL.

hard frost See black frost.

harmattan /har-mă-tan/ A dry and oftenstrong northeasterly wind that blows fromthe Sahara desert to the Gulf of Guinea, af-fecting countries such as Mali, Senegal,Guinea, and Sierra Leone in West Africa. Itis hot, dusty, and unpleasant when experi-enced inland in the Sahel zone but the dry-ness of the harmattan can bring welcomerelief from the high humidity experiencedalong the coastlands of the Gulf of Guinea.In light of its beneficial health effects, theharmattan is referred to locally as the DOC-TOR. Usually, but not exclusively, affectinglocations north of the Equator, the wind’ssouthern limit in the northern winter is typ-ically 5°N; this limit is 17°N in the north-ern summer.

Hawaiian high See North Pacific high.

haze A suspension of very small dry par-ticulate matter in the atmosphere such aspollutants and/or natural aerosols (dust,salt particles, and smoke, for example),that produces a milky sky. It is used offi-cially as a term if the concentration of par-ticles is sufficient to give the sky an‘opalescent’ look. The particle sizes aresuch (less than 1 µm) that they contributeto dramatic colors at sunrise and sunset bytheir differential scattering of sunlight trav-eling through the hazy air.

HCFC See hydrochlorofluorocarbon.

heat The energy that is transferred fromone body or region to another as a result ofa difference in temperature. It is measuredin joules or calories.

heat balance A component part of theenergy balance. In a limited sense, the heatbalance may provide an estimate of the en-ergy balance of the Earth’s surface by con-

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verting other components into units ofheat. This was a favored method of analy-sis in the mid-twentieth century. See alsoglobal energy balance.

heat burst A rare and short-livedweather phenomenon that sometimes de-velops in the evening or at night in a weak-ening thunderstorm in which there is asudden downdraft of hot dry air. The jumpin temperature can be around 20°F and isassociated with a sharp fall in humidityand gusty wind conditions.

heat capacity (thermal capacity) Theamount of heat required to change the tem-perature of a substance by 1°C. Substanceswith a high heat capacity, such as water,require a large amount of heat to producea small rise in temperature. Thus changesin ocean surface temperatures are slow,compared with those in the atmosphere.The specific heat capacity is the amount ofheat required to raise the temperature ofunit mass of a substance by 1°C, usuallyexpressed in joules per kilogram.

heat index (HI, apparent temperature) An index, devised by the US NationalWeather Service, to provide a warning ofconditions in which a combination of hightemperatures and high relative humidity,as in a heat wave, can be dangerous tohuman health. It gives, in °F, a measure ofwhat the recorded temperature in combi-nation with the relative humidity feels likein reality. The heat index chart can be usedto find the HI; for example, if the air tem-perature is 96°F (35.5°C) and the relativehumidity is 55%, then the HI is 112°F.

heat island A dome of warm, frequentlypolluted, air above an urban area in whichthe temperature is higher than that in thesurrounding rural areas. Urban areas, irre-spective of their size, may be warmer thantheir surroundings since they are centers ofenergy storage (buildings) and energy ac-tivity (such as domestic heating, industrialprocessing, and transportation). A study inAtlanta, during a one-month period in July1996, found that temperatures averaged8–10°F higher in the urban area than in the

surrounding rural areas. This can create lo-calized weather effects, such as convection,producing thunderstorms. The heat islandeffect is most pronounced around sunsetduring spells of calm weather. This occurswhen the greater vegetation surface area inrural locations allows greater radiativecooling to take place.

heat lightning Lightning observed fromsuch a considerable distance away that anyassociated thunder is inaudible. Heat light-ning is frequently associated with the mis-conception that the warmth of a cloudlesssummer evening is, of itself, sufficient tocause the diffuse lightning in the absence ofa thundercloud.

heat low (thermal low) A synoptic-scaleregion of low pressure or CYCLONE formedby strong heating of land masses, for ex-ample, in spring and summer in subtropi-cal regions. The marked warming of thesurface leads to a vertical expansion of theoverlying air, and an associated fall of sur-face pressure. There are strong seasonalheat lows over the western Sahara andsouthwestern US, and also occasional de-velopments over the Iberian peninsula.

heat wave A prolonged period of ab-normally hot and humid weather. Theseevents can pose serious health issues as theyoung and elderly, in particular, are vul-nerable to the impact of excessive heat.

Heaviside–Kennelly layer See E-layer.

hectopascal /hek-tŏ-pass-kăl/ (hPa) Aunit of ATMOSPHERIC PRESSURE that is a mul-tiple (100) of the PASCAL, the SI unit ofpressure. One hectopascal (hPa) is equal to1 MILLIBAR (mb). While the millibar (mb) isthe c.g.s. unit still often used in everydaysituations (e.g. by meteorological observersand in television weather forecasts), it hasbeen replaced by the hectopascal in mostscientific work and in publications, such asscientific journals.

heiligenschein /hÿ-li-gen-shÿn/ An opti-cal phenomenon in which the shadow of anobserver’s head, seen on a rough surface or

heat burst

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vegetation such as grass wet with dew, isenveloped by a bright glow of sparklingwhite light. The glow is centered on the an-tisolar point and is most marked when theSun is low in the sky. It occurs on dew-cov-ered vegetation when the near-sphericaldew drops act as lenses and focus sunlightonto the leaves of grass; some of the light isscattered back through the drop to createthe heiligenschein. The effect over roughsurfaces is produced by a different mecha-nism and is less bright. It occurs when, nearthe antisolar point surrounding theshadow of the observer, the irregular ele-ments of the surface (e.g. gravel or drygrass) obscure their own shadows so ap-pear brighter than the surrounding areaswhere both illuminated surfaces and shad-ows are seen. [German: ‘halo’]

Heinrich event /hÿn-rik/ An event char-acterized by the marked discharge of largenumbers of icebergs from the easternNorth American ice sheet during glacialperiods. They occur at the end of coolingperiods, about every 7000–10,000 years,and are generally indicative of a shift to-ward warmer conditions. Evidence forthese events from marine core samples inthe Atlantic was first observed by the Ger-man scientist Hartmut Heinrich in 1988.

Heinrich layer Sediments occurring inmarine deposits in the North Atlantic thatcontain a high proportion of ice-rafted de-bris to foraminifera deposits, and thus pro-vide evidence of surges in the NorthAmerican ice sheet, and increased calvingof icebergs, pointing to frequent shifts inthe Earth’s climate.

helicity /hee-liss-ă-tee/ In a moving fluid,such as air, the component of spin of a par-ticle on the direction of propagation. It isused in the forecasting of the formation ofMESOCYCLONES.

helm wind A strong, cold, and gustyeast to northeast wind in northern Englandthat blows down the western slopes of theCross Fell Range (maximum elevation 893m (2930 ft)) into the Vale of Eden; thisbeing the valley that separates the Pennine

Mountain Range from the Lake District. Itoccurs most often in late winter and spring,and when it blows, a stationary helm cloud(a BANNER CLOUD) envelops the mountaintop. Downwind in the Vale of Eden andparallel to the helm cloud, a thin roll ofcloud (helm bar) forms as a result of eddiesand turbulence in the lee of the high es-carpment (an example of LENTICULARIS).

heterogeneous nucleation /het-ĕ-rŏ-jee-nee-ŭs/ 1. The formation of a pure waterdroplet in the atmosphere by condensationfrom a supersaturated vapor with the aidof atmospheric aerosols (see cloud conden-sation nucleus).2. The formation of an ice crystal in the at-mosphere from liquid water that collectsand freezes around a foreign particle, forexample dust, clay, or an atmosphericaerosol. It occurs mostly at temperaturesless than –10°C (14°F). See freezing nu-cleus.

Compare homogeneous nucleation.

heterosphere /het-ĕ-rŏ-sfeer/ The regionof the atmosphere found above the TUR-BOPAUSE that contrasts markedly with theunderlying HOMOSPHERE. The structure ofthe heterosphere depends on temperature,which varies in response to solar activity.The lower part is characterized by largenumbers of free oxygen atoms produced bythe photodissociation of diatomic oxygen.The heavier gases by molecular weighttend to settle in the lower levels of the het-erosphere; the higher levels are dominatedby the lighter gases, such as helium and hy-drogen.

HFC See hydrofluorocarbon.

HI See heat index.

high See anticyclone.

high clouds In CLOUD CLASSIFICATION,those clouds that have a base height abovethe surface of roughly 5000 m (16,000 ft)or more (in middle latitudes). This crite-rion varies seasonally (higher in the sum-mer) and latitudinally (higher at lowlatitudes). High clouds are composed pre-

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dominantly of ice crystals and/or super-cooled water droplets and are usually thinand white but reflect the colors of red, or-ange, and yellow at sunrise and sunset.They all have the prefix CIRRO-. See alsocirrocumulus; cirrostratus; cirrus.

high-index circulation A type of at-mospheric circulation that occurs when theair flow on a large scale is principally west-erly, i.e. when it has a large value of theZONAL INDEX.

highland climate The climate of thehighland regions of the world, such as theAndes of South America, Cascades, SierraNevada, and Rocky Mountains of NorthAmerica, the Himalayas, and central Bor-neo and New Guinea. The climates of thehighlands vary considerably but in general,temperature, humidity, and pressure all de-crease with height, and receipt of solar ra-diation is increased by the greatertransparency of the atmosphere, due tolower dust content with height.

high-latitude climate The group of cli-mates of the high-latitude regions of theworld identified in the STRAHLER CLIMATE

CLASSIFICATION: the boreal forest climate,tundra climate, and ice sheet climate,which are dominated by the polar and arc-tic air masses.

high-precipitation storm (HP storm) A SUPERCELL thunderstorm that producesvery heavy precipitation often with pro-longed DOWNBURST events, major FLASH

FLOODS, and damaging hail episodes. Theheaviest precipitation falls from the trailingside of the MESOCYCLONE within the super-cell. Compare low-precipitation storm.

high-resolution picture transmission(HRPT) A method of encoding andtransmitting high-resolution imagery,which is broadcast continuously, from thePOLAR-ORBITING SATELLITES. Compare auto-matic picture transmission.

high-resolution satellite imagery Thebest quality images provided by satellitesensors. For meteorological satellites, the

high-resolution images typically have PIX-ELS of about 1 km square at the SUBSATEL-LITE POINT. There is no great needoperationally to have a resolution betterthan this for mapping cloud and water-vapor features.

high Sun 1. The period of around onehour either side of the maximum elevationof the Sun in the sky in summer in middle-latitude climates.2. In general usage, the high elevation ofthe Sun in low latitudes.

hill fog Fog that occurs in hilly uplandareas when the cloud base of STRATUS cloudis lower than the high ground; the tops ofhills are occasionally shrouded in strati-form cloud for many hours – or sometimesfor longer. Compare upslope fog.

hill wave See lee wave.

hoar frost (white frost) A deposit ofpatterned ice crystals with a feathery andneedle-like appearance that forms on sur-faces near the ground, especially on vegeta-tion, when the temperature of the surface isbelow 0°C (32°F) and the air is saturated(i.e. at the FROST-POINT). Hoar frost is com-posed partly of ice crystals that are de-posited directly from water vapor on to thesurfaces by sublimation. Radiation coolingof the surface on calm and clear nights en-sures that any condensation that occursfreezes when the temperature drops below0°C (32°F). See also frost; dew.

hodograph /hod-ŏ-graf, -grahf/ A dia-gram using polar coordinates upon whichwind vectors are plotted. The center of thechart represents the position of the stationtoward which all winds blow. Once thevectors are plotted, the points are joined.This diagram represents the differences inwind speed and direction between differentlayers in the atmosphere.

homogeneous nucleation /hoh-mŏ-jee-nee-ŭs/ 1. The formation of a pure waterdroplet by condensation from a supersatu-rated vapor without the presence of atmos-pheric aerosols to act as nuclei. Water

high-index circulation

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droplets rarely form in natural clouds byhomogeneous nucleation.2. The formation of an ice crystal from liq-uid water molecules without the presenceof atmospheric aerosols to act as nuclei.Homogeneous nucleation is most effectivein high-level clouds at temperatures below–40°C (–40°F).

Compare heterogeneous nucleation.

homosphere /hoh-mŏ-sfeer/ The well-mixed region in the atmosphere lyingbelow the TURBOPAUSE. The composition ofthis layer, which extends from the Earth’ssurface to around 85 km (50 miles), tendsto be constant with an even mix of gases in-cluding nitrogen, oxygen, and argon(water vapor, and small concentrationgases, such as carbon dioxide and ozone,are exceptions). This region contrastssharply with the overlying HETEROSPHERE.

hook echo A RADAR echo pattern in theform of a distinct small-scale feature re-sembling a hook that frequently appearson radar images in tornadic situations. Thehook shape is related to a narrow band ofprecipitation that is being drawn in to partof a tightly curved vortex in the flow asso-ciated with a SUPERCELL. Such hook echoesare sometimes, but not invariably, indica-tors of the presence of a TORNADO; they arealways associated with severe weather andturbulence.

horizontal visibility See visibility.

horse latitudes The latitudes of the SUB-TROPICAL ANTICYCLONES, at around 30° to40° across the oceans. The name is said tooriginate from the practice of throwinghorses overboard if the trading sailing ves-sels of the time were becalmed for consid-erable periods. This occasionally occurredin the light winds near the centers of thehighs.

hot tower Large cumulonimbus cloudsfound at the INTERTROPICAL CONVERGENCE

ZONE that draw latent and sensible heatprimarily from underlying warm oceansurfaces up into the troposphere. Collec-tively, these hot towers represent the as-cending limb of the HADLEY CELL.

hPa See hectopascal.

HRPT See high-resolution picture trans-mission.

Humboldt Current /hum-bohlt/ SeePeru Current.

humid continental climate The cli-mate of the areas between about 30° and60°N in central and eastern North Americaand Asia, within the POLAR FRONT zone, inwhich there is conflict between the polarand tropical air masses. Summers are

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Hodograph

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warm and winters cold with large temper-ature ranges between the seasons. Precipi-tation is abundant, generally cyclonic inthe winter, with a summer maximum fromconvectional rainfall. Tornadoes occurmost often in the early summer, especiallyin the US. The weather associated with thisclimate is characteristically changeable. Itis denoted by Dfa, Dfb, Dwa, and Dwb inthe KÖPPEN CLIMATE CLASSIFICATION.

humidity The concentration of WATER

VAPOR in the atmosphere, expressed in me-teorology in a variety of ways. The mainoperational method of measuring it is bysensing the WET-BULB TEMPERATURE, nor-mally measured within a weather screen.Comparing this temperature with the si-multaneously measured DRY-BULB TEMPER-ATURE is the basis for calculating the air’sRELATIVE HUMIDITY or the nearness to satu-ration of the air. A value of 100 means theair is saturated, while one of 30 to 40 is typ-ical for the air in a house. Meteorologistsfind an absolute measure of the amount ofwater vapor very useful. Its concentrationin grams per kilogram, for example, is es-sential for calculations relating to howmuch precipitation may occur. See also ab-solute humidity; specific humidity.

humidity mixing ratio see mixingratio.

humid subtropical climate The cli-mate of the eastern parts of the continentsbetween 20° and 30° latitude N and S, inNorth America (extending from the Car-olinas to E Texas), South America (parts ofBrazil, Uruguay, and Argentina), Australia,and parts of China, Taiwan, and southernJapan. Moist air (mT) flows from the westsides of the subtropical anticyclones lo-cated over the oceans. Precipitation is gen-erally uniform throughout the year;temperatures in the warmest months arearound 27°C and in the coldest month5°–12°C. In winter frontal cyclones de-velop along the polar front; TORNADOES

occur in North America along the zone be-tween the polar and tropical maritime air.The climate is denoted by Cfa and Cwa inthe KÖPPEN CLIMATE CLASSIFICATION.

humilis /hyoo-mă-liss/ A cloud type orspecies of low CUMULUS in which the cloudsare relatively small and do not grow muchvertically. They are normally scattered,have sharp outlines with white or light-gray flat bases all at the same level, andflattened tops – the vertical growth is usu-ally limited by the presence of a SUBSIDENCE

INVERSION. Cumulus humilis is often alsoknown as FAIR-WEATHER CUMULUS. See alsocloud classification.

hurricane The name (of Spanish/Por-tuguese origin) for a TROPICAL CYCLONE

with wind speeds in excess of 64 knots (73mph), force 12 on the BEAUFORT SCALE, thatis particularly associated with storms oc-curring in the North Atlantic and Gulf ofMexico, as well as the eastern seaboardNorth Pacific Ocean. In the North Atlanticthese storms usually develop off the westcoast of North Africa and movewest/northwest toward the Caribbean andsouthern US. Hurricanes develop overwarm water of at least 27°C (81°F) permit-ting appreciable evaporation of watervapor and subsequent latent heat releaseon cooling, which generates their motion.Their rotation results from the CORIOLIS EF-FECT, therefore they cannot form on theEquator where the Coriolis acceleration iszero. Their development is in the IN-TERTROPICAL CONVERGENCE ZONE wherethe trade winds meet. This zone changeswith the season from about 15°N betweenJuly and October to 10°–15°S betweenJanuary and March. These periods repre-sent the principal cyclone season in the re-spective hemispheres.

Hurricanes range from 80 km (50miles) to 800 km (500 miles) across and thewinds form an almost circular vortex witha slight inward deflection near the oceansurface. Sustained wind speeds in excess of87 knots (100 mph) are common in full-scale storms and intense storms of this typeare persistent over the sea but dissipatewhen deprived of the oceanic moisturesource when they reach large land or evensmall islands. Central pressures below 880hPa have been recorded at the center ofhurricanes although a 960 hPa centralpressure with peripheral pressure of about

humidity

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1020 hPa is more common. These valuesare no more extreme than a severe mid-lat-itude CYCLONE but an intense tropical cy-clone is much smaller in diameter with amuch steeper pressure gradient. On aver-age some 6 hurricanes occur during a sea-son (June 1–November 30). The 2005Atlantic hurricane season was the most ac-tive on record with 15 hurricanes, 7 ofwhich became major hurricanes of cate-gory 3 or higher on the Saffir–SimpsonHurricane Scale. Hurricane Katrina, one ofthe most devasting hurricanes in US his-tory, caused considerable flooding and de-struction over the New Orleans area andalong the Mississippi Gulf coast, and an es-timated 1200 deaths. It made landfall as acategory 1 hurricane on August 25 overFlorida, but after entering the Gulf of Mex-ico strengthened considerably and madelandfall again as a category 4 hurricaneover Louisiana on August 29, 2005. Seealso typhoon.

hurricane modification An attempt toalter the intensity of hurricanes. The mostcommon technique is that of seeding usedin the US in the 1960s, 1970s, and 1980s in

attempts to modify the structure of hurri-canes. This technique utilizes silver iodide,which causes supercooled water to freezeremoving some of the latent heat energyused by the hurricane as fuel; as a result thehurricane is weakened. In recent years hur-ricane modification has been viewed withcaution as hurricanes play an importantrole in balancing the Earth’s energy bud-get. Altering hurricanes could thereforehave wide-reaching implications.

hurricane strength scale See Saffir–Simpson Hurricane Scale.

HWRP See Hydrology and Water Re-sources Program.

hydraulic jump An abrupt wave thatoccurs in the flow of a fluid. It occasionallyoccurs in the atmosphere when, for exam-ple, the middle and upper troposphericflow approaches a mountain chain alignedat right angles to the wind’s direction.There is then a relatively marked jump inthe fluid as it passes up and over the elon-gated ridge. The fairly steep ascent on theupwind side, followed by more-or-less hor-

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band

eyewallrain

band

subsidence

Vertical cross-section of a hurricane (tropical cyclone)

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izontal flow in the upper tropospheredownwind, is often marked by a very ex-tensive sheet of CIRROSTRATUS that streamsaway from the mountain ridge.

hydrocarbon A range of organic com-pounds formed of hydrogen and carbon,for example, fossil fuels. Some hydrocar-bons are serious air pollutants.

hydrochlorofluorocarbon /hÿ-droh-klor-ŏ-floo-ŏ-rŏ-kar-bŏn, kloh-roh-/(HCFC) A compound containing hydro-gen, chlorine, fluorine, and carbon. TheHCFCs were introduced as temporary re-placements for the CHLOROFLUOROCARBONS

(CFCs), which were to be phased out underthe MONTREAL PROTOCOL. The HCFCs arethemselves also ozone-depleting sub-stances, but not at the levels of the CFCs,and they have shorter atmospheric life-times. Their production is limited by theMontreal Protocol with phasing out oftheir use: production is prohibited in devel-oped countries after 2020 and in develop-ing countries after 2030.

hydrofluorocarbon (HFC) A com-

pound containing carbon, hydrogen, andfluorine. The hydrofluorocarbons were in-troduced as replacement compounds forchlorofluorocarbons (CFCs), which arebeing phased out under the MONTREAL PRO-TOCOL, and are used, for example, as re-frigerants in refrigeration andair-conditioning units and in aerosols.They do not destroy ozone but are power-ful GREENHOUSE GASES and were mentionedin the KYOTO PROTOCOL as one of six keygreenhouse gases.

hydrologic cycle /hÿ-drŏ-loj-ik/ (hydro-logical cycle, water cycle) The continu-ous events through which water passes inthe Earth-atmosphere system, encompass-ing EVAPORATION from water bodies andland and TRANSPIRATION from land-basedplants, cloud formation through CONDEN-SATION, PRECIPITATION to the Earth’s sur-face, accumulation in glaciers or soil,runoff from land surfaces to rivers, lakes,and seas, and re-evaporation. Over 99% ofthe Earth’s water is stored in the oceans,glaciers, and icecaps. The atmospherestores a minute 0.001% and the remainder

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ice and snow

sublimation

water vapor intothe atmosphere condensation

advection

precipitation ontoland 119,000 km3

precipitation ontoocean 458,000 km3

evaporation fromocean 502,800 km3

transpiration

evaporation fromland 74,000 km3

lake

rock

river runoff

groundwater flow

infiltration

Hydrologic Cycle: the annual turnover of water on Earth

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is stored on land in lakes, rivers, andground water.

While the total quantity of water in theglobal hydrologic cycle, estimated to besome 1.4 × 1018 cubic meters, remains con-stant, the hydrologic cycle of a region willbe largely determined by its climate. It willalso be influenced by the local topography,geology, and vegetation. The frequencyand nature of the precipitation, the varia-tions in local temperature and humidity,and the extent of its exposure to solar radi-ation are clearly of great importance in theinteraction between the weather and alocal hydrologic cycle. Human activities ofvarious kinds are playing an increasing rolein determining the flows in local hydro-logic cycles.

hydrologic network A network of hy-drologic monitoring sites incorporatingstations measuring, for example, precipita-tion and other meteorological elements,groundwater flow, and streamflow. Thedata collected may be used in flood fore-casting.

hydrology /hÿ-drol-ŏ-jee/ The scientificstudy of the properties of water on andwithin the ground, especially with relationto the effects of PRECIPITATION and EVAPO-RATION upon the occurrence and propertiesof water on or below the land surface, in-cluding that held in rivers and lakes. Hy-drology encompasses studies of runoff,ground water, soil moisture, the hydrolog-ical balance, and snow and ice accumula-tion.

Hydrology and Water Resources Pro-gram (HWRP) A program of the WORLD

METEOROLOGICAL ORGANIZATION (WMO).Its aim is to promote the measurement ofbasic hydrological elements from networksof hydrological and meteorological sta-tions and the collecting, publishing, andstorage of data, and to further close coop-eration between meteorological and hydro-logical services. It has an overall objectiveof applying hydrology to the sustainabledevelopment and use of water and relatedresources; to the mitigation of water-re-lated disasters; and to effective environ-

mental management at national and inter-national levels.

hydrometeorology /hÿ-droh-mee-tee-ŏ-rol-ŏ-jee/ The interdisciplinary study ofthe interrelationships between water in theatmosphere and on land as it movesthrough the hydrologic cycle. It includesthe study of the water budget, atmosphericmoisture and precipitation, infiltration andgroundwater flow, streamflow characteris-tics, fluvial geomorphology. Hydrometeo-rology has applications, for example, in thedesign of flood-control measures and plan-ning of dams and reservoirs.

hydrometeors /hÿ-droh-mee-tee-er/ Allthe forms of water associated with CON-DENSATION or SUBLIMATION of atmosphericWATER VAPOR. Examples of hydrometeorsinclude rain and drizzle, which fall as liq-uid precipitation; hail, graupel, sleet, andsnow, which fall as frozen precipitation;virga, which evaporates before it reachesthe ground; fog and cloud, which are sus-pended in the air; drifting snow; and dewand hoar frost.

hydrosphere /hÿ-drŏ-sfeer/ The Earth’swater in any of its states: liquid, solid, orgaseous. The hydrosphere includes land,sea, and atmospheric water sources. Ingeneral, the amount of water in the hy-drosphere is constant. About 71% of theEarth’s surface is covered by water, ofwhich about 97% of this is contained inthe oceans and seas, while the remaining3% can be found on land in the form ofpolar snow and ice, ground water, rivers,and in the flora and fauna.

hydrostatic equation /hÿ-drŏ-stat-ik/ An equation that quantifies the rate of de-crease of ATMOSPHERIC PRESSURE withheight in the atmosphere. In a hydrostaticatmosphere, the downward force due togravity balances the upward PRESSURE GRA-DIENT FORCE, so that the net resolution offorces acting in a vertical plane is zero.There is thus no vertical acceleration in ahydrostatic atmosphere. If p is the pressurein PASCALS, z is the height (m), ρ is the air

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density (kg/m3), and g is the acceleration offree fall (9.8 m s–2), then:

∂p/∂z = –ρg.The hydrostatic equation can be com-

bined with the ideal gas equation to derivethe hypsometric equation, an importantformula that allows the vertical distance(THICKNESS) between two pressure levels tobe calculated if the mean temperature ofthat layer is known:

Z2 – Z1 = [(Rd Tv)/g] [loge (p1/p2)]where Z2 and Z1 are the height of the topand bottom of the atmospheric layer re-spectively, Rd is the gas constant for dry air(287 J K–1 kg–1), Tv is the mean virtualtemperature of that layer (K), and p1 (p2) isthe pressure at the bottom (top) of thelayer. For example, to find the 1000–500mb thickness when the mean virtual tem-perature of this layer is 282 K:

Z500 –Z1000 =(287 × 282/9.8)loge(1000/500).

hyetograph /hÿ-ĕ-tŏ-graf, -grahf/ 1. Amap or chart displaying the temporal orareal distribution of precipitation.2. A type of self-recording RAIN GAUGE.

hygrogram /hÿ-grŏ-gram/ See hygro-graph.

hygrograph /hÿ-grŏ-graf, -grahf/ A HY-GROMETER that continuously measures andrecords atmospheric humidity. The stripchart produced is known as a hygrogram.See also hair hygrometer.

hygrometer /hÿ-grom-ĕ-ter/ An instru-

ment used for measuring the humidity ofair. The most common type of hygrometeris the dry- and wet-bulb PSYCHROMETER.Other types include the HAIR HYGROMETER

(the simplest form of the instrument), theDEW-POINT HYGROMETER, and those thatuse the moisture-absorbing properties ofcertain chemicals.

hygrometric table /hÿ-grŏ-met-rik/ Atable used to obtain vapor pressure, rela-tive humidity, and dew-point from wet-and dry-bulb temperatures.

hygroscopic nucleus /hÿ-grŏ-skop-ik/The very small particle that forms the coreonto which water vapor condenses duringthe process of cloud-droplet and ice-crystalformation. They have by definition anaffinity for water (hygroscopic). See cloudcondensation nucleus.

hygrothermograph /hÿ-groh-ther-mŏ-graf, -grahf/ (thermohygrograph) An in-strument consisting of a combinedHYGROGRAPH and THERMOGRAPH, continu-ously measuring atmospheric humidity andtemperature and recording these values ona single chart.

hypsithermal /hip-să-ther-măl/ See al-tithermal.

hythergraph /hÿ-th’er-graf, -grahf/ Aclimatic graph showing the relationship be-tween selected meteorological elements;for example, temperature and rainfall ortemperature and humidity.

hyetograph

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ICAO standard atmosphere See stan-dard atmosphere.

ice Water in its solid crystalline state. Itis formed from the freezing of liquid water,which, for pure water at 1013.24 hPa pres-sure, occurs at 0°C (32°F), or by direct sub-limation from the gaseous state of water(i.e. water vapor). Within the atmosphere,ice occurs in a number of forms, includingDIAMOND DUST, GRAUPEL, HAIL, ICE CRYS-TALS, ICE FOG, ICE PELLETS, ICE PRISMS, andSNOW. It is deposited onto surfaces asGLAZE, HOAR FROST, ICING, and RIME.Within and above water bodies it forms asFRAZIL, grease ice, and PANCAKE ICE. Onland and/or water, accumulations of iceform extensive ice sheets, ice caps, iceshelves, glaciers, ice bergs, and pack ice.

ice age (glacial period, glaciation) A pe-riod of extensive glaciation during theEarth’s geologic history, during whichlarge areas were covered by vast ice sheetsand glaciers. The most recent ice age wasthat of the late Cenozoic (often known asthe Ice Age), which is estimated to haveended around 10,000 years ago, althoughit is believed that the present time may bean interglacial. At least seven ice ages haveoccurred throughout geologic history, thefirst occurring 2500 million years ago dur-ing the Archean, and three more in the Pro-terozoic (between 900 and 600 millionyears ago). Others occurred toward theend of the Ordovician, and in the Car-boniferous–Permian periods. The ice ageshave generally lasted for around 100,000years with oscillations of ice advance(glacial conditions), and of ice retreat (in-terglacial conditions).

A combination of processes is believedto be responsible for ice ages. These include

the location and extent of continents andoceans, and mountain building (resultingfrom continental drift and plate tectonics),the changes in incoming solar radiation re-sulting from variations in the Earth’s orbit(see Milankovitch cycles), and variations inatmospheric concentrations of carbondioxide and other greenhouse gases.

ice blink A bright white or yellowish-white glare seen on the underside of lowcloud that is produced by light being re-flected onto the cloud from an ice-coveredsurface, such as pack ice. On a water sur-face, the appearance of ice blink can indi-cate the presence of ice that is beyond therange of vision when seen in contrast to thedarker appearance of clouds above water(water sky).

ice-cap climate See ice-sheet climate.

ice core A cylindrical core removedfrom a section of ice for the purpose ofstudying past climate conditions. Ice coresfrom glaciers or sea ice may be examinedfor stratification, to determine seasonalchanges, or chemically analyzed to esti-mate changes in carbon dioxide and tracegases over a period. See also Greenland IceCore Project; Greenland Ice Sheet Project2.

ice crystal A minute spicule of ice thatforms from water in the atmosphere on anICE NUCLEUS at temperatures below thefreezing point of 0°C (32°F). Ice crystalshave a variety of shapes, depending on thetemperature range at their formation, thatincludes needles, hexagonal prisms, andstars, and they increase in number by splin-tering. Their growth occurs by the diffu-sion of water vapor onto them – and they

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collide with other ice crystals to formsnowflakes.

ice fog A type of fog consisting of a sus-pension of ice crystals in the atmospherethat forms when mild maritime air blowsacross an ice or snow-covered surfaces. Ifconditions are extremely cold, condensa-tion will occur as ice crystals rather thanwater droplets to produce a fog. Theseoccur mostly in Arctic regions, occasion-ally in coastal stretches of Alaska, for ex-ample. Ice fog can also form artificially inthese areas, for example from the burningof hydrocarbon fuels and from the exhaustgases of aircraft and motor vehicles.

icehouse period A time during theEarth’s geologic history during which glac-iers were at their most extensive, and sealevels were low. Compare greenhouse pe-riod.

Icelandic low The feature on averagedmean-sea-level pressure maps that ex-presses where the traveling CYCLONES

(lows) of the North Atlantic reach theirdeepest (minimum) value on average. Itdoes not mean a low is always centeredover or near Iceland, but the region does lieon the major storm track of the North At-lantic. It is at its most intense during thenorthern-hemisphere winter and earlyspring when it is generally centered overIceland and S Greenland. During summerit is weaker and may separate into two:west of Iceland and over the Davis Straitbetween Greenland and Baffin Island. Seealso Aleutian low.

ice nucleus A minute solid particle sus-pended in the atmosphere that provides thesurface onto which an ICE CRYSTAL canform. They are less numerous than CLOUD

CONDENSATION NUCLEI and are mostly par-ticles that possess a geometry/lattice struc-ture similar to that of ice crystals. Someclay minerals like kaolinite have such astructure and become active nuclei at tem-peratures of around –15°C (5°F).

ice pellets A form of PRECIPITATION con-sisting of small spherical or irregularly

shaped pellets of ice with diameters of lessthan 5 mm (0.2 in). They are formed fromfrozen RAINDROPS or from SNOWFLAKES thathave almost totally melted and then re-frozen.

ice prisms Tiny unbranched ice crystalsthat form at temperatures below –30°C(–22°F) in the atmosphere. They may havethe form of needles, platelets, or hexagonalcolumns.

ice rind A stage in the growth of SEA ICE

during which ice crystals combine to forma brittle skin.

ice-sheet climate (ice-cap climate) Aclimate in which mean monthly tempera-tures throughout the year are below 0°C(32°F), occurring north and south of about65° latitude over the ice caps of Greenlandand Antarctica and over the frozen ArcticOcean, which are the source regions for theANTARCTIC AIR MASS and ARCTIC AIR MASS.Temperatures are extremely low, especiallyin the Antarctic region, where –89.2°C(–128.6°F) was recorded at the Russianmeteorological station at Vostock in 1983(the coldest ever recorded temperature; seecold pole), and precipitation is light, occur-ring as snow, or ice crystals or pellets. Theclimate is designated EF in the KÖPPEN CLI-MATE CLASSIFICATION.

ice storm Conditions during which iceforms on exposed surfaces, commonly in-cluding power and telephone cables andtheir supports. Supercooled water droplets(i.e. water droplets that are liquid belowthe usual freezing temperature of bulkwater) freeze on contact with surfaces. Al-though relatively rare in Europe, there areice storms in eastern Canada and thenortheastern states of the US more fre-quently.

icicle A suspended spike of clear iceformed by the freezing of dripping water.

icing The development of a coating ofice on an exposed object, such as the wind-shield of a motor vehicle or the wings of anaircraft. Icing occurs when supercooled

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water droplets in a cloud or fog come intocontact with the surface to form RIME orwhen droplets freeze on impact, leading toGLAZE.

ideal gas laws The laws obeyed by ideal(or perfect) gases that relate their volumechanges to variations in temperature andpressure; BOYLE’S LAW and CHARLES’ LAW

are the principal ideal gas laws. They canbe combined into the single equation, PV =RT for a single mole of an ideal gas, whereP is its pressure, V its volume, and T itsTHERMODYNAMIC TEMPERATURE. R is theuniversal gas constant, which has the value8.315 J K–1mol–1. These gas laws are notperfectly obeyed by real gases; they strictlyapply only to ideal gases.

IFOV See instantaneous field of view.

IGAC See International Global Atmos-pheric Chemistry.

imaging A technique using remote sens-ing to record and interpret images of fea-tures, such as the surface of the Earth, fromthe sensing of various ranges of electro-magnetic radiation emitted by the source.For example, an image of the distributionof clouds over a large region at one instant,may be mapped from a satellite by sensingthe thermal infrared radiation being emit-ted into space by the clouds and cloud-freesurfaces in view. The resulting map is animage or a representation of the naturalscene.

inches of mercury (in Hg) A unit ofmeasure of surface ATMOSPHERIC PRESSURE

used in the US. Inches refers to the heightof the column of mercury in the tube of amercury barometer, a response to the forceof atmospheric pressure. Average air pres-sure at mean sea level is 29.92 inches. At-mospheric pressure aloft is measured inHECTOPASCALS (hPa) or MILLIBARS (mb); 1hPa (or mb) = 0.2953 (in Hg).

incus /ink-ŭs/ A supplementary cloudfeature (see cloud classification) that refersspecifically to the spreading anvil CIRRUS

that streams out of and away from the

upper reaches of CUMULONIMBUS clouds.[Latin: ‘anvil’]

index cycle The way in which the large-scale circulation in the free TROPOSPHERE

tends to evolve over some weeks from ahigh ZONAL INDEX, when the circulation isprincipally west to east, to one of a lowzonal index, when it is much more parallelto the lines of longitude.

Indian Ocean The third largest of theEarth’s major oceans. It is located betweenAfrica, India, and Australia, and covers anarea of approximately 68,500 sq km(26,500 sq miles). A unique reversal occursin the currents of the northern IndianOcean; in summer during the southwestMONSOON the Southwest Monsoon Cur-rent flows eastward, while in winter thenortheast monsoon occurs with winds andcurrents flowing from the northeast to thesouthwest. A counterclockwise gyre flowsin the south Indian Ocean.

Indian summer A warm period of anti-cyclonic weather with clear skies and coolnights frequently occurring during Octoberor November. The term was first used inthe US in the late 18th century and in theUK at the beginning of the 19th century.Traditionally, it marks the end of the lastvestiges of summer weather and usuallyfollows the first frost. No climatic reasonhas been identified as to why such a periodof warmth should occur in most years.

indifferent equilibrium See neutralstability.

industrial smog See smog.

inert gas A chemically inactive gas; inertgases include the gases argon, neon, he-lium, and xenon, which are present in thesmall quantities in the atmosphere.

inferior mirage See mirage.

inflow band See feeder band.

inflow jets Localized near-surface jets

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of air that flow into the base of TORNA-DOES.

infrared radiation ELECTROMAGNETIC

RADIATION in the approximate wavelengthfrom 0.7 µm (700 nm) to 1000 µm. In theelectromagnetic spectrum it is between thered end of the visible light and microwaves;near-infrared (0.7–4.00 µm) is in that partof the spectrum closest to visible lightwhereas far-infrared is closest to mi-crowave radiation. Although about 50%of solar radiation falls within the infraredband, the term is often loosely used as analternative for longwave radiation (terres-trial radiation).

infrared remote sensing The techniquethat images the Earth and its atmosphere ininfrared wavebands. Meteorological satel-lites, for example, produce cloud images bysensing the planet in the THERMAL IN-FRARED, and water vapor images by sensingin the WATER VAPOR waveband. Both ofthese lie in the infrared part of the electro-magnetic spectrum.

infrared satellite imagery Images ofthe Earth and its cloud cover within spe-cific infrared BANDS, obtained by satellites.Two such bands commonly used inweather imaging are the THERMAL IN-FRARED window and the WATER VAPOR.These sense the intensity of infrared radia-tion that is being emitted by the Earth’ssurface and its clouds, and the water vaporconcentration in the middle TROPOSPHERE,respectively. Infrared imaging has the ad-vantage over visible satellite imagery inthat, because the surface, clouds, andwater vapor radiate ceaselessly, it is con-stantly available.

infrared thermography A remote-sensing technique using sensors and cam-eras sensitive to emitted thermal radianceto map the temperature of a surface; thetechnique can be used on an aircraft or onthe ground. This technique has applica-tions, for example, in producing images ofthe spatial variation of the temperature ofa sea surface, or in images of the heat lossfrom an urban area.

insolation /in-sŏ-lay-shŏn/ In climatol-ogy, the intensity of short-wavelengthSOLAR RADIATION received over a given pe-riod of time on a horizontal surface, mea-sured in watts per square meter (W m–2). Itmay also refer to measurements on inclinedplanes not necessarily at the Earth’s sur-face. Insolation is influenced by the time ofthe year, latitude, and atmospheric condi-tions, such as cloud amount. [From: Latininsolatio and incoming solar radiation]

instability See absolute instability.

instantaneous field of view (IFOV) The size of the smallest PIXEL that can bepractically imaged by the sensor system ofa satellite, given its orbital elevation andthe physical dimensions of the active partof the sensor on-board. For the AVHRRinstrument on the NOAA satellites, theIFOV is an angle of 1.3 ± 0.1 milliradians,which equates, at its elevation of 850 km(530 miles), to a scan spot that is 1.1 kmacross at the SUBSATELLITE POINT.

instrument shelter (Stevenson screen) A box-like shelter, designed originally bythe Scottish engineer Thomas Stevenson(1818–87), used to accommodate meteoro-logical thermometers for the measurementof air temperatures unaffected by directsolar radiation, precipitation, and conden-sation. It consists of a white wooden boxwith a hinged door, which faces poleward,sited approximately 1.25 m (4 ft) above theground, and allowing indirect ventilationthrough the bottom of the screen, louveredsides, and a double roof. The screen housesthe wet- and dry-bulb thermometers andthe maximum and minimum thermometer;larger versions also house a thermographand hydrograph.

insular climate The climate of a smallisland or group of islands. Seasonal tem-perature change is low, especially in thelow latitudes.

intensification A strengthening of thewinds in association with a simultaneousstrengthening of the PRESSURE GRADIENT

within a weather-producing system such as

infrared radiation

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a HURRICANE or a FRONTAL CYCLONE. Seealso deepening. Compare weakening.

intensity The strength of the radiationsignal emitted or received by a remote-sensing instrument. It is quantified in wattsper square meter.

intensity of rainfall See rainfall inten-sity.

interception The process by which PRE-CIPITATION is captured by vegetation andprevented from reaching the ground. Someof the intercepted moisture may be evapo-rated directly back into the atmospherefrom the plant surface. If the RAINFALL IN-TENSITY is high, the precipitation will over-come the carrying capacity of the plantsand drip to the ground.

interglacial /in-ter-glay-shăl/ A periodof warm climate between two glacial peri-ods during which continental glaciers re-treat. During the last 1,000,000 yearsinterglacials have had a periodicity of100,000 years, each lasting for approxi-mately 10,000 years.

Intergovernmental Panel on ClimateChange (IPCC) An international panelthat was established in 1988 by the UnitedNations Environment Program (UNEP)and the World Meteorological Organiza-tion (WMO) to assess information on pos-sible global warming. Scientific authorsand editors produce periodic reports forthe IPCC, which are then reviewed by in-ternational experts. After publication,many governments view reports as the of-ficial state of knowledge on climate changeissues, and in most cases policy formula-tion revolves around this information. TheIPCC produced its First Assessment Reportin 1990. This was followed by the SecondAssessment Report: Climate Change 1995,then the Third Assessment Report: ClimateChange 2001; the Fourth is scheduled forcompletion in 2007.

International Cloud Atlas See cloudclassification.

International Global AtmosphericChemistry (IGAC) A core project spon-sored jointly by the Commission on At-mospheric Chemistry and Global Polution(CACGP) of the International Associationof Meteorology and Atmospheric Sciences(IAMAS) and the International Geo-sphere–Biosphere Program (IGBP). It aimsto develop understanding of the processesdetermining atmospheric composition andof interactions between the chemical com-position and physical and climaticprocesses. It also aims to predict the impactof natural and anthropogenic forcing onthe chemical composition of the atmos-phere. Among the project’s areas of studyare atmosphere–surface chemical ex-changes, ozone in the troposphere, andaerosols and their radiative effects.

International Practical TemperatureScale See thermodynamic temperature.

international synoptic code See syn-optic code.

interpluvial /in-ter-ploo-vee-ăl/ A pe-riod of geologic time characterized by a dryclimate between PLUVIALS.

interstade /in-ter-stayd/ (interstadial) Awarmer interval during a glacial periodwhere the temperatures are not sufficientlywarm or prolonged to form an INTER-GLACIAL.

Intertropical confluence /in-ter-trop-ă-kăl/ A broadly east–west aligned zone inwhich the NORTHEAST and SOUTHEAST

TRADE WINDS flow generally smoothly to-gether from either side of a stretched-outasymptote. A CONFLUENCE refers only tothe direction of the flow; it does not in-clude changes of speed.

Intertropical Convergence Zone(ITCZ) The broadly east–west alignednarrow zone in which the NORTHEAST andSOUTHEAST TRADE WINDS flow generallysmoothly together and converge. TheITCZ migrates into the northern hemi-sphere during the northern summer andinto parts of the southern hemisphere dur-

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ing the southern summer, following the an-nual movement of the zenithal Sun with atime lag of about six weeks. The distribu-tion of land and sea considerably modifiesthis simple pattern. Over most of the Pa-cific Ocean, the ITCZ remains in the north-ern hemisphere all year round. The ITCZ ismost clearly developed over the tropicaloceans (especially the eastern portions ofthe Pacific and Atlantic Oceans), where thenortheast and southeast trade winds bothslow down markedly as they flowsmoothly in to the zone, known here as theDOLDRUMS. The marked convergence andthe piling up of air that slows down in thelowest kilometer or so as it enters the ITCZis connected to significant VERTICAL MO-TION. Scattered along the zone are very vig-orous CUMULONIMBUS clouds that produceheavy rainfall. The zone is sometimes alsoknown as the Intertropical Front over thecontinents. See also South Pacific Conver-gence Zone.

Intertropical Front See IntertropicalConvergence Zone.

intortus /in-tor-tŭs/ A cloud variety re-lated uniquely to CIRRUS, the filaments ofwhich are highly twisted or contorted intotangled shapes. See also cloud classifica-tion.

inversion (temperature inversion) Anincrease in temperature with increasing al-titude. This is a reversal from the normalexpected decrease of temperature with in-creasing height. A temperature inversionresults in a warmer layer overlying a coolerlayer. Inversions are a characteristic of theTROPOPAUSE and MESOPAUSE. See also inver-sion layer.

inversion layer (capping inversion, tem-perature inversion layer) A layer of theatmosphere in which there is an INVERSION

of temperature, i.e. temperature increaseswith height, a reversal of the expected de-crease of temperature with altitude. In thetroposphere, an inversion layer is generallystable and tends to act as a lid or cap thatinhibits vertical motion, resulting in an ab-sence of turbulence and general atmos-

pheric stability. If the air under an inver-sion layer is moist, the inversion can act asa cap to a layer of stratified cloud, restrict-ing the development of convective cloudforms. Inversion layers can also trap dust,smoke, and other atmospheric pollutants;as a result, precipitation and visibility aregreatly reduced below the layer. Inversionsmay occur with the passage of a cold front(see frontal inversion), the influx of rela-tively cold sea air via an onshore breeze(see surface inversion), the overnight radia-tive cooling of surface air (see radiation in-version), and through the presence of ahigh-pressure system (see subsidence inver-sion).

ionosphere /ÿ-on-ŏ-sfeer/ The region ofthe upper ATMOSPHERE within which thereis a high concentration of free electronsthat affects the propagation of electromag-netic radio waves. It extends through theupper MESOSPHERE and THERMOSPHERE toindefinite heights, becoming most distinctbeyond about 80 km (50 miles) above theEarth’s surface. It is characterized by highconcentrations of ions (hence its name) andelectrons formed when solar X-ray and ul-traviolet radiation ionize molecules oratoms into ion pairs (a negative electronleaves a neutral atom which then becomespositive while the electron attaches itself toanother atom producing a negative ion).These ions and electrons are at sufficientlyhigh concentrations to reflect and/or re-fract radio waves. The structure of theionosphere is affected by the cycle ofsunspot activity, as well as by geographicallocation and seasonal and diurnal changes.

Several layers or regions are identifiedwithin the ionosphere, based on the propa-gation of electromagnetic radio waves,with electron density increasing in generalwith altitude from the D-LAYER through theE-LAYER and with especially high ionizationin the F-LAYERS. Records of the ionosphereshow that it is an indicator of greenhouseeffects. Increases in greenhouse gases in thelower atmosphere cause warming but inthe upper atmosphere cooling and contrac-tion takes place as less heat is being re-flected from below. The height of the

Intertropical Front

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ionosphere is lowered by a small amounteach year.

IPCC See Intergovernmental Panel onClimate Change.

iridescence /i-ră-dess-ĕns/ The subtlytinted patches of red, green, and occasion-ally blue and yellow that appear aroundthe edges of HIGH CLOUD within around 30°of the Sun. The colors are produced by thediffraction of sunlight by small cloud par-ticles.

irradiance /i-ray-dee-ăns/ (radiant fluxdensity) The rate of incident energy onunit area of a surface, measured in wattsper square meter (W m–2).

isallobar /ÿ-sal-ŏ-bar/ A line on a mean-sea-level weather map (synoptic chart) thatconnects points at which the time rate ofchange of atmospheric pressure is thesame. A typical value (normally quoted asa rise or fall (or steady) over the threehours leading up to the observation hour)would be 2.0 hPa per 3 hours. A veryrapidly deepening FRONTAL CYCLONE may,however, have falls of up to 10.0 hPa per 3hours in extreme cases.

isanomal /ÿ-să-noh-măl/ A line that con-nects places that experience the sameanomaly of a particular variable within aparticular period: for example, the differ-ence (or anomaly) of one year’s rainfalltotal from its long-term annual mean rain-fall.

isentropic /ÿ-sen-trop-ik/ See isentropicsurface.

isentropic analysis A technique inweather forecasting or research in whichSYNOPTIC-SCALE air motions (e.g. withinFRONTAL CYCLONES are mapped across sur-faces of constant dry-bulb or wet-bulbPOTENTIAL TEMPERATURE, known as ISEN-TROPIC SURFACES. It is more realistic to dothis because the ascending and descendingcirculation on such a scale is ADIABATIC (i.e.no heat is exchanged by the air parcel withits environment); such upslope and downs-

lope motion glides three dimensionallyalong the surfaces of constant potentialtemperature. The air does not move acrosshorizontal ISOBARIC SURFACES (i.e. surfacesat which all points are of the same atmos-pheric pressure, which are often used to de-pict upper-air features, such as the LONG

WAVES, but instead, the air moves up anddown through the isobaric surfaces. Seealso analysis; isobaric analysis.

isentropic lift The lifting of air that fol-lows an upward-sloping ISENTROPIC sur-face. Situations producing isentropic liftare associated with widespread stratiformclouds and precipitation.

isentropic surface (isentropic) A sur-face in space on which all points are at thesame ENTROPY or constant POTENTIAL TEM-PERATURE at a given time. See also isen-tropic analysis.

isobar /ÿ-sŏ-bar/ (isobaric surface) Aline on a chart connecting places with thesame ATMOSPHERIC PRESSURE. Isobars aretypically drawn on a MEAN-SEA-LEVEL PRES-SURE (MSLP) chart at either 2 hPa, 4hPa, or8 hPa intervals. Unlike the surface, wherethe meteorologist plots the spatial varia-tion of pressure at a fixed height, upper-aircharts show the variation in the height of afixed pressure level, for example 850 hPa,500 hPa. Lines on such charts are calledisohypses or CONTOUR lines, rather thanisobars.

isobaric analysis An ANALYSIS of at-mospheric conditions for an ISOBARIC SUR-FACE. On surface isobaric charts, theisobars are traditionally plotted at 4 hPaintervals centered on 1000 hpa (i.e. 996,1000, 1004, 1008 hPa, etc). Once plottedthe centers of high and low pressure can beidentified and are indicated by the letters H(high pressure) and L (low pressure). Otherfeatures, such as troughs of low pressure,are also indicated. See also isentropicanalysis; surface weather chart.

isobaric chart (constant-pressurechart) A SYNOPTIC CHART that is pro-duced for a particular ISOBARIC SURFACE

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(e.g. 1000, 850, 700, 500, 300, or 200hPa). The atmospheric variables usuallyplotted on the isobaric chart include theheight of the pressure surface, temperature,moisture content, and wind speed and di-rection.

isobaric surface (constant-pressure sur-face) A surface in space on which allpoints are at the same or constant ATMOS-PHERIC PRESSURE at a given time. See alsoisobaric analysis.

isobront /ÿ-sŏ-bront/ A line that con-nects the location of THUNDERSTORMS thatexist at a specific time over a particular re-gion.

isoceraunic line /ÿ-so-sĕ-raw-nik/ Acontour joining places that have the samepercentage frequency of days in the year onwhich THUNDER was heard.

isocheim /ÿ-sŏ-kÿm/ A line on a map orchart that connects places that experiencethe same mean winter temperature.

isohaline /ÿ-soh-hay-lÿn/ In oceanogra-phy, a line on a chart connecting points ofequal salinity.

isohel /ÿ-sŏ-hel/ A line that connectsplaces that observe equal durations ofbright sunshine for a particular period (e.g.for one day, one week, or one month).

isohyet /ÿ-sŏ-hÿ-ĕt/ A line that joinsplaces that observe equal rainfall totalsover a particular period (e.g. one hour, oneday, or one month).

isokinetic /ÿ-sŏ-ki-net-ik/ See isotach.

isoline /ÿ-sŏ-lÿn/ (isopleth) A contourthat joins places that have the same valueof some variable, such as an ISOTHERM

(DRY-BULB TEMPERATURE) or an ISOBAR

(MEAN-SEA-LEVEL PRESSURE).

isomer /ÿ-sŏ-mer/ 1. In meteorology, aline that connects places with the same

value of an individual month’s rainfall, ex-pressed as a percentage of their annual av-erage rainfall.2. In chemistry, compounds with the samemolecular formula but a different arrange-ment of the atoms, which may make themcompletely different compounds, e.g.CH4N2O may be either urea, CO(NH2)2,or ammonium cyanate, NH4CNO.

isoneph /ÿ-sŏ-mef/ A contour joiningplaces of equal total CLOUD AMOUNT.

isonif /ÿ-sŏ-nif/ A line connecting sitesthat have the same snow depth.

isopleth /ÿ-sŏ-pleth / See isoline.

isopycnic /ÿ-sŏ-pik-nik/ A line (or a sur-face) of constant atmospheric density.

isoryme /ÿ-sŏ-rÿm/ A contour connect-ing places that experience the same inten-sity of frost.

isotach /ÿ-sŏ-tak/ (isokinetic) A line thatjoins places that have the same wind speed.

isothere /ÿ-sŏ-theer/ A contour that con-nects places that have the same summermean temperature.

isotherm /ÿ-sŏ-therm/ A contour thatconnects places with the same DRY-BULB

TEMPERATURE.

isothermal layer /ÿ-sŏ-ther-măl/ A layerin the atmosphere with zero LAPSE RATE oftemperature, i.e. there is no change in tem-perature with height and it is thus charac-terized by a constant temperature.

isothermobath /ÿ-soh-ther-mŏ-bath/ Inoceanography, a line drawn to connect allpoints of equal temperature on a diagramshowing the vertical temperature structureof the ocean.

ITCZ See Intertropical ConvergenceZone.

isobaric surface

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Jacob’s ladder See crepuscular rays.

Japan Current See Kuroshio Current.

jet-effect wind See canyon wind.

jet maximum The elongated core of aJET STREAM, in which the wind speed isstrongest. In the winter the maximumspeed in the POLAR FRONT JET STREAM canreach 200 knots (100 m s–1 or 230 mph) ormore on rare occasions. The air speeds upas it approaches the core in the ENTRANCE

REGION, and slows down in the EXIT RE-GION.

jet streak A small-scale zone of maxi-mum wind speed – a JET MAXIMUM – thatruns through the snaking pattern of the JET

STREAM. The jet maximum is most often amore-or-less stationary feature within thejet stream, through which the air blowsvery strongly. A jet streak is often associ-ated with changes in surface-pressure de-velopments, such as rapid deepeners (seeexplosive cyclogenesis).

jet stream A long snaking ribbon of fastflowing air, normally in the upper TROPOS-PHERE. It is often thousands of kilometerslong, a few hundred kilometers wide, andabout one or two kilometers deep. It hasmarked WIND SHEAR above, below, and tothe sides of the very elongated core, alongwhich there are one or more speed maxima(see jet maximum). Significant CLEAR-AIR

TURBULENCE is associated with the windshear. The principal upper tropospheric jetstreams are the SUBTROPICAL JET STREAM,POLAR FRONT JET STREAM, and TROPICAL

EASTERLY JET STREAM. The subtropical jet isa geographically relatively fixed westerlyflow feature over days and weeks, but mi-

grates seasonally with the SUBTROPICAL AN-TICYCLONES. The tropical easterly jet is ge-ographically fixed and markedly seasonal,forming as part of the summer MONSOONsof southern Asia and West Africa (see alsoAfrican monsoon; Asian monsoon). Its EN-TRANCE REGION is above the Indonesianarea while its EXIT REGION lies above sub-Saharan Africa. The polar front jet isstrongest in winter when the POLAR FRON-T’s temperature contrasts are largest. It sin-uous shape and day-to-day change inlocation across middle-latitude oceansmeans, unlike the previous two jets out-lined, it does not show up as a distinct fea-ture on monthly or seasonal mean maps ofthe upper tropospheric flow. See alsoPolar-Night Jet Stream.

jet stream cirrus An extensive sheet orelongated striations of cirrus associatedwith vertical motions around the core of a jet stream. Deep ascent often occurs on the warm low-latitude side of a jetstream with deep descent on the cold high-latitude flank. The ascent leads to CIRRI-FORM cloud that exhibits a sharp edgecoinciding roughly with the jet core. Thecloud moves very quickly with the rapidflow at jet level.

JGOFS See Joint Global Ocean FluxStudy.

Joint Global Ocean Flux Study(JGOFS) An international programlaunched in 1987 to study the fluxes of car-bon within the oceans (which are believedto contain 50 times as much carbon as theatmosphere), and exchanges of carbon be-tween the atmosphere and oceans, on aglobal scale. It was a core project of the In-ternational Geosphere–Biosphere Pro-

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gram(IGBP). The Program also aimed todevelop the capacity to predict the global-scale response to anthropogenic perturba-

tions, especially those related to climatechanges. The study was completed in2003.

Joint Global Ocean Flux Study

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Kamchatka Current /kam-chat-kă/ Acold ocean current that flows from theBering Sea along the pensinsula of Kam-chatka into the North Pacific Ocean andforms the OYASHIO CURRENT.

Kansan I and II /kan-za˘n/ A glacial pe-riod, the second of four glacial stages inNorth America, which started about620,000 years ago, and lasted for about60,000 years. During this period extensiveareas of northern and central Europe andNorth America were covered in ice.

katabatic wind /kat-ă-bat-ik/ (drainagewind, gravity wind, mountain breeze) Alocal cold downhill wind that blows downvalleys and mountain slopes (e.g. Norwe-gian fjords); the term is also applied to coldair that flows outward from icecaps, suchas those in Antarctica and Greenland. Akatabatic wind results from the radiativecooling of the upland ground surface atnight, which in turn cools the air in contactwith the slope. As the air becomes denser,it starts to move downhill by gravity flowbeneath warmer less dense air, its directionbeing controlled almost exclusively by theorography. In a valley-wind system, inwhich cooling may occur over distances of5 km (3 miles) or less, the katabatic flow’sthickness rarely exceeds 10 m (30 ft); inAntarctica it could extend up to 500 m(1600 ft) into the atmosphere. See alsobora.

kata-front A WARM FRONT or COLD

FRONT that is overrun by drier air, or inwhich the warm air subsides, so that anycloud and precipitation tend to be sup-pressed. In this sense, they are generally in-active fronts. Compare ana-front.

kelvin /kel-vin/ (K) The SI unit of THER-MODYNAMIC TEMPERATURE (absolute tem-perature); it is equal to 1/273.16 of thethermodynamic temperature of the triplepoint of water. The units of Kelvin andCentigrade degrees are equal. The unit isnamed for William Thomson, BaronKelvin (1824–1907). See also Kelvin tem-perature.

Kelvin–Helmholtz instability /helm-holts/ A phenomenon that occurs occa-sionally in an atmospheric layer withinwhich the wind speed increases rapidlywith height. Kelvin–Helmholtz waves formin the layer of strong WIND SHEAR; thewaves can break and are marked by a trainof clouds that have the appearance ofbreaking ocean waves. The phenomenon isnamed for the British phyicist WilliamThomson, Baron Kelvin (1824–1907) andthe German scientist Hermann vonHelmholtz (1821–94).

Kelvin temperature The THERMODY-NAMIC TEMPERATURE (absolute tempera-ture) of a substance measured in KELVINS.The lowest temperature theoretically possi-ble (see absolute zero) is 0 K, and the melt-ing point of ice is 273.15 K, which is 0.01K below the triple point of water.

Kelvin wave See El Niño Southern Os-cillation.

kernlose winter See coreless winter.

Kew barometer A type of MERCURY

BAROMETER. Designed in 1854 by P. Adie,this is a portable marine barometer with agraduated scale. The scale is designed totake account of any unsteadiness in thelevel of the mercury which may occur as a

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result of changes in air pressure throughgusting winds or due to the rolling of aship.

khamsin /kam-sin, kam-seen/ A LOCAL

WIND in Egypt that can persist for severaldays. The khamsin is a hot and drysoutherly wind, which often transportslarge quantities of dust from the Saharadesert into the country’s capital Cairo,sometimes pulling in air from Arabia andthe Gulf of Aden. The khamsin occursmost frequently in April, May, and June;the visibility is often so poor that lights aresometimes required at midday. Synopti-cally, the khamsin is a product of the ad-vection of warm air that occurs on theeastern limb of a low-pressure system thattracks either through the MediterraneanBasin or over North Africa from west toeast.

knot A unit of WIND speed; 1 knot = 1.15mph = 0.515 m s–1 = 1.85 km h–1. The knotis equivalent to one nautical mile per hour,the word being derived from the methodthat sailors used to estimate a ship’s speedin the days before accurate instruments be-came available. This consisted of a ropewith regularly spaced knots, attached to alog-line. The ship’s speed was proportionalto the number of knots that were run offthe reel in a fixed amount of time. The knotis used in weather observations and fore-casting and is still widely used in shippingand aviation.

kona storm /koh-nă/ The local name fora storm in the Hawaiian Islands (North Pa-cific Ocean), characterized by strongsoutherly winds and heavy rain associatedas a depression passes north of the islands.[Polynesian: ‘leeward’]

Köppen climate classification /kop-ĕn,kœp-/ An empiric CLIMATE CLASSIFICATION

system devised by the German botanist andclimatologist, Wladimir Peter Köppen(1846–1940), first published in 1900, andsubsequently revised and published in1918. Further revisions were made byKöppen throughout his life and it has alsobeen revised by other climatologists. The

classification was based on the major vege-tation zones and their climatic require-ments. The climates of the world aredivided into five major groups, each repre-sented by capital letters (A, B, C, D, and E)and defined by temperature, with the ex-ception of the B group, in which the con-trolling factor is dryness. Group A: tropicalmoist (rainy) climate with average temper-atures above 18°C (64.4°F) and no realwinter or summers seasons. Group B: dryclimates in which evaporation exceeds pre-cipitation. Group C: warm temperate rainyclimates with the coldest month between–3°C (26.6°F) and 18°C (64.4°F) and thewarmest month greater than 10°C (50°F).Group D: cold boreal forest climates withthe coldest month less than –3°C (26.6°F);in North America this is modified to 0°C(32°F), and the warmest month greaterthan 10°C (50°F). Group E: polar climates,which are subdivided into tundra climates(ET) with temperatures in the warmestmonth between 0°C (32°F) and 10°C(50°F) perpetual frost climate (EF), withtemperatures in the warmest month lessthan 0°C (32°F).

The major groups are further subdi-vided on the basis of seasonal precipitationand designated with the additional capitalor lower-case letters. These include: f – ab-sence of a dry season and adequate precip-itation in all months; s – a dry summerseason; w – a dry winter season; and m –monsoon climate with rain, except duringshort dry season (A group climates only).The arid climates (group B) are subdividedinto: S – semiarid (steppe type), and W –arid (desert). Further additions to the codeare lower-case letters designating tempera-ture, which include: a – warmest monthmean temperature is greater than 22°C(71.6°F); b – warmest month mean tem-perature is below 22°C (71.6°F); c – lessthan 4 months are over 10°C (50°F); d –coldest month is below –38°C (–36.4°F); h– mean annual temperature is greater than18°C (64.4°F); and k – mean annual tem-perature is less than 18°C (64.4°F). TheKöppen climate classification has beenwidely used, although it has been criti-cized. The main criticisms are that it ex-cludes other meteorological factors, such

khamsin

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as sunshine and wind, which are importantto vegetation, and the poor correlation be-tween the groups and actual vegetation dis-tribution in the world. See also Flohnclimate classification; Strahler climate clas-sification; Thornthwaite climate classifica-tion.

kosava /kos-ă-vă/ (kossava) A cold eastto southeast wind experienced in centraland eastern Europe. The kosava affectscountries, such as Bulgaria and Serbia, usu-ally blowing from the Aegean Sea region;for example, a kosava is a ravine windblowing on the Danube to the southeast ofBelgrade. See also canyon wind.

Kuroshio Current /koo-rosh-ee-oh/(Kurosiwo Current, Japan Current) Awarm current in the Pacific Ocean thatflows northeastward along the east coastof Japan. This current is a northward-flow-ing continuation of the Pacific north EQUA-TORIAL CURRENT between the Philippinesand the east coast of Japan. Dense fogtends to develop at its boundary with theOYASHIO CURRENT due to the temperaturecontrast produced. The relative warmthfrom the Kuroshio Current moderates theclimate of Taiwan and Japan as air iswarmed as it flows toward the mainland.

Kyoto Protocol /kee-oh-toh/ An inter-national agreement to reduce worldwideemissions of greenhouse gases, originally

formulated in 1997 at the Third Confer-ence of Parties (COP 3) to the UNITED NA-TIONS FRAMEWORK CONVENTION ON

CLIMATE CHANGE (UNFCC) in Kyoto,Japan. The agreement commits individualindustrialized nations to specified reduc-tions in six greenhouse gases or classes ofgases: CARBON DIOXIDE (the most promi-nent), METHANE, NITROUS OXIDE, HYDRO-FLUOROCARBONS, PERFLUOROCARBONS, andSULFUR HEXAFLUORIDE. In addition, the pro-tocol introduced the concept of emissionscredits and emissions trading. Before beingput into force, the Protocol required ratifi-cation by 55 countries, including those re-sponsible for at least 55% of the developedworld’s 1990 carbon dioxide emissions.This was achieved after ratification byRussia in November, 2004, and the Proto-col came into force on February 16, 2005.Those countries that have opted not to rat-ify the protocol include the US (estimatedto be responsible for 36.1% of carbonemissions in 1990, and which indicated in2001 that is was considering counter pro-posals), Australia, and Canada. The firstphase of the Protocol expires in 2012, thedeadline for emissions reductions to beachieved. Signatories to the Protocolagreed at the 11th Conference of the Par-ties to the UN Framework Convention onClimate Change in Montreal in December,2005, to extend the treaty beyond thisdate.

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Labrador Current A cold current in theATLANTIC OCEAN that flows south along theLabrador coast through the Grand Banks,where it splits to flow east and west. Theeastward-flowing current joins the NORTH

ATLANTIC DRIFT and the west branch flowsinto the Gulf of St Lawrence. The LabradorCurrent is a product of the convergence ofthe West Greenland and Baffin Island Cur-rents and inflow from Hudson Bay.

lacunosus /lak-yŭ-noh-sŭs/ A variety ofCUMULIFORM cloud in which the normallyrather thin sheets, layers, or patches ofcloud, most commonly CIRROCUMULUS andALTOCUMULUS but also STRATOCUMULUS,display a more-or-less regular scattering ofcircular holes, often with fringed or raggededges. The cloud ‘units’ and holes com-bined present a pattern like a honeycombor a net. See also cloud classification.

lake breeze A phenomenon particularlyevident on the Great Lakes of North Amer-ica (although lake breezes occur globally)in calm summer conditions, when the sur-face temperature of the lake is cool relativeto the land. A mesoscale anticyclone (high)develops over the lake; air consequentlyblows from the higher pressure over thelake to the lower pressure caused by heat-ing over the land. The pressure gradientcan sometimes be in excess of 2 hPa. Thebreeze’s leading edge, the lake breeze front,a zone of atmospheric convergence and in-creased cloud development, may be felt upto 40 km (25 miles) inland from the lake.

Lamb’s classification (Lamb catalog) A catalog of the daily weather types expe-rienced by the British Isles that was devel-oped by the British climatologist HubertHorace Lamb (1913–97). The catalog runs

from January 1, 1861 to February 3, 1997.The classification has eight directionaltypes (NE, E, SE, S, SW, W, NW, and N),two vorticity types (anticyclonic and cy-clonic), and an unclassifiable type.

Lamb’s dust veil index See dust veilindex.

laminar flow The very smooth flow ofthe air in which each particle follows ex-actly that of its predecessor. There is nomixing whatsoever of adjacent fluid.

land breeze The opposite of a SEA

BREEZE, the land breeze is a diurnal LOCAL

WIND that typically begins around mid-night, and is a product of the more rapidcooling of the land surface relative to thesea. This offshore wind is most developedin calm conditions around dawn in tropicaland mid-latitude locations when thegeostrophic flow is slack.

Landsat A US series of very high resolu-tion Earth-surface monitoring satellitesthat grew out of the Earth Resources Tech-nology Satellite 1 (ERTS–1) launched inJuly 1972 and renamed Landsat–1. Thecurrent version in the series is Landsat–7,launched in 1999, which images the Earthwith a MULTISPECTRAL SCANNER, sensing ineight narrow spectral BANDS. It orbits at aheight of 705 km (438 miles) with a periodof 99 minutes and, unlike operational ME-TEOROLOGICAL SATELLITES, has a repeatcycle (imaging the same region) of 16 days.

langley (ly) A unit of energy per unitarea, equivalent to 1 calorie per square cen-timeter. It is named for the American sci-entist Samuel Langley (1834–1906) of theSmithsonian Institute, a pioneer in solar ra-

L

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diation studies. The unit has been super-seded by the SI unit W m–2 internationallybut is still used in the US (1 langley = 698W m–2).

Langmuir theory of raindrop growth/lang-myoor/ A variant of the COLLI-SION–COALESCENCE PROCESS of raindropgrowth in clouds. Experiments by theAmerican scientist Irving Langmuir(1881–1957) revealed that larger waterdroplets with their greater terminal veloci-ties overtook and in some cases absorbedsmaller droplets.

La Niña /lah neen-yah/ A climatic fluc-tuation in the equatorial Pacific in whichthere is an upwelling of cooler than normalwaters off the coast of Peru and Ecuador(in contrast to EL NIÑO, which is character-ized by unusually warm waters). It may fol-low an El Niño event but occurs with lessfrequency. The cold current affects theweather in the middle latitudes of the west-ern Pacific Ocean and causes very hot sum-mers in Japan. Whereas El Niño tends toreverse the normal climatic conditions ofsome regions (e.g. wet places become dry,and dry become wet), La Niña enhancesnormal conditions, for example, wet placesbecome wetter. In the 1988–89 La Niñaevent, a build-up of exceptionally cold airin the Arctic produced North America’shighest atmospheric pressure reading of1075 hPa at Northway, Alaska. During the1995–96 event, Winnipeg, Canada, whichnormally experiences cold winters, hadday-time temperatures remaining below–20°C (–4°F) for over a week. Overnightlows of –30°C (–22°F) were experiencedfor 19 consecutive days. [Spanish: ‘the lit-tle girl’]

lapse rate The rate of change of a prop-erty (usually temperature) with height inthe atmosphere. The overall average lapserate is about 6.5°C km–1 up through thetroposphere but there are widespread vari-ations with height, season, and location. Itis described as positive when the tempera-ture decreases with height, and negativewhen the temperature increases withheight (see inversion). See also dry adia-

batic lapse rate; environmental lapse rate;saturated adiabatic lapse rate.

latent heat The amount of heat ab-sorbed or released in the change of phase ofa substance at constant temperature. Inmeteorology, latent heat is the amount ofheat absorbed or released in the transfor-mations between the three phases of water:ice, liquid water, and water vapor. Duringthe process of evaporation a change ofphase from liquid (water) to gas (watervapor) takes place. The energy required forthe transformation, the latent heat of va-porization, depends on temperature but isabout 2.4 × 106 J kg–1 at 0°C. This energy,unlike some other transformations is re-coverable through condensation in the at-mosphere. The latent heat of condensationis the energy released during condensationfrom water vapor to liquid water. The la-tent heat of fusion is the energy releasedwhen water freezes to ice, or absorbedwhen ice melts to water. The latent heat ofsublimation is the energy absorbed or re-leased when water vapor changes directlyto ice, or vice versa. Latent heat is trans-ported around the world and plays an im-portant role in maintaining the energybalance. Compare sensible heat.

latent heat flux The movement ofwater vapor (transporter of latent heat)from one location to another, for examplefrom the tropics toward the poles, wherethere is an energy deficit. Globally, pole-ward latent heat flux reaches a maximumof 1.5 x 1015 watts at 38° N and at 40° S.

latent instability A vertical tempera-ture profile (sounding) that is STABLE nearthe Earth’s surface but unstable above, typ-ically occurring at night. Although CON-VECTION is absent at the time of thesounding, it may be released during day-time hours as a result of surface heating.

latitude The angular distance of a pointon the Earth’s surface north or south of theEquator. (latitude 0°). It is a major factor,together with the tilt of the Earth on itsaxis (see axial tilt), in the maldistributionof solar radiation over the globe. Indi-

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rectly, it produces a deficit of 130 W m–2 atthe poles and a surplus of 30 W m–2 at theEquator. The climate system, through theGENERAL CIRCULATION, seeks to redress thisimbalance.

layer cloud See stratiform.

layered haze A distinct layer of hazecapped above and below by air with bettervisibility. Layering is associated with rela-tively thin slabs of STABLE air that may be afew hundred meters deep.

leader A preliminary electrical dischargethat precedes the formation of a LIGHTNING

channel, frequently propagating in stepsuntil, in the case of a CLOUD-TO-GROUND

DISCHARGE, it meets an upward-advancingstreamer. A leader is faintly luminous; it isnot associated with appreciable currentflow, but its rapid propagation is essentialto the formation of the final visible light-ning channel through which the majorityof the charge flows. See also dart leader.

lee See leeward.

lee depression (lee trough, orographic de-pression) A low-pressure feature thatforms preferentially to the lee (or down-wind) of a mountain barrier when air flowis at right angles to the barrier. The air col-umn is ‘squashed’ by vertical contractionas it crosses the barrier. After crossing, thecolumn of air expands to its original (up-stream) depth and, as it does so, tends todevelop a strong spin about the verticalaxis that is related to the conservation ofabsolute vorticity. The spin to the lee ofmountains is manifested as a low-pressurecenter. This process is linked to lows thatoccasionally form to the lee (east) of theRocky Mountains with westerly winds (seeAlberta clipper) and to the south of thewestern Alps under northerly flow.

lee trough See lee depression.

leeward (lee) The downwind side or sidefacing away from the wind. For example,the side of a range of mountains that issheltered from the PREVAILING WIND, such

as the Patagonian region in South America,which is located on the leeward side of theAndes, and hence is sheltered from themid-latitude westerlies.

lee wave (mountain wave, hill wave) Amechanically induced circulation compris-ing a stationary (standing) wave or wavesthat occurs above and on the LEEWARD sideof a mountain range or obstacle. Air flow-ing over the mountain or obstacle plungesdownward and upward in a series of crestsand troughs analogous to the standingwaves sometimes seen in a stream. Leewaves only develop in a stably stratified at-mosphere when there is a strong and con-stant horizontal wind flow of at least 15knots (17 mph). Their wavelength, whichcan be up to 40 km (25 miles), althoughtypically between 10 and 20 km (6 and 12miles), is only dependent on the atmos-phere and is independent of the orography,being calculated by means of theBrunt–Vaisala frequency equation. Leewaves may give rise to severe downslopewinds and FÖHN conditions. WAVE CLOUDS

form when the wave’s crest reaches thecloud condensation level.

Examples include the Sierra wave thatforms over the Sierra Nevada in Californiawhen a westerly airflow increases speedover the crest as a cold front approachesfrom the northwest; ‘stationary’ cloudsover the leeward valleys indicate the pres-ence of the lee wave. The helm wave ofCumbria, England, is another smaller-scaleexample of a lee wave.

lenticular cloud /len-tik-yŭ-ler/ Seelenticularis.

lenticularis /len-tik-yŭ-lar-is, -la-ris/ (len-ticular cloud) A type or species of cloudthat has the shape of a smooth lens or al-mond, horizontally aligned, and frequentlywith well-defined outlines. Lenticularisoccur quite often as WAVE CLOUDS in thecrests of LEE WAVES generated downstreamof hills and mountains – so can occur overa large region downwind of uplands. Theycan also occur in layers, sometimes resem-bling a stack of pancakes, in which the hu-midity, temperature, and wind profiles

layer cloud

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favor their development. ALTOCUMULUS

lenticularis is the most common type al-though the form can occur with CIRROCU-MULUS and STRATOCUMULUS too. See alsocloud classification.

leste /lest/ A dry hot southerly, south-easterly, or easterly wind that affects theCanary Islands, Madeira, and the NorthAfrican mainland. Often laden with dustfrom north-central African desert regions,the leste blows as a result of the pressuregradient established between high pressureto the east and an approaching area of lowpressure from the Atlantic. In the CanaryIslands, the word levanto is used specifi-cally for a hot dry southeasterly wind.

levanter /lĕ-van-ter/ (solano) An easterlywind that blows from the MediterraneanSea through the Straits of Gibraltar be-tween Spain and Morocco. It occurs mostfrequently between July and November.The levanter generates LEE WAVES and aBANNER CLOUD on the western flank of theRock of Gibraltar. As the wind velocity in-creases, this cloud lifts and disperses; undersuch exceptional easterly storm conditionsthe levanter is referred to as the levantades.

leveche /lĕ-vesh/ A hot dry south orsoutheast sand- and dust-laden SCIROCCO-

type wind that is experienced in southeast-ern parts of Spain, for example in the Ali-cante district, extending only a few milesinland. It originates over the deserts ofNorth Africa and Saudi Arabia. Its proper-ties and synoptic climatology are similar tothose of the LESTE wind, occurring mostfrequently during the summer.

libeccio /li-bech-ee-oh/ A LOCAL WIND

experienced in the central MediterraneanBasin. The libeccio blows from the west orsouthwest, advecting moist Atlantic airmasses into the region. Occurring most fre-quently during the winter, the libeccio canbring stormy conditions and even snowwhen the air stream is forced to ascend,such as when it approaches the west coastof Corsica.

lidar /lÿ-dar/ A remote-sensing tech-nique, similar to RADAR. It consists of atransmitter from which a laser pulse isemitted into the sky. The light particles(photons) scattered back (e.g. fromaerosols or water vapor within the atmos-phere) are received by a telescope and mea-sured at the receiver of the lidar instrument(see diagram). Lidar has a number of ap-plications in meteorology and climatology;these include cloud-base recorders; detect-ing and studying pollutants, aerosols, and

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lidar

laser transmitter

receiver telescope

computer

detector

laser light pulse

light scattering particles

Lidar atmospheric sensing

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water vapor within the atmosphere; andstudies of cloud physics (e.g. the airbornecloud physics lidar operated by NASA).[From: light detection and ranging]

lifetime See atmospheric lifetime.

lifted index (LI) A measure of atmos-pheric INSTABILITY. The lifted index is de-termined by calculating the temperaturethat air near the Earth’s surface wouldhave after it has been lifted to a higher level(usually the 500 hPa level) and then com-paring it to the actual temperature ob-served at that level. Negative lifted indexvalues indicate instability; positive valuesindicate STABILITY. High negative liftedindex values indicate a very UNSTABLE at-mosphere, often leading to strong updraftswithin any developing THUNDERSTORM. Seeelevated convection.

lifting condensation level The level inthe atmosphere at which an unsaturatedair parcel that is lifted at the DRY ADIABATIC

LAPSE RATE would become saturated.

lightning A transient electrical phenom-enon leading to substantial exchange ofcharge between two regions of the atmos-phere. A lightning path is typically kilome-ters long with considerable luminosityalong the path; it may generate THUNDER.Lightning is commonly associated withTHUNDERSTORMS and conveys charge fromthe cloud to the surface (CLOUD-TO-GROUND DISCHARGE), between clouds(CLOUD-TO-CLOUD DISCHARGE), to the air(CLOUD-TO-AIR DISCHARGE) and aboveclouds (see sprite). See ball lightning;beaded lightning; forked lightning; heatlightning; sheet lightning. See also light-ning conductor.

lightning conductor An electrical con-nection to an elevated spike mounted at thehighest point of a building, structure, ship,or aircraft intended to carry lightning cur-rents harmlessly or away from critical re-gions.

line scanner An instrument used in re-mote sensing in which a mirror is used to

sweep the ground surface normal to theflight path of the platform to produce animage. For example, it is used on-board theNOAA and TIROS meteorological satellites toproduce cloud imagery. As the satellite or-bits the Earth, the spinning mirror of theline scanner samples extremely rapidlyalong one line swiped at right angles to thetrack of the satellite. As it orbits, the rate ofspin of the mirror is organized in such away that the next scan line just touches itspredecessor so that a continuous image isbuilt up line-by-line. The line scanner onthese satellites produces 360 lines everyminute.

line source The linear origin of pollu-tants typically associated with transportroutes, such as roads (e.g. the emissionsfrom a line of traffic along a highway) andflight paths of aircraft. Compare areasource; point source.

line squall See squall line.

liquid-in-glass thermometer See ther-mometer.

Little Climatic Optimum See climaticoptimum.

little dry season A break in the summeror monsoon rains of the tropics and sub-tropics, sometimes known by local names(see veranillo). The poleward movement ofthe Intertropical Convergence Zone(ITCZ) is followed by the poleward migra-tion of the main rains in some parts ofAsia, Africa, and Central America. As thezenithal Sun begins its retreat back towardthe Equator, there may be a noticeable less-ening in the rains along the 10°–15° N lat-itude.

Little Ice Age A period between about1550 and 1850 AD during which the tem-peratures in Europe, North America, andAsia were cooler than at present and con-ditions were harsher. During this periodthere was increased glaciation aroundmountainous areas, such as the Alps andSierra Nevada, as well as Alaska. Rivers,such as the Thames in London, froze over.

lifetime

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local climate The climate of a smallarea as distinguished from the climate ofthe surrounding area. See also microcli-mate.

local storm report (LSR) In the US, astorm report produced by local NationalWeather Service (NWS) offices to informusers of reports of severe and/or significantweather-related events.

local wind A wind that affects a limitedgeographical area and is usually seasonallyspecific. There are two main causes of localwinds: (1) A regional wind-flow patternthat is modified by the disposition of relief,an example being the MISTRAL. (2) TheFÖHN effect resulting from the adiabaticcompression of descending air to the LEE-WARD of a mountain range, one examplebeing the CHINOOK. Despite having similargeneric origins, a vast plethora of namesexist for local winds dependent on the re-gional setting.

lofting In pollution studies, a pattern offlow that occurs when the top of a plumefrom a chimney stack disperses intoslightly turbulent or neutral airflow condi-tions, while the lower part of the plume isprevented from dispersing down towardthe surface by a stable boundary layer, es-pecially at night.

long-range weather forecastingWeather predictions that are for periodssignificantly longer than those handled bythe MEDIUM-RANGE FORECAST. It usuallyrefers to periods of one month or longerahead, overlapping into the realm of SEA-SONAL PREDICTION.

long wave (Rossby wave) A very large-scale wave on an upper-air isobaric analy-sis of the middle and upper TROPOSPHERE.Long waves consist of a sequence of RIDGES

and TROUGHS, with a wavelength that istypically a few thousand kilometers. Theystretch right around the Earth, principallyin middle latitudes, with typically four orfive waves around a hemisphere. Longwaves are strongly linked to surfaceweather patterns and the POLAR FRONT JET

STREAMS are located along the central axisof the waves in the upper troposphere.

longwave radiation (terrestrial radi-ation, infrared radiation) ELECTROMAG-NETIC RADIATION, in the INFRARED part ofthe spectrum, that is emitted from the sur-face of the Earth and the atmosphere in theabsolute temperature range 200–300 K. Ithas a maximum intensity at about 10 µmwith a range of 3–100 µm. Compare short-wave radiation.

low A usually extensive region, fromseveral hundred to several thousand kilo-meters across, of relatively low barometricpressure, linked to cyclonic inflow of air,its large-scale ascent, and often cloudy andwet conditions. See also cyclone.

low cloud In CLOUD CLASSIFICATION,cloud with a base not more than 2000 m(6500 ft) above the surface. The class in-cludes CUMULUS, CUMULONIMBUS, STRATUS,and STRATOCUMULUS.

low-latitude climates The group offive climates of the low-latitude regions ofthe world identified in the STRAHLER CLI-MATE CLASSIFICATION: wet equatorial cli-mate, trade wind littoral climate, tropicaldesert and steppe climates, west-coastdesert climate, and tropical wet-dry cli-mate.

low-level jet A maximum of wind speedin the lower TROPOSPHERE, associated witha marked horizontal temperature gradient.Examples occur just ahead of some strongsurface COLD FRONTS and along the coast ofSomalia in northeast Africa. This latter So-mali Jet is a component part of the summerASIAN MONSOON; it turns from southeast-erly, to southerly, to a southwesterly cur-rent along its axis as it blows across theArabian Sea toward the Indian subconti-nent. Its JET MAXIMUM lies about 1 km (0.6mile) above the surface and although theJET STREAM width is narrow, it is responsi-ble for about one half of the air’s masstransported across the entire Equator intothe northern (summer) hemisphere.

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low-precipitation storm (LP storm) ATHUNDERSTORM, occurring along or near adry line, that seldom produces significantprecipitation. These storms have a promi-nent bell-shaped main tower and are capa-ble of producing tornadoes and very largehail and much lightning. They commonlyoccur over the High Plains of the US. Com-pare high-precipitation storm.

low-pressure system See cyclone.

low Sun The time of year, usually win-ter, when the elevation of the Sun at noon(solar time) is relatively low in the sky and

when the receipt of radiation is likely to fallbelow about 4 MJ m–2day.

luminous night cloud See noctilucentcloud.

ly See langley.

lysimeter /lÿ-sim-ĕ-ter/ An instrumentused to study aspects of the hydrologiccycle, including the assessment of EVAPO-TRANSPIRATION losses from soils and thesoil moisture content. It consists of a suit-able container placed below ground sur-face to intercept and collect water movingdown through the soil.

low-precipitation storm

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mackerel sky A complete or very exten-sive cover of high ALTOCUMULUS or CIR-ROCUMULUS cloud, with ripples and patchesseparated by small areas of blue sky, thatresembles the pattern on the skin of amackerel. It applies particularly to thespecies of cloud VERTEBRATUS, but the termis commonly applied to other clouds thatexhibit similar patterns.

macroburst /mak-roh-berst/ See down-burst.

macroclimate /mak-roh-klÿ-mit/ Theclimate of an extensive geographic region,such as part or all of a continent, hundredsto thousands of kilometers across. Com-pare mesoclimate; microclimate.

macroclimatology The study ofMACROCLIMATES, i.e. the climates of exten-sive regions.

macrometeorology /mak-roh-mee-tee-ŏ-rol-ŏ-jee/ The study of large-scale meteo-rological processes, such as those occurringover a large region, continent, or the wholeEarth. These include monsoons, long-wavetroughs, and the general circulation of theatmosphere.

macroscale /mak-roh-skayl/ Denotingthe largest-scale areas or processes in mete-orology, from an extensive region to conti-nental to global. Compare mesoscale;microscale.

maestro A cold northwesterly wind thataffects the central Mediterranean Basin. Itis experienced in the Tyrrhenian, Adriatic,and Ionian seas, and along the west coastof Corsica and Sardinia. It blows when alow-pressure system is located to the east

over the Balkan Peninsula, the result of thisbeing that this region is no longer in thewarm sector of the low.

magnetopause /mag-nee-toh-pawz/ Theboundary that surrounds the MAGNETOS-PHERE, separating the flow of the solarwind from the charged particles of themagnetosphere.

magnetosheath /mag-nee-tŏ-sheeth/ Theintervening layer between the MAGNE-TOPAUSE and the shock front caused by thesolar wind.

magnetosphere /mag-nee-tŏ-sfeer/ Theregion of the atmosphere, encompassingthe EXOSPHERE and most of the IONOSPHERE

in which the Earth’s magnetic field influ-ences the motion of ionized particles. Themagnetic field acts as a shield for the Earthdeflecting and preventing solar winds fromreaching the surface. The shape of the mag-netosphere is distorted by the influence ofthe solar wind: on the daylight side facingthe Sun the magnetic field lines are com-pressed by the solar wind; on the night side(away from the Sun) they are elongated toform a long magnetotail.

Mai-yu season /may-yoo, mÿ-yoo/ (Bai-useason) The time that the ‘plum rains’occur across much of central China, afterthe cessation of the northeast trade airflowfrom the Pacific Ocean. The rains tend tooccur once the southwest airstream has be-come established. By mid-June the warmhumid air lies across much of centralChina. Small disturbances produce heavyrainfall of a non-frontal nature, these arecalled the plum rains, or Mai-yu (Mei-yu),since they arrive as the plums are ripening.

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Weekly rainfall may exceed 60 mm (2.36in).

Maloja wind /mah-loy-ah/ A warmföhn-type wind that blows down the UpperEngadine valley by day from the summit ofthe Maloja Pass in Switzerland; at night thewind blows up valley.

mamma /mah-mă/ (mammatus) A sup-plementary cloud feature consisting ofhanging rounded udder-like protuberanceson the underside of another cloud, perhapsmost commonly observed on the undersideof laterally spreading anvils on top of CU-MULONIMBUS clouds. Their appearance canbe dramatic when illuminated from belowby the setting Sun. Mamma clouds alsooccur beneath ALTOCUMULUS, ALTOSTRATUS,STRATOCUMULUS, CIRRUS, and CIRROCUMU-LUS. See also cloud classification.

mammatus /mă-mat-ŭs/ See mamma.

manometer /mŏ-nom-ĕ-ter/ An instru-ment consisting of a liquid column gaugeused for measuring pressure differences.The MERCURY BAROMETER, in which mer-cury is balanced against atmospheric pres-sure, is a type of manometer.

mares’ tails A common name for a typeof CIRRUS in the form of elongated wispystreaks.

marin /ma-rin/ A warm SCIROCCO-typewind that blows across the Gulf of Lionsinto southeastern France. Usually oppres-sive, it can be accompanied by overcastconditions and heavy rain.

marine climate See marine west coastclimate; maritime climate.

marine meteorology The study of theatmospheric phenomena over ocean andcoastal areas and the interaction of the at-mosphere and fluid surface of the ocean. Itincludes wind and wave measurement,wind systems, boundary-layer processes,mesoscale coastal phenomena, sea breezes,and tropical cyclones.

marine west coast climate The climateof the west coastal regions of the conti-nents, between 35° and 60° N and S. Pre-cipitation is abundant all year with awinter maximum, and prevailing westerliesand cyclonic storms can occur throughoutthe year. Annual temperature ranges arelow with mild winters and generally mod-erate summers. In western Europe these cli-mates can extend inland into centralEurope; in North and South America, Aus-tralia, and New Zealand mountain rangesaligned generally north–south confine theclimates to the coastal margins. The cli-mate is designated Cfb and Cfc in the KÖP-PEN CLIMATE CLASSIFICATION.

maritime air An AIR MASS that is gener-ally moist and mild or warm from itsSOURCE REGION over an ocean, or throughmodification after passage across an oceansurface.

maritime climate (marine climate) Theclimate of a region that is strongly influ-enced by its location in relation to theocean. The moderating influence of theocean gives these climates their characteris-tically low diurnal and annual temperaturerange, while the availability of moisture isresponsible for comparatively high precip-itation and humidity.

maritime polar air mass (mP) A cooland moist AIR MASS that has swept acrossmany hundreds to a few thousand kilome-ters of middle- or higher-latitude oceanfrom a cold continental SOURCE REGION.The process of flowing across an oceanafter leaving the source region means thatthese air masses are strongly influenced byhuge SENSIBLE HEAT and LATENT HEAT

FLUXES.

maritime tropical air mass (mT) Awarm and moist AIR MASS that originatesover the subtropical oceans beneath theSUBTROPICAL ANTICYCLONES, out of whichthe air spirals toward higher and lower lat-itudes. Maritime tropical air tends to beladen with stratiform cloud, and bringsmild and humid conditions followed gener-

Maloja wind

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ally by light precipitation by the time it ar-rives in middle latitudes.

marsh gas See methane.

Maunder minimum /mawn-der/ A pe-riod of relative solar inactivity from 1645to 1715 during which it is believed therewere fewer sunspots than have occurredafter this period, according to evidence putforward in the 1890s by the British as-tronomer E. Walter Maunder (1851–1928). It coincided with the period ofcolder than average temperatures of theLITTLE ICE AGE. Evidence suggests that otherperiods of similar inactivity have occurredin the more distant past.

maximum and minimum thermome-ter A THERMOMETER used to measureboth maximum and minimum tempera-tures over a given time period. See alsomaximum thermometer; minimum ther-mometer.

maximum temperature The highesttemperature achieved during a specific pe-riod, typically over 24 hours. It is usuallyrecorded with a standard MAXIMUM THER-MOMETER.

maximum thermometer A THER-MOMETER designed to record the highesttemperature reached over a given time pe-riod between settings. This is usually amercury-in-glass thermometer in which aconstriction above the bulb allows the mer-cury to expand and flow past it on expan-sion with a rise in temperature but stops itflowing back in the opposite directionupon contraction. Resetting is carried outby shaking the thermometer vigorouslyuntil the mercury is below the constriction.See also maximum and minimum ther-mometer; minimum thermometer.

mb See millibar.

MCC See mesoscale convective com-plex.

MCS See mesoscale convective system.

mean sea level (MSL) The average levelof the surface of the sea for all stages of thetide. The level of the sea is constantlychanging through tidal oscillations, waves,swell, and atmospheric pressure changes.The mean sea level is calculated from ob-served heights taken at hourly intervals,generally over a 19-year period, usingmethods that take account of the fluctua-tions in the surface level. The world meansea level is calculated from many observa-tions made globally and can change in re-lation to eustatic (see eustasy) changes andalso changes in the height of land surfaces(isostatic changes).

mean-sea-level pressure The value ofATMOSPHERIC PRESSURE after it has been re-duced from its station value (i.e. the actualobservation) to the common datum ofmean-sea-level. This eliminates the verymarked influence of elevation on the valueof recorded pressure; near sea level thepressure decreases by about 1 hPa for every10 meters of ascent.

mean temperature The average tem-perature taken over a specified period. Forexample; mean daily temperature is the av-erage temperature observed at regular timeintervals throughout the solar day.

mediocris /med-ee-ok-riss/ A species ofCUMULUS cloud showing moderate verticalextent and small protuberances on thetops. See also cloud classification.

Mediterranean climate A climatic typethat is distinguished by its unique precipi-tation cycle, with hot dry summers andmild moist winters. It occurs around lati-tude 30°–45°N and S along the west coastsof continents and poleward of the easternside of the subtropical anticyclones (highs).During the summer the poleward migra-tion of the anticyclones into the Mediter-ranean climate regions brings drycontinental tropical air. In winter the cli-mate is influenced by westerly airflowbringing moist air, cyclonic storms, andabundant rainfall. Total annual rainfallamounts vary considerably depending on

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the location, decreasing over continentallocations and toward the tropics.

The largest extent of this climate type isaround the Mediterranean Sea, in Europe,extending from Portugal and Morocco inthe west to Eurasia in the east. Mediter-ranean climates also occur in central andsouthern California in North America; inthe southern hemisphere it occurs alongthe coast of Chile, South Africa, and south-western Australia. It is denoted by Csa andCsb in the KÖPPEN CLIMATE CLASSIFICATION.

Mediterranean front That part of thePOLAR FRONT when located over theMediterranean–Caspian Sea areas in win-ter. It separates Mediterranean air to thenorth from the continental tropical airflowing northeastward from the Sahara inthe south. Temperatures may vary by asmuch as 12–16°C across the front in latewinter.

Mediterranean water A region of deepdense water in the Mediterranean. It isformed by the excessive evaporation ofsurface water and the resultant instability,which culminates in the sinking andspreading out of water at the bottom of theMediterranean Basin. Mediterraneanwater also overflows across the submarinesill at the Straits of Gibraltar where it sinksto depths of about 1000 m (3000 ft) andspreads out on the bottom of the AtlanticOcean.

medium-range weather forecastingThe process of predicting the weather on aglobal scale from three to seven or even upto ten days ahead. The EUROPEAN CENTRE

FOR MEDIUM-RANGE WEATHER FORECASTS

(ECMWF), for example, specializes in thisperiod; other centers, including the US NA-TIONAL WEATHER SERVICE (NWS), fulfil thesame role.

megathermal climate /meg-ă-ther-măl/ A climate type with high temperatures. Inthe KÖPPEN CLIMATE CLASSIFICATION it is theA group of climates (humid subtropical ortropical), with monthly mean temperaturesabove 18°C (16.4°F). The same name is

also used in the THORNTHWAITE CLIMATE

CLASSIFICATION.

meltemi /mel-tay-mee/ See etesian wind.

melting band See bright band.

melting level In the atmosphere, thelevel at which the air temperature is 0°C(32°F) and ice crystals and snowflakesbegin to melt as they fall. See also brightband.

melting point The temperature atwhich a substance changes from a solid toa liquid. The melting point of pure water is0°C (32°F, 273.15 K) at standard atmos-pheric pressure. Compare freezing point.

mercury barometer (mercurial barome-ter) A type of BAROMETER used for mea-suring atmospheric pressure. It consists ofa vertical column of mercury contained ina glass tube that has been evacuated andsealed at the top and marked with a scale.The weight of the mercury in the tube isbalanced by the atmospheric pressure act-ing on the exposed cistern of mercury atthe base of the column, the height of thecolumn varying with changes in atmos-pheric pressure. Readings from the mer-cury barometer need to be corrected totake into account air temperature and localvalue of the acceleration of free fall and ad-justed to mean-sea-level. Although thistype of barometer is still widely used it isbeing superseded in many weather stationsby solid-state barometers.

The mercury barometer, historically thefirst instrument used to measure atmos-pheric pressure, was invented in 1643 bythe Italian mathematician and physicistEvangelista Torricelli (1608–47). See alsoFortin barometer; Kew barometer.

mercury-in-glass thermometer A THER-MOMETER containing mercury, which actsas a heat-measure medium. The mercury iscontained within a glass bulb or reservoirattached to a thin tube within a glasssheath. With temperature changes the mer-cury expands as it is heated or contracts asit is cooled and moves up or down the at-

Mediterranean front

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tached tube. Changes in the height of themercury column can be read from a gradu-ated scale etched onto the side of thethermometer. The mercury-in-glass ther-mometer is unsuitable for recording excep-tionally low temperatures because ofmercury’s freezing point of –38.9°C(–38°F); a thermometer containing alco-hol, which has a freezing point of–114.4°C (–174°F), is used instead torecord very low temperatures.

meridional circulation /mĕ-rid-ee-ŏ-năl/ The component of the large-scale at-mospheric motion that is parallel tomeridians or lines of longitude and thusshows large north–south movement. Com-pare zonal flow.

meridional temperature gradient Thetemperature gradient in the direction of thegeographic meridian, for example the dif-ference in temperature between a locationin the north and one on the Equator.

mesoclimate /mess-ŏ-klÿ-mit, mee-/ Theclimate of a small geographical area cre-ated, for example, by differences in thevegetation cover (e.g. forest, crops), differ-ences in elevation (e.g. upland areas), dif-ferences in slope aspect (e.g. in valleys),water bodies, or urban areas. Such climatesoccur on the scale of tens to hundreds ofkilometers and within the broader exten-sive climate regions of the MACROCLIMATES.Compare microclimate.

mesocyclone /mess-ŏ-sÿ-klohn, mee-/ A cyclonic circulation consisting of a col-umn of rapidly rotating air associated withsome deep convective storms. The mesocy-clone, which is a feature of all SUPERCELL

thunderstorms, occurs within the cumu-lonimbus cloud and is usually up to 10 km(6 miles) in diameter. A TORNADO may de-velop from the mesocyclone’s rotation andextend below the cloud to the ground.

mesohigh /mess-ŏ-hÿ, mee-/ (mesoscalehigh) A MESOSCALE high-pressure featurethat is associated with the cold dense cur-rent of air that flows out of a THUNDER-STORM as a DOWNDRAFT. The cold air

radiates horizontally away from under-neath a thunderstorm, leading to a suddenjump in surface pressure (apart from thetemperature drop and sudden onset ofgusty winds).

mesolow /mess-ŏ-loh, mee-/ (mesoscalelow, subsynoptic low) A MESOSCALE low-pressure region.

mesometeorology /mess-ŏ-mee-tee-ŏ-rol-ŏ-jee, mee-/ The study of middle-scaleweather and atmospheric phenomena,such as tropical cyclones and polar lows,thunderstorms, mesoscale convective sys-tems (MCS), and tornadoes. These featuresare intermediate between microscale andsynoptic scale or macroscale. Comparemacrometeorology; micrometeorology.

mesonet /mess-ŏ-net, mee-/ A networkof environmental monitoring stations de-signed to measure the environment at thesize and duration of mesoscale weatherevents, i.e. at the scale of few kilometers toseveral hundred kilometers and of a fewminutes to several hours duration.

mesopause /mess-ŏ-pawz, mee-/ Theboundary that lies about 80 km (50 miles)above the Earth’s surface and caps theMESOSPHERE, at which point temperaturesstop decreasing.

mesoscale /mess-ŏ-skayl, mee-/ Denot-ing meteorological phenomena or weathersystems that range in size from about 80 toseveral hundred km (50 to several hundredmiles) horizontally. Such features includesquall lines, mesoscale convective com-plexes, and mesoscale convective systems.They are smaller than the SYNOPTIC-SCALE

weather systems but larger than MI-CROSCALE. Mesoscale is sometimes appliedto smaller-scale systems, for example, be-tween 10 and 100 km (6–60 miles). Seealso macroscale.

mesoscale convective complex (MCC)A large and persistent MESOSCALE CONVEC-TIVE SYSTEM, which establishes inertiallystable circulations and does not readilydecay. It is classified in terms of the areal

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extent of the cloud shield as observed oninfrared satellite imagery: the area of cloudtop with temperature below –32°C mustexceed 100,000 km2; that with tempera-ture below –52°C must exceed 50,000km2. In addition, it must last at least 6hours.

mesoscale convective system (MCS) A deep convective system of a number ofthunderstorms that can extend for up toaround 500 km (300 miles) in horizontalextent and last for 6–12 hours. It is distin-guished in synoptic observations by anextensive middle- or upper-level strati-form/cirrus shield. There is frequentCLOUD-TO-CLOUD LIGHTNING within the sys-tem. See also mesoscale convective com-plex.

mesoscale system (mesosystem) Seemesoscale; mesoscale convective complex;mesoscale convective system.

mesosphere /mess-ŏ-sfeer, mee-/ Theatmospheric layer that lies above theSTRATOSPHERE, separated from it by theSTRATOPAUSE at about 40–50 km (25–30miles) above the Earth’s surface, and belowthe thermosphere, from which it is sepa-rated by the MESOPAUSE at approximately80 km (50 miles) above the Earth’s surface.It is the coldest atmospheric layer withtemperatures falling rapidly with height toabout –100°C at 80 km (50 miles). This fallis a result of the lack of water vapor, dust,and ozone, which absorb incoming solarradiation.

mesothermal climate /mess-ŏ-ther-măl,mee-/ A climate type with moderatetemperatures. In the KÖPPEN CLIMATE CLAS-SIFICATION it is the C group of climates(warm temperate rainy climates), in whichthe monthly mean temperature for thecoldest month is between –3°C (27°F) and18°C (64°F) and for the warmest month isgreater than 10°C (50°F). The same nameis used in the THORNTHWAITE CLIMATE CLAS-SIFICATION.

METAR A special SYNOPTIC CODE usedfor routine weather reports from airfields

and airports that are made quickly avail-able to aviation.

METEOR A long-lived series of Russ-ian near polar-orbiting meteorologicalsatellites.

meteoric water Water on the surface ofthe Earth that is derived from atmosphericPRECIPITATION.

meteorological computer model /mee-tee-ŏ-rŏ-loj-ă-kăl/ A computerized modelused to simulate certain meteorologicalprocesses.

Meteorological Operational SatelliteSee METOP.

meteorological satellite (weather satel-lite) An artificial satellite in orbit aroundthe Earth, most commonly in either aPOLAR ORBIT or a GEOSTATIONARY ORBIT,with the specific operational aims that in-clude mapping cloud distribution and sea-surface temperature, and providing verticaltemperature and humidity profiles togetherwith cloud motion vectors.

Meteorological Service of Canada Afederal organization, a part of ENVIRON-MENT CANADA, that undertakes day-to-dayweather prediction and research into fu-ture climate. It also monitors water and airquality, and the extent of sea ice aroundCanadian waters.

meteorology /mee-tee-ŏ-rol-ŏ-jee/ Thestudy and science of all the phenomena andprocesses of the atmosphere, and the appli-cation of such knowledge to the forecast-ing of weather. It includes the physical andchemical processes that take place withinthe atmosphere, the motions (dynamics),and the interactions of the atmospherewith the oceans and surface of the Earth.See also climatology; dynamic meteorol-ogy; physical meteorology; synoptic mete-orology; weather forecasting.

Meteosat /mee-tee-ŏ-sat/ The Europeancommunity geostationary satellite series,operated by EUMETSAT. A Meteosat is posi-

mesoscale convective system

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tioned above Europe and Africa at approx-imately 0° longitude as one of the opera-tional meteorological GEOSTATIONARY

SATELLITES (currently five or six) stationedabove the Equator at 0° to provide cover-age of the whole Earth. The first satellite ofthe series, Meteosat–1, was launched bythe European Space Agency (ESA) in 1977.The Meteosats scan using the wavelengthsof: visible (0.5–0.9 µm); thermal infrared(10.5–12.5 µm); and water vapor (5.7–7.1µm).

The Meteosat Second Generation(MSG) series came into operation duringthe early years of the 21st century. TheMSG system is to comprise four geosta-tionary meteorological satellites, the firstof which, Meteostat-8, was launched in2002, followed by the launch of the second(MSG-2), to be renamed Meteostat-9 afterits test phase, in 2005. The satellites carrya radiometer – the Spinning Enhanced Vis-ible and InfraRed Imager (SEVIRI) – andthe Geostationary Earth Radiation Budget(GERB) instrument.

methane (CH4, marsh gas) A colorless,odorless, and flammable hydrocarbon gas,which is a TRACE GAS in the Earth’s atmos-phere. Emissions of methane are producedthrough the venting of natural gas and pro-duction of petroleum, decomposition oflandfill and animal waste, animal diges-tion, rice cultivation, and coal production.Methane is a potent GREENHOUSE GAS, andconcentrations within the atmosphere havebeen rising at the rate of about 0.6% peryear.

methane clathrate (methane hydrate) A CLATHRATE in which a water–ice latticetraps METHANE gas molecules. Deposits ofmethane clathrate occur on or below seafloors on the continental shelf at depths ofover 300 m (1000 ft) where there is suffi-cient pressure, temperatures are near 0°C(32°F), and there is the presence of or-ganic-rich marine sediments to formmethane. Deposits of methane clathrateare also extensive below ground in Arcticpermafrost regions such as those of NorthAmerica and Siberia. Studies of polar-icecores have revealed that during times of cli-

mate change from glacial to interglacialconditions rapid rises in levels of methane(which is also a potent greenhouse gas)have occurred. This methane may havebeen released from methane clathrate melt-ing in the warmer temperatures. Marineand permafrost deposits of methaneclathrate are recognized as potentialsources of fossil fuel.

methyl bromide (CH3Br) An odorlesscolorless gas that has been widely used as apesticide and fumigant to control a numberof pests. It contributes to ozone depletionin the stratosphere and has been includedin the list of substances to be phased outunder the requirements of the MontrealProtocol: by 2005 in the developed coun-tries and by 2015 in developing countries.

Met Office The national weather ser-vice in the UK, based in Exeter (formerybased in Bracknell until 2004). The organi-zation can be traced back to 1854, when itoriginated as a small department of theBoard of Trade. Its role has expanded re-cently from one focusing primarily on theweather to a broader service that includesthe impact of weather on the environmentwith interests in the other environmentalsciences, such as hydrology and oceanogra-phy.

MetOp (Meteorological OperationalSatellite) A new series of polar-orbitingmeteorological satellites under develop-ment, which will carry instruments for theEUROPEAN SPACE AGENCY (ESA), EUMETSAT,NOAA, and the French space agency(CNES). The first satellite of the series,MetOp-A, is scheduled for launch in 2006.The satellite series is intended to eventuallyform part of a NOAA–EUMETSAT sys-tem.

microburst /mÿ-kroh-berst/ A narrowDOWNBURST that is either dry or wet. Mi-crobursts appear in association with shaftsof precipitation that fall from the base ofCUMULONIMBUS clouds. Microbursts are ofshort duration (less than five minutes) andaffect a small area of less than 4 km (2.5miles). A wet microburst occurs when the

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heavy rainfall drags a column of air downwith it, hitting the surface and spreadingout horizontally as a very gusty current ofair. A dry microburst is one that is com-mon across the High Plains in the US,where the cloud base is high and the air be-tween the base and the ground is normallydry. In these conditions, the rain evapo-rates before reaching the surface, so cool-ing the air it falls through. This leads to amicroburst that also splays out sidewayson reaching the ground. Microbursts areparticularly hazardous to aircraft, espe-cially when landing or taking off.

microclimate /mÿ-kroh-klÿ-mit/ Theclimate of a small distinct area, usually inthe lowest layer of the atmosphere abovethe ground surface, in which features of thesurface, such as vegetation and buildings,influence temperature, humidity, andwind. Different microclimates may occurat different layers within a plant canopy, oron either side of a building, for example.Such climates typically occur at scales ofless than 100 meters. Compare macrocli-mate; mesoclimate.

microclimatology /mÿ-kroh-klÿ-mă-tol-ŏ-jee/ The study of a MICROCLIMATE, en-compassing temperature, moisture andwind profiles within the lowest level of theatmosphere, including the effects of vegeta-tion, soil moisture content, and urbaniza-tion.

micrometeorology /mÿ-kroh-mee-tee-ŏ-rol-ŏ-jee/ The study of small-scale atmos-pheric phenomena that occur in the lowestlevels of the atmosphere, in the PLANETARY

BOUNDARY LAYER; it includes the processesof exchanges of heat, moisture, and mo-mentum with the surface of the Earth; theSURFACE BOUNDARY LAYER; and TURBU-LENCE. Compare macrometeorology; me-someteorology.

micrometer /mÿ-krom-ĕ-ter/ (µµm) TheSI unit of length equal to 10–6 meter (for-merly known as the micron). It is encoun-tered in meteorology, for example, in thediameters and radii of aerosols, water

droplets, and particles, and in the wave-lengths of electromagnetic radiation.

micron /mÿ-kron/ A former name forthe MICROMETER.

microscale /mÿ-kroh-skayl/ Denotingthe smallest scale of meteorological phe-nomena, ranging in size from a few cen-timeters up to several kilometers. The termis also applied to small-scale phenomena ofshort duration of a few minutes. Comparemacroscale; mesoscale.

microthermal climate /mÿ-krŏ-ther-măl/A climate type with low temperatures,short summers, and long cold winters. Inthe KÖPPEN CLIMATE CLASSIFICATION it is theD group of climates (cold boreal forest cli-mate), with mean monthly winter tempera-tures of less than –3°C (27°F). The samename is also used in the THORNTHWAITE

CLIMATE CLASSIFICATION.

microwave sounding unit (MSU) Seeadvanced microwave sounding unit.

middle clouds In CLOUD CLASSIFICATION,cloud with a base between approximately2000 and 7000 m (6500 and 23,000 ft)above the surface. These values vary sea-sonally and latitudinally such that they arehigher in summer and in low latitudes. Theclass includes ALTOCUMULUS, ALTOSTRATUS,and NIMBOSTRATUS.

middle-latitude steppe and desert cli-mate The climates of the cool desertsand steppe in North and South Americaand Asia, which are located within the con-tinental interiors far from sources of mois-ture and often bounded by mountainbarriers on the windward side. Cooldeserts extend from about 35° latitude asfar north as 50° latitude N and S; the coolsteppes extend nearly to 60°N in Canada.Winters are cold with little precipitation;summer precipitation tends to be cyclonic.The climates are denoted by the letters BSkand BWk in the KÖPPEN CLIMATE CLASSIFI-CATION.

mid-latitude climate The group of five

microclimate

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climates of the middle-latitude regions ofthe world identified in the STRAHLER CLI-MATE CLASSIFICATION: humid subtropical,marine west-coast, Mediterranean, mid-latitude desert and steppe, and humid con-tinental, which are controlled by bothtropical and polar air masses.

mid-latitude cyclone See cyclone.

mid-latitude depression See cyclone.

mid-level cooling Localized cooling ofair in the middle TROPOSPHERE between2400 and 7500 m (8000 and 25,000 ft).This cooling process is able to destabilizethe entire atmosphere, for example when amid-level cold pool approaches a region.

midnight Sun The phenomenon thatoccurs in the polar regions, as a result ofthe Earth’s tilt about its axis, in which theSun remains visible above the horizon forover 24 hours. The Sun remains above thehorizon from about March 20 throughSeptember 23 at the North Pole, and fromabout September 23 through March 20 atthe South Pole. The length of time forwhich the midnight Sun occurs decreasestoward the Arctic and Antarctic Circles.Along the Arctic Circle (66° 32′N) the Sunremains above the horizon on June 21, thenorthern hemisphere summer solstice;along the Antarctic Circle (66° 32′S) thisoccurs on December 22, the southernhemisphere summer solstice.

Mie scattering A theory of the SCATTER-ING of ELECTROMAGNETIC RADIATION byparticles in the atmosphere, proposed bythe German physicist Gustav Mie(1868–1957) in 1908. The degree of scat-tering of direct radiation depends uponparticle size. Whenever particles of a simi-lar size to the wavelength of the incidentradiation are present, Mie scattering willoccur; this is predominantly in the forwarddirection in all wavelengths, especially in aturbid (dusty) atmosphere. This produces agrayish sky as no wavelengths in the visibleband are preferentially scattered. The par-ticles causing Mie scattering are sphericaland include water vapor, dust, smoke, and

pollen. It occurs primarily in the lower at-mosphere See also non-selective scattering;Rayleigh scattering.

Milankovitch cycles /mi-lank-ŏ-vich/ (as-tronomical theory of climate change) Atheory of the cycle of long-term climatechange, and the ICE AGES, correlated withthe distribution of insolation that is deter-mined by variations in the Earth’s orbitaround the Sun. The theory was suggestedduring the mid 1800s and subsequentlyformulated and published in 1920 by theSerbian astronomer and mathematicianMilutin Milankovitch (1879–1958).

There are three main variables that in-fluence the amount and distribution of thesolar radiation received by the Earth: (1)Changes in the tilt of the Earth’s axis,which affects the severity of seasons. Thetilt or inclination varies between about21.8° and 24.4° on a cycle of some 41,000years. (2) The eccentricity of the Earth’sorbit about the Sun, which varies on aprimary periodicity of approximately100,000 years and a secondary periodicityof 400,000 years. (3) Changes in theEarth’s axis of rotation, which affects thetiming of the equinoxes – the precession ofthe eqinoxes – and the timing of the APHE-LION and PERIHELION, and occurs on a pe-riod of approximately 22,000 years. Thisalters the timing of the aphelion and peri-helion in relation to the seasons. The com-binations of these cyclical variations affectthe severity of the seasons.

millibar (mb) The unit of ATMOSPHERIC

PRESSURE that is frequently used by meteo-rologists. The millibar is defined as onethousandth (10–3) of one BAR. A pressure of1 mb is equivalent to 0.7501 mm of mer-cury. The millibar has been largely re-placed in scientific work by theHECTOPASCAL (hPa) but remains in generaluse for weather forecasting. 1 mb is equalto 1 hPa.

minimum temperature The lowesttemperature achieved during a specifiedperiod of time, typically over a 24-hour pe-riod. It is usually recorded with a standardMINIMUM THERMOMETER within an instru-

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ment shelter; additional measurements ofminimum temperatures are made over con-crete, over soil, and over grass.

minimum thermometer A THERMOME-TER used for measuring the lowest temper-ature attained over a given time period(usually a day). These thermometers areusually alcohol-in-glass thermometers; thealcohol has a lower freezing point of–114.4°C (–174°F) compared to that of–38.9°C (–38°F) for mercury. An indexmarker is contained within the thin bore ofthe thermometer. As the alcohol cools andcontracts the marker is carried down by thealcohol’s meniscus toward the bulb of thethermometer, but as it warms and expandsthe marker remains unmoved. The end ofthe marker furthest from the bulb indicatesthe lowest temperature reached since it waslast reset. The thermometer is reset by tilt-ing it until the marker is returned to posi-tion at the meniscus. See also maximumand minimum thermometer; maximumthermometer.

mirage An optical effect of the lowestatmosphere in which images of objects ap-pear displaced from their actual positionand/or distorted as a result of the differen-tial refraction of light rays passing fromone density of air to another. A range of ef-fects occurs according to the variations indensity of the air, which are determined bytemperature and humidity. An inferior mi-rage occurs when the image appears belowits actual position. This results when the airclosest to the ground surface is stronglyheated, for example, above the surface ofan asphalt road on a hot day or above adesert surface. Light rays near the groundsurface are strongly refracted upward andproduce the visual effect of shimmeringpools of water. A superior mirage occurswhen a layer of warm air overlies a muchcooler layer directly above the ground sur-face, as occurs when warmer air flowsabove an ice surface. The light rays arestrongly refracted downward resulting inthe effect that the image appears to beabove its actual position. In polar regions,for example, distant objects may appear to

hang above the snow or ice surface. Seealso fata morgana.

mist A condition in which the horizontalvisibility is reduced by small suspendedwater droplets or wetted hygroscopic nu-clei such that it is greater than 1000 m(3000 ft) but less than 2000 m (6500 ft); ifthe visibility is less than 1000 m it is de-fined as FOG. The obscuring particles havetypical diameters of a few tens of microm-eters. See also Scotch mist.

mistbow See See fogbow.

mistral /mis-trahl/ A strong cold dryLOCAL WIND that can be experienced alongthe Mediterranean coast from Barcelona toGenoa, although it is most frequently ob-served in the Rhône delta region. It occursmost often during the winter season, forexample, Marseille, France, is subject tothe mistral on 100 days in an average year.Synoptically, the mistral blows as a resultof the pressure gradient established by highpressure over central Europe and/or thewestern Mediterranean and a deep low-pressure area situated in the Gulf of Lionsor Gulf of Genoa. The resulting north tonorthwesterly wind is funneled down theRhône Valley between the Massif Centraland Alpine ranges, transferring cold polarmaritime air southward. The mistral oftenattains speeds at the surface in excess of 30knots (35 mph), sometimes being strength-ened by KATABATIC effects; the sky is typi-cally devoid of clouds at the time and thewind can reduce the temperature to below0°C (32°F). WINDBREAKs, such as thickhedges of cypress trees, have been plantedthroughout the region to protect crops, or-chards, and gardens from the damagingforce and coldness of the mistral. Occa-sionally the mistral has crossed theMediterranean Sea and been felt in suchNorth African countries as Tunisia.

mixed cloud A cloud composed of bothliquid water droplets and ice crystals. AL-TOSTRATUS, CUMULONIMBUS, and NIMBO-STRATUS are examples.

mixed layer (turbulent boundary layer)

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That part of the PLANETARY BOUNDARY

LAYER that is dominated by turbulent diffu-sion caused by eddies generated by frictionwith the surface and thermals arising fromsurface heat sources (see eddy diffusion).Surface heating during the day and the ab-sence of temperature inversions allow com-ponents of the air within the planetaryboundary layer to exhibit mainly randomvertical movements. Such movements maybecome more organized into gusts of windand dust devils during the afternoon. De-spite being random, the turbulent move-ments allow the transfer of atmosphericproperties, such as heat, water vapor, mo-mentum, and air pollutants, from the nearsurface up through the planetary boundarylayer.

mixing condensation level The lowestlevel at which condensation occurs whentwo air masses possessing different initialtemperature and DEW-POINT profiles aremixed together. The process of verticalmixing has the effect of averaging the prop-erties of the two original air masses, result-ing in over-saturation and thereforecondensation.

mixing depth (mixing layer) The verti-cal extent of the atmospheric layer near theEarth’s surface in which convection andturbulence are pronounced, causing themixing of air and any pollutants present.

mixing fog See evaporation fog.

mixing ratio (humidity mixing ratio) Ameasurement of atmospheric moisturecontent, usually expressed as the dimen-sionless ratio of mass of WATER VAPOR in aparcel of air to the unit mass of dry air(typically grams of water vapor per kilo-gram of dry air).

mizzle /miz-ăl/ See Scotch mist.

mock Moon See paraselene.

mock Sun See parhelion.

model A representation of a system orprocess. A tool used to simulate or forecast

the behavior of the atmosphere–ocean sys-tem. Such models are used to produceweather forecasts from hours to days orlong-term climate conditions over periodsof years to thousands of years. Examples ofmodels include CLIMATE MODELS, ENERGY

BALANCE MODELS, GENERAL CIRCULATION

MODELS, RADIATIVE-CONVECTIVE MODELS,and STATISTICAL-DYNAMICAL MODELS.

moist adiabatic lapse rate See satu-rated adiabatic lapse rate.

moisture advection The predomi-nantly horizontal movement of atmos-pheric water vapor from one location toanother. This water vapor flux is impor-tant for maintaining a location’s moisturebudget, a rapid flux being required if sub-stantially high precipitation totals are to begenerated.

moisture index A measure of the an-nual water balance for an area, incorpo-rated in a new CLIMATE CLASSIFICATION

emphasizing the concept of EVAPOTRANSPI-RATION that was devised by the Americanclimatologist, Charles Warren Thornth-waite (1899–1963), in 1948. Slightly re-vised in 1955, the moisture index includedevapotranspiration and is expressed by theformula:

Im = 100 × (S – D)/PE,where Im is the moisture index, S is thewater surplus in months when precipita-tion exceeds evapotranspiration, D is thewater deficit in months when evapotran-spiration exceeds precipitation, and PE ispotential evaporation. It has a value of zerowhen precipitation is equal to potentialevapotranspiration; a positive value indi-cates a moist climate, and a negative valueindicates a dry climate. See also Thornth-waite climate classification.

monsoon The seasonal change in winddirection associated with differential heat-ing of land and sea areas. The word, fromthe Arabic mawsim, season, was originallyapplied to the seasonal reversal of winds inthe Arabian Sea that blow for about sixmonths from the northeast and six monthsfrom the southwest. The monsoon is asso-

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ciated with the seasonal changes that takeplace in the pressure systems together withthe positions of the upper wind patternsand jet streams. In winter the intense high-pressure system, the Siberian anticyclone,is located over central Asia, and winds flowout from this (the northeast monsoon).During summer it is absent, low-pressure iscentered over the Middle East (the Asiaticlow), and the southwest monsoon is domi-nant. The Indian subcontinent is especiallyaffected by the monsoon but monsoonsalso occur in Africa, northern Australiaand to a limited extent in the southwesternUS. The word is now more generally usedin the tropics and subtropics for any regionaffected by a reversal of wind, particularlywhen accompanied by copious amounts ofrainfall. See also African monsoon; Asianmonsoon. See also burst of the monsoon.

monsoon climate See tropical mon-soon and trade-wind littoral climate.

monsoon current The humid south-west flow of air that crosses the southernArabian Sea and heralds the onset of thesouthwest MONSOON in India. Theairstream usually reaches Kerala state inearly June, and proceeds northeast towardCalcutta, before moving erratically north-west up the Indo-Gangetic Plain along thefoothills of the Himalayas.

monsoon depression A low-pressuresystem that normally forms in the Bay ofBengal north of 18°N latitude and moveswest-northwest across the central andnorthern parts of India producing wide-spread rains, especially in the southern sec-tor of the traveling low. Between five andseven every year form in this way. Thesedepressions rarely develop into devastatingcyclonic storms.

monsoon trough An elongated area oflow pressure positioned on averageENE–WNW across the plains of northIndia during the summer MONSOON season.It may be regarded as the most northerlyposition of the INTERTROPICAL CONVER-GENCE ZONE (ITCZ) in India and marks theapproximate line along which the majority

of MONSOON DEPRESSIONS travel duringJune to late September.

month degrees In climate classifica-tions, a parameter that is sometimes usedto indicate the conditions suitable for veg-etation growth, in which the excess ofmean monthly temperatures greater than6°C (43°F) are summed and used as accu-mulated temperature.

Montreal Protocol (Montreal Protocolon Substances that Deplete the OzoneLayer) The international agreementdrawn up in Montreal in 1987 by theUnited Nations Environment Program(UNEP), and ratified by most of the indus-trialized nations in 1989, under whichozone-depleting substances are regulated.The original agreement sought a 50% re-duction in the use of chlorofluorocarbons(CFCs) by 1992, relative to 1986 levels.Subsequent amendments expanded the listof substances to be regulated, and broughtforward the dates for these to be phasedout. The Protocol uses several interna-tional bodies to report on the science ofozone depletion and provide informationfor policy development. See also KyotoProtocol.

morning glory A very long spectacularnarrow ROLL CLOUD that sweeps acrosspart of the Gulf of Carpentaria in northernAustralia. It can stretch up to 1000 km(600 miles) in length and to 3000 m(10,000 ft) in height, and may sometimesbe as low as 50 m (160 ft). It is orientedNNW–SSE and advances from east to westat speeds of 25–45 knots (30–50 mph). Themorning glory brings squall-like winds andfine mist, but little precipitation. One the-ory for its formation is that it is somehowrelated to the double sea breeze penetra-tion that occasionally occurs from eithercoast of the Cape York peninsula on theeastern flank of the Gulf of Carpentaria.

mother cloud The type of cloud fromwhich another one develops. For example,anvil cirrus can be associated, as a mothercloud, with MAMMA.

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mother-of-pearl cloud See nacreouscloud.

mountain breeze See mountain wind.

mountain gap wind See canyon wind.

mountain meteorology The study ofatmospheric phenomena in mountainousregions and areas of complex terrain. Thescale of the phenomena and processes varyfrom a few meters to thousands of kilome-ters. It includes gravity waves, convectioncells, and mesoscale mountain circulations.

mountain wave See lee wave.

mountain wind (mountain breeze) AKATABATIC WIND that is most developed invalley systems during the middle of thenight when it flows down from the moun-tains, filling the valley in the late night andearly morning period.

Compare valley wind.

Mozambique Current /moh-zam-beek/ A warm west Indian Ocean cur-rent, that flows between Madagascar andAfrica, influencing the climate of the islandand the mainland. South of Madagascar,the Mozambique Current feeds into theAGULHAS CURRENT.

MSL See mean sea level.

multicell storm A thunderstorm that

contains more than one cell, i.e. more thanone updraft and downdraft system. Theremay be a number of updrafts and down-drafts within the storm in close proximity.See also squall line.

multiple-vortex tornado (multivortextornado) A TORNADO that possesses twoor more condensation funnels or debrisclouds.

multispectral imaging The process ofremotely sensing a natural scene from asatellite or a planetary surface in a largenumber of narrow wavebands. Each BAND

is normally selected to map a different fea-ture from other bands. See also remotesensing.

multispectral satellite imagery A setof simultaneous satellite images of thesame scene that are recorded in differentnarrow wavebands, usually to stress thenature of Earth surface features.

multispectral scanner An on-boardsatellite instrument that is capable of imag-ing the Earth in a number of narrow wave-bands simultaneously, to build up aline-by-line image of the surface. LANDSAT,for example, uses such a scanner.

mutatus /myoo-tah-tŭs/ Denoting aMOTHER CLOUD changed in such a way thatthe entire cloud is affected.

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nacreous cloud (mother-of-pearl cloud)A rare kind of cloud that occurs in thestratosphere, mainly at high latitudes, andat elevations around 24 km (15 miles).They are normally of lenticular form andexhibit brilliant IRIDESCENCE at angleswithin some 40° of the Sun soon after sun-set or before sunrise. They appear to beorographic in origin and are likely to beforced by flow over mountains that pro-duces wavy undulations at great heights.Their formation is thought to require atemperature of –80°C to –90°C (–112°F to–130°F) at cloud level.

nadir /nay-der/ The point on the celestialsphere diametrically opposite to theZENITH.

NAO See North Atlantic Oscillation.

National Ambient Air Quality Stan-dards (NAAQS) In the US, the targetsset for an acceptable concentration of spe-cific pollutants in air. Under the 1970Clean Air Act and amendments, nationalPrimary Ambient Air Quality Standards,set by the Environmental ProtectionAgency (EPA), define levels of air qualitydesigned with an adequate margin of pub-lic health safety. The EPA is required to setprimary standards for the major air pollu-tants at levels that are necessary to protectthe public health with an adequate marginof safety. Secondary Ambient Air QualityStandards are defined with a view to pro-tecting the public welfare from any knownor anticipated adverse effects of a pollu-tant.

National Centers for EnvironmentalPrediction (NCEP) A US organization,part of the NATIONAL OCEANIC AND ATMOS-

PHERIC ADMINISTRATION’s (NOAA’s) Na-tional Weather Service, that is made up ofnine national centers with the aim of pro-viding timely and accurate forecasts. Thecenters include the TROPICAL PREDICTION

CENTER, STORM PREDICTION CENTER, andCLIMATE PREDICTION CENTER, as well as oth-ers, such as the Aviation Weather Center inKansas City, Missouri. It was establishedin 1958 as part of the former National Me-teorological Center.

National Environmental Satellite,Data, and Information Service (NES-DIS) In the US, an organization that ispart of the NATIONAL OCEANIC AND ATMOS-PHERIC ADMINISTRATION (NOAA); it man-ages its environmental and operationalweather satellite program, including theprocessing, storage, and distribution of thehuge volume of data provided by the satel-lite network.

National Fire Danger Rating System(NFDRS) A method of rating the poten-tial danger of FIRE for large areas across theUS. Operational, since 1972, this systemuses daily fire weather observations andforecasts as well as integrating and inter-preting seasonal weather trends to deter-mine the risk of fire.

National Hurricane Center See Trop-ical Prediction Center.

National Oceanic and AtmosphericAdministration (NOAA) A major or-ganization, within the US Department ofCommerce. Its role is to conduct researchand to gather data about the global oceans,atmosphere, space, and the Sun. It aims todescribe and predict changes in the Earth’senvironment and to conserve and wisely

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manage coastal resources. Two of its manyconstituent services are the NATIONAL

WEATHER SERVICE (NWS) and the NATIONAL

ENVIRONMENTAL SATELLITE, DATA, AND IN-FORMATION SERVICE (NESDIS).

National Severe Storms Laboratory(NSSL) In the US, a research laboratoryin Norman, Oklahoma, sponsored by theNATIONAL OCEANIC AND ATMOSPHERIC AD-MINISTRATION (NOAA); it has the role of‘enhancing NOAA’s capabilities to provideaccurate and timely forecasts and warningsof hazardous weather events (e.g. bliz-zards, ice storms, flash floods, tornadoes,lightning, etc.)’. Its work includes under-taking research to improve understandingof weather processes associated with suchsevere phenomena, as well as improvingthe techniques of prediction and warning.

National Weather Service (NWS) TheUS operational weather service, forming apart of the NATIONAL OCEANIC AND ATMOS-PHERIC ADMINISTRATION (NOAA); it ismade up of a headquarters, national cen-ters, and regional offices, which togetherprovide expertise in the fields of meteorol-ogy, hydrology, and climate science. TheUS regions and their HQ offices are:Alaska (Anchorage), Pacific (Honolulu),Western (Salt Lake City), Central (KansasCity), Southern (Fort Worth) and Eastern(Bohemia). There are also 13 regional riverforecast offices. The five centers, includingthe STORM PREDICTION CENTER and theTROPICAL PREDICTION CENTER have the roleof delivering analysis and forecast productsfor all regional offices and other US gov-ernment agencies.

natural hazard A natural process orevent that causes harm to people and/orproperty. Natural hazards connected withthe weather and climate include drought,fire, floods, tropical cyclones, tornadoes,severe thunderstorms, and snow or icestorms. These, however, only constitutenatural hazards if they pose a threat, risk,danger, or peril to human activity, other-wise they are seen as extreme naturalevents.

natural-siphon rainfall recorder Seetipping-bucket rain gauge.

NCEP See National Centers for Envi-ronmental Prediction.

nebulosus /neb-yŭ-loh-sŭs/ A type orspecies of cloud that appears as a nebulouslayer or veil with no distinct detail. It ap-plies only to STRATUS and CIRROSTRATUS.See also cloud classification.

negative energy See convective inhibi-tion.

negative tilt trough The angular dis-placement of a trough line from a meridianin which the axis of the trough is rotatedclockwise from the meridian.

neoglacial /nee-oh-glay-shăl/ A series ofrelatively small glacier readvances duringthe last few years of the Holocene geologicepoch.

nephanalysis /nef-ă-nal-ă-sis/ A tech-nique to produce an analysis of cloud typefrom studying satellite cloud images orsynoptic charts. The subjective method isno longer used, though there are ways toassess the type and height of cloud auto-matically from such data.

nephoscope /nef-ŏ-skohp/ An instru-ment used to measure the speed and direc-tion of cloud movement relative to a pointon the ground directly below it.

neritic /ni-rit-ik/ Denoting the region ofshallow water lying directly above the con-tinental shelf, usually less than 200 m (660ft) deep.

NESDIS See National EnvironmentalSatellite, Data, and Information Service.

net radiation (Q*, net all-waveradiation) The net value of downward in-coming shortwave and upward outgoinglongwave radiation fluxes at the Earth–at-mosphere interface. This is the most im-portant surface energy exchangemeasurement or parameter since it repre-

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sents the limit to the available energy ei-ther as source or as sink. For a daytimesurface energy budget:

Q* = (Kdown – Kup) + (Ldown – Lup) =K* + L*,

where Q* is net (all-wave) radiation, K isshortwave radiation, and L is longwave ra-diation, down is incoming radiation andup is outgoing radiation. At night K*equals zero. See also radiation budget.

net radiometer An instrument thatmeasures net upward and downward long-and short-wave radiation flux; i.e. the dif-ference in intensity between radiation en-tering and leaving the Earth’s surface.

neutral stability (indifferent equilibrium)A state of atmospheric STABILITY when it isneither STABLE nor UNSTABLE. It occurs ei-ther when the ENVIRONMENTAL LAPSE RATE

(ELR) of a layer of unsaturated air is equalto the DRY ADIABATIC LAPSE RATE (DALR),or when the environmental lapse rate of asaturated air layer is the same as the SATU-RATED ADIABATIC LAPSE RATE (SALR). If anair parcel is perturbed in a neutral stabilitystate, it will remain stationary, neithermoving up or down. Neutral stabilityrarely occurs in the atmosphere, usuallyonly prevailing for short periods of time.

nevados /nĕ-vah-dohs/ A local name foran intensely cold KATABATIC WIND experi-enced in the valleys of Ecuador, SouthAmerica. The nevados is caused partly bynocturnal cooling of air by radiation andalso by its contact with snow and ice overthe Andean mountain range.

névé /nay-vay/ See firn.

NEXRAD (Next Generation WeatherRadar) A network of DOPPLER RADARS,operated by the NATIONAL WEATHER SER-VICE (NWS) that virtually covers the wholeof the contiguous US. The radar providesinstantaneous maps, which are producedevery 5–6 minutes, of the accurate locationand intensity of most PRECIPITATION withinabout 120 km (75 miles) of the system, andof intense precipitation within some 200

km (124 miles). Additionally, the radialcomponent of the wind toward or awayfrom the radar is mapped. [From: next gen-eration radar]

Next Generation Weather Radar SeeNEXRAD.

nightglow The weak radiance that ispresent in the sky at night, excluding lightfrom the Moon and stars, that results fromthe from radiation emitted by atmosphericreactions occurring in ionized particles.

nimbostratus /nim-boh-stray-tŭs/ Oneof the cloud genera (see cloud classifica-tion); a type of LOW CLOUD that may extendinto the middle levels and from which pre-cipitation falls. It is often a deep grayragged sheet or layer of cloud that obscuresthe Sun. It is the typical cloud on a very wetdull day and is associated with virtuallycontinuous light to moderate precipitationof rain or snow. Low, ragged FRACTUS

clouds may form in the SUBCLOUD LAYER. Itusually follows ALTOSTRATUS in the se-quence of clouds ahead of an approachingWARM FRONT.

Nimbus /nim-bŭs/ A series of Sun-syn-chronous METEOROLOGICAL SATELLITES thatwere operated by NASA, the first of which(Nimbus 1) was launched in 1964 and thelast (Nimbus 7) in 1978. The series wasone of second-generation meteorologicalsatellites, following on from the originalTIROS series. See also POES.

nitrogen (N) The most abundant gas inthe Earth’s atmosphere constituting ap-proximately 78.08% of the volume of dryAIR; it is colorless and odorless.

nitrogen cycle The global-scale ex-change of nitrogen and its compounds be-tween different reservoirs. Nitrogen gas inthe atmosphere finds its way into the soil inthe form of nitrates as a result of nitrogenfixation by bacteria and by lightning. Fromthe soil it passes to plants in protein andother molecules and thence to animals,who consume the plants. From dead plantsand animals it returns by nitrification back

net radiometer

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into the soil, where some of it returns to theatmosphere as a result of the activities ofdenitrifying bacteria.

nitrogen oxides Gases consisting of ni-trogen combined with oxygen, formed dur-ing the high-temperature combustion offossil fuels. The two principal products ofthis are nitrogen dioxide (NO2) and nitricoxide (NO). Their concentration in urbanareas is typically ten to a hundred timesthat in the rural areas because they are pro-duced, for example, by motor vehicles andelectric power stations. Nitrogen dioxidecan react with water vapor to produce ni-tric acid, adding to the ACID RAIN problem;if its concentration is great enough, it canproduce a reddish-brown tinge to the skyin dry conditions.

nitrous oxide (N2O) A colorless TRACE

GAS within the atmosphere that acts as apowerful GREENHOUSE GAS. It is producedby the burning of vegetation, agriculturalpractices (such as the use of commercialand organic fertilizers), combustion of fos-sil fuels, and industrial emissions in nitricacid production.

NOAA See National Oceanic and At-mospheric Administration.

NOAA meteorological satellite SeeGOES; POES.

noctilucent cloud /nok-tă-loo-sĕnt/ Lit-erally, clouds that are bright at night,against a black sky. These rare very high-altitude clouds are observed occasionallypoleward of about 50° latitude around themidnight hours in the summertime whenthey appear with a bluish-white glow. InNorth America they may be seen from mid-Canada and further north. They occur atan altitude of 80–85 km (50–53 miles) andmove extremely rapidly at between100–300 knots (115–350 mph). They arecomposed of minute ice particles with di-ameters of 0.1–0.3 µm, possibly composedof water that has entered the atmosphereon meteoric dust.

nocturnal radiation Radiative ex-

changes during the hours of darkness; sincethere is no solar radiation available this in-volves only longwave radiation. Nocturnalradiation represents the excess radiationemitted by the Earth’s surface at night overthat received by the Earth’s surface fromthe atmosphere (clouds and greenhousegases). It is indicated by negative values inNET RADIATION.

non-attainment level The reducedlevel of emissions or of ambient pollutionconcentration aimed for but that is un-likely to be achieved given current eco-nomic policies or best available techniquesnot entailing excessive costs (BATNEEC).

non-frontal depression A synoptic-scale low-pressure system that is not asso-ciated with fronts and therefore has nosignificant AIR MASS changes within its cir-culation. A HEAT LOW and a CUT-OFF LOW

are examples of non-frontal lows.

non-selective scattering The SCATTER-ING of photons of ELECTROMAGNETIC RADI-ATION, usually by molecules and particlesin the atmosphere that are larger than thewavelength of the incident radiation, forexample water droplets and dust particles.This scattering takes place in all directionsand may be upward or downward with re-spect to the Earth’s surface, and primary,secondary, or tertiary. See diffuse radia-tion. See also Mie scattering; Rayleigh scat-tering.

nor’easter (northeaster) A cyclonic stormthat affects the east coast of North Amer-ica, especially during winter, with maxi-mum intensities near New England. Itdevelops in the lower middle latitudes,30°–40°N, some 160 km (100 miles) offthe coast. Wind gusts can be in excess of 64knots (i.e. hurricane force) and given itsNorth Atlantic source region, a nor’easteris usually cold, generally moist in charac-ter, and is frequently accompanied bymuch cloud bringing precipitation in theform of heavy snow and/or rain.

normal In meteorology and climatol-ogy, the average value of an element (e.g.

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temperature or precipitation), over a givenperiod. The timescale of 30 years is themost frequently used.

norte /nor-tay/ (el norte) 1. An outbreakof cold polar air, a continuation of theNorth American NORTHER, experiencedduring the winter in Central America caus-ing a drop in temperature of the order of6–9°C. It is particularly evident on the nar-row 250-km (150-mile) wide Isthmus ofTehuantepec in southeast Mexico, wheresuch a wind can reach GALE force and blowincessantly throughout the winter season.It is generally accompanied by heavy rain,but the airflow becomes markedly drier onthe Pacific side of the Isthmus.2. A cold northerly wind that affects east-ern Spain during the winter, blowing whenhigh pressure is resident over the IberianMesata.3. A warm and humid northerly wind ex-perienced in Argentina.

North Atlantic Drift (North AtlanticCurrent) The relatively warm currentthat flows northeastward in the North At-lantic Ocean from approximately 40°N,45°W off the Grand Banks of Newfound-land, Canada, toward Europe. The NorthAtlantic Drift is frequently called the GULF

STREAM but is more correctly an extensionof this latter current. Off the coast of theBritish Isles it splits, the southerly currentbecomes the CANARIES CURRENT and thenortherly current flows along the coast ofnorthwestern Europe.

North Atlantic Oscillation (NAO) Alarge-scale oscillation in the pressure sys-tems over the North Atlantic that particu-larly affects winter weather, especially ofEurope. The oscillation centers on the rela-tionship between two pressure systems: thelow-pressure system located over Green-land and the high-pressure system locatedover the North Atlantic west of the Azores,between about 35° and 40°N. During theNorth Atlantic Oscillation’s positive (high)index phase, the ICELANDIC LOW across thehigh latitudes of the North Atlantic isdeeper than normal, while the subtropicalanticyclone (high) over the central North

Atlantic – the AZORES HIGH – is strongerthan normal. As a result of the increasedpressure difference, the winter storms arestrong and take a more northerly track.Milder wetter conditions with above nor-mal temperatures occur across northernEurope and the eastern US; cold dry win-ters occur over Canada and Greenland.Below normal precipitation occurs acrosssouthern and central Europe. Duringstrong negative (low) index phases of theNorth Atlantic Oscillation, the Azores highand the Icelandic low are weak and there isa correspondingly reduced pressure gradi-ent. The storms follow a more west–easttrack bringing moist air across to southernEurope, the Mediterranean, and into theMiddle East, while Greenland is milderand a northerly and easterly flow of Arcticair brings cold winter conditions to north-west Europe. The winter North AtlanticOscillation tends to show interannual andinterdecadal variability. See also El NiñoSouthern Oscillation.

northeaster See nor’easter.

northeast trade winds (northeasttrades) The markedly persistent andstrong TRADE WINDS that are best developedacross the subtropical oceans of the north-ern hemisphere. They flow toward the IN-TERTROPICAL CONVERGENCE ZONE (ITCZ)and are most pronounced between 5° and15°N. They flow smoothly toward theSOUTHEAST TRADE WINDS into this major re-gion of CONVERGENCE and associated as-cent in the ITCZ.

North Equatorial Current See Equa-torial Current.

norther /nor-ther/ A northerly wind thatcauses a sudden plunge in temperature inthe southern US, sometimes of the order of20°C in only 24 hours. It is caused by polarair, which sweeps southward on the west-ern side of a low-pressure system. Thenorther can lead to very squally conditions,and is often accompanied by hail andthunderstorms. Wind speeds may exceed60 knots (70 mph) and it is a hazard forfarmers and growers. The norther is not al-

norte

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ways moist, however. In the Central Valleyof California, for instance, descent of theairflow from the mountains often producesa dry and sometimes dusty wind. In light oftheir similarities, the terms norther andNORTE are sometimes used interchange-ably.

northern lights See aurora.

North Pacific Current (North PacificDrift) A warm ocean current that flowseastward across the North Pacific Ocean,as an extension of the KUROSHIO CURRENT.

North Pacific high (North Pacific anticy-clone, Hawaiian high, Hawaiian anticy-clone) The SUBTROPICAL ANTICYCLONE

over the North Pacific Ocean, often in thevicinity of the Hawaiian Islands and be-yond. During the winter it is weak andshrinking and migrates to lower latitudesaround 20°N, in summer the anticycloneintensifies, expands, and migrates north-ward to around 40°N.

nor’wester (northwester) 1. A moderateto strong northwesterly wind that bringscool to cold conditions to regions east ofthe North American Rocky Mountains.2. A warm, dry, and often strong FÖHN-type wind that descends the LEEWARD

slopes of the Southern Alps in South Island,New Zealand, to affect regions to the south and east, such as the CanterburyPlains.3. A type of squall, often generating violentthunderstorms, that affects regions ofNorthern India, including Assam and Ben-gal, and Myanmar (Burma). It usually oc-curs during March, April, May, and Juneand accounts for the majority of the rain-fall in these regions during this season.4. A GALE-force northwesterly wind thatoften affects the Cape Region of SouthAfrica, especially during the winter.

nowcasting /now-kas-ting/ A modernmethod of predicting the weather fromsome few minutes to up to about six hoursahead. Special schemes have been devel-oped for such very short-term prediction,involving the use of radar observations of

precipitation, satellite cloud and watervapor images, wind profilers, and rela-tively dense and frequent surface weatherobservations (in some areas, such as theOklahoma Mesonet). One thrust of thenowcasting method is to draw togetherthese various types of data into a user-friendly display system for forecasters, en-abling them to monitor very frequently theevolution of SYNOPTIC and smaller-scalefeatures. The essence of the method is,however, a basic extrapolation of pre-ex-isting features of the meteorological fields.

NSSL See National Severe Storms Labo-ratory.

nuclear winter A scenario in which aseries of nuclear explosions raises dust intothe TROPOSPHERE and lower STRATOSPHERE,leading to long periods of dense overcastskies with very little light penetrating to theEarth’s surface. The thermal structure ofthe atmosphere would be completely al-tered by such an event.

nucleation The process by which achange of state is initiated in a substance:from a supercooled liquid to a solid (e.g.supercooled water to ice) or from a super-saturated solution or vapor to a solid. It isan important mechanism in cloud physicsfor the formation of ice crystals from su-percooled water droplets, and waterdroplets from supersaturated water vapor.See also heterogeneous nucleation; homo-geneous nucleation.

nucleus (plural nuclei) In meteorology,a tiny particle suspended in the atmos-phere. The smallest and most plentiful par-ticles are known as AITKEN NUCLEI and havea radius of less than 0.1 µm. Large nucleihave radii of 0.2–1 µm and the largest, thegiant nuclei, have radii of over 1 µm. Seealso cloud condensation nucleus; freezingnucleus.

numerical modeling A MODEL that usesmathematical techniques to represent andinvestigate physical processes. Such modelsare usually computer simulations, al-though they may range from simplistic bal-

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numerical weather prediction

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ance models (e.g. estimating the input andoutput of radiation through the atmos-phere) or sophisticated GENERAL CIRCULA-TION MODELS.

numerical weather prediction (NWP,numerical weather forecasting) Themethod, using complex computer models,of forecasting the evolution of the atmos-phere and weather over periods that varyfrom less than one hour to some twoweeks. It is numerical because the proce-dure involves the mathematical processingof an extremely large number of meteoro-logical measures in order to predict theweather. The pioneer of numerical weatherprediction was the British mathematicianand metereologist Lewis Fry Richardson(1881–1953) who, during World War I,undertook the first ever mathematically

based prediction of a pressure field acrossparts of western Europe. He founded thesystem of assimilating scattered valuesonto a regular network of latitude/longi-tude grid points, which formed the essen-tial framework for calculating futurechanges of temperature, wind, etc. Modernnumerical weather forecasting featuresfeeding a large range of different observa-tions into the world’s fastest computers inorder to calculate the forecast as quickly aspossible.

nutrient cycle See biogeochemicalcycle.

NWP See numerical weather prediction.

NWS See National Weather Service.

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objective analysis A critical techniqueused in WEATHER FORECASTING that pro-duces the best possible fields of meteoro-logical variables given that the originalobservations are unevenly scatteredaround the globe. It is a completely auto-matic mathematical method that is used toset the initial conditions from which theweather forecast is run. As part of theprocess, erroneous observations are elimi-nated from these starting conditions.

obscuration (obscured sky cover) Anatmospheric condition in which visibilityof the sky is prevented by a weather phe-nomenon at the surface, such as fog.

observations (weather observations) Awide range of meteorological measure-ments made at the surface by eye or byusing instruments every hour, on the hour,at busy airports for example (see stationmodel). UPPER-AIR STATIONS provide rou-tinely timed observations from the RA-DIOSONDES they release, while other datafrom commercial aircraft and POLAR-ORBIT-ING SATELLITES are produced at asynoptictimes (i.e. not exactly on the hour).Weather RADAR and satellites in GEOSYN-CHRONOUS ORBITS also observe aspects ofthe weather at routine times.

occluded front /ŏ-kloo-did/ (occlusion) A complex front that occurs during thelater stage of the evolution of a FRONTAL

CYCLONE (frontal depression). It indicatesthat the warm, MARITIME TROPICAL air atthe surface in the WARM SECTOR has beenlifted away from the surface into the lowerand/or middle TROPOSPHERE by the colderdenser air of the cold front. At the surface,the front separates polar air on either sideof it and is often a region of deep cumu-

lonimbus cloud and precipitation, some-times with fierce squalls and hail and thun-der. Warm occlusions are those withrelatively cooler air ahead of the warmfront compared to the cold air behind thecold front, while cold occlusions exhibitthe opposite thermal contrasts. See alsocold front; warm front.

occluded mesocyclone A MESOCY-CLONE in which air from the rear-flankdowndraft has cut off the inflow of warmunstable low-level air.

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0 km 200

1008

1004

verycold air

wind

wind

snoworrain

rain orsnow

wind

precipitationregion

wind1000

coldair

Occluded front

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occlusion /ŏ-kloo-zhŏn/ See occludedfront.

occult deposition The interception ofacidic cloud droplets by vegetation, chieflyin hilly areas. This may constitute about10% of the total deposition. Concentra-tion of sulfur oxides (SOx) and nitrogenoxides (NOx) in such droplets may bemuch higher than in washout (rainfall)conditions. See also acid rain.

ocean The mass of salt water that coversnearly 71% of the Earth’s surface. Al-though only one basin, the ocean is split,geographically, into major oceans; theseinclude the ATLANTIC, PACIFIC, and the IN-DIAN OCEANS. To the north and south of theglobe are the ARCTIC and ANTARCTIC

OCEANS, respectively.

ocean climate See maritime climate.

ocean current The movement or flowof ocean water along a defined path. Thismovement can be permanent or semi-per-manent. The currents are generally wind-driven or thermohaline, i.e. produced bythe sinking of high-density water createdby temperature and/or salinity differences.The major ocean currents have an impor-tant influence on the climate due to thehigh specific heat capacity of water; theytransport heat from the tropics to higherlatitudes, and return cool water. In thenorthern hemisphere, those flowing gener-ally northward, toward the North Pole, arewarm and act to raise coastal tempera-tures, for example the GULF STREAM and theNORTH ATLANTIC DRIFT; those flowingsouthward, toward the Equator, are coldand lower than the temperatures alongcoastal regions, for example, the OYASHIO

and CALIFORNIA CURRENTS.

oceanicity /oh-shă-niss-ă-tee/ The de-gree to which the climate of a location is in-fluenced by the sea, as opposed to land.Compare continentality.

oceanography /oh-shă-nog-ră-fee/ Thescientific study of the seas and oceans. It ismainly divided into geologic oceanogra-

phy, physical oceanography, chemicaloceanography, and marine biology.

ocean weather ship An ocean-goingvessel equipped with meteorological in-struments to carry out and record meteo-rological observations.

offshore current 1. A current in the sea or ocean that flows parallel to thecoast.2. Any current that flows away from shore.

okta /ok-tă/ The internationally usedbasic unit for assessing cloud cover forboth total cloud amount and the cover as-sociated with individual cloud layers. Oneokta is the same as one eighth of the celes-tial dome. Eight oktas means the sky isovercast, 4 oktas means that it is half cov-ered, 0 oktas means the sky is clear, and 9oktas means that the sky is obscured – byblowing snow or fog for example – so thatthe observer cannot see the sky to reportthe cloud cover.

opacus /oh-pahk-ŭs/ A cloud variety, asheet or layer of cloud that is sufficientlythick or opaque to blot out the Sun or theMoon. It is used in conjunction with STRA-TUS, STRATOCUMULUS, ALTOCUMULUS, andALTOSTRATUS. See also cloud classification.

open pan evaporimeter See evapor-imeter.

orbital forcing The influence exertedby variations in the orbit and rotation ofthe Earth on climate. See also Mi-lankovitch cycles.

orographic /ô-rŏ-graf-ik/ Of or relatingto mountains; for example weather phe-nomena, such as clouds and rainfall, thatare generated as a result of a mountain bar-rier interfering with the physics and dy-namics of the atmospheric flow.

orographic cirrus HIGH CLOUD pro-duced by the flow in the LEE WAVE of moistair in the middle and/or upper troposphereacross a mountain barrier, normally at ahigh angle to the alignment of the line of

occlusion

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mountain peaks and ridges. The moist airis sometimes forced to ‘jump’ up and overthe barrier, which can lead to the produc-tion of extensive CIRRUS and CIRROSTRATUS

that streams away downwind from the lineof the watershed. The cloud persists for aslong as the conditions pertain, which maybe some hours or half a day to a day andcan stretch many hundreds of kilometersdownwind, producing overcast skies a verylarge distance from the mountains.

orographic cloud Cloud produced bythe forced uplift of a flow of moist air upand over hills and mountains. The reducedpressure as the air ascends is associatedwith adiabatic cooling and, if conditionsare right, to the condensation of myriads ofcloud droplets.

orographic depression See lee depres-sion.

orographic lift (orographic uplift) Theforced ascent of air over a barrier of hills ormountains. If the air is moist the uplift cangive rise to adiabatic cooling leading to sat-uration and condensation and the forma-tion of OROGRAPHIC CLOUD.

orographic precipitation (orographicrainfall, relief rainfall) PRECIPITATION

caused by the vertical uplift of a moist airstream as it flows over a topographic bar-rier, such as a mountain range. The airstream cools to DEW-POINT as it ascends,with a layer of OROGRAPHIC CLOUD formingif the condensation level is reached. If suf-ficient water vapor is present, considerableamounts of rain are deposited on the highground. The generation of orographic pre-cipitation is favored when: (1) the moun-tain range is extensive and oriented at rightangles to the direction of the air flow (e.g.the Southern Alps of New Zealand) and (2)cyclonic or convective processes are al-ready active in the atmosphere (the oro-graphic component is normally weak andmerely acts as a trigger mechanism).

Compression and consequently warm-ing of the air stream on the leeward slopesof a mountain range results in the forma-tion of a RAIN SHADOW. For example, the

eastern slopes of the Western Ghats inIndia are sheltered from the southwestmonsoon and in some years receive lessthan 600 mm (24 in) of rainfall, whereastotals on the windward slopes can exceed5000 mm (200 in) per annum.

oscillation /oss-ă-lay-shŏn/ In meteorol-ogy, a variation from the mean value for ameteorological parameter or phenomenonthat takes place on a cyclical basis. See alsoNorth Atlantic Oscillation; Southern Os-cillation.

outer convective band See feederband.

outflow boundary The boundary be-tween the cooled outflow air from aTHUNDERSTORM and the surrounding envi-ronmental air, similar in characteristics toa cold weather front. New thunderstormsoften develop along these outflow bound-aries.

overcast The condition of cloud cover,for any level of cloud or mixture of levelsof cloud, when the sky is completely cov-ered.

overrunning The action of one layer ofair, often relatively warm, moving over an-other, more dense layer at the surface, as inthe case of a WARM FRONT.

overshooting top In the cumulonimbuscloud of a THUNDERSTORM or a SUPERCELL

thunderstorm with a vigorous updraft, abulge at the top of the ANVIL produced asthe updraft rises above the top of the cloud.Overshooting tops are generally short-lived but those that persist are a strong in-dication that the storm’s updraft is strongenough to produce severe weather.

oxygen (O) The second most abundantgas in the Earth’s atmosphere. It exists intwo forms: the diatomic molecule (O2),constituting approximately 20.95% of thevolume of dry AIR, and triatomic oxygen,known as OZONE (O3), which occurs intrace amounts but is of considerable im-portance in protecting the Earth from the

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harmful effects of the Sun’s ultraviolet ra-diation.

oxygen-isotope analysis In paleocli-matology, the technique of obtaining evi-dence of past climates, including ice ages,from stable oxygen-isotope concentrationsin cores from sources, such as marine sedi-ments, ice sheets, tree trunks, and stalac-tites and stalagmites within caves. It isused, for example, in estimating the tem-perature of water at the time the shells ofmarine organisms were deposited from theoxygen retained in the calcium carbonateof the shells. This is based on the ratio ofthe stable oxygen isotopes, 18O and 16O,which is temperature-dependent in water;18O concentrations increase with a lower-ing of temperature as a result of the lighter16O being preferentially evaporated. Stableoxygen isotopes are also used to extractpast climatic information from ice cores.To a large degree the isotope concentrationcan be considered as a function of the tem-perature at which the water condensed. Seealso oxygen-isotope ratio.

oxygen-isotope ratio The ratio of thetwo main stable isotopes of oxygen: 16Oand 18O. Variations of the ratio are used asan indicator of paleotemperatures (thetemperatures of past climates). Differencesin the mass of the two isotopes result in thelighter 16O being evaporated from waterbodies in preference to the heavier 18O; at-mospheric water thus has a deficiency of18O.

Oyashio Current /o-yah-shee-oh/ (OyaCurrent, Kuril Current) A cold current inthe Pacific Ocean that flows southwestfrom the Bering Sea and along the eastcoast of Japan, where it meets theKUROSHIO CURRENT.

ozone (O3) A TRACE GAS consisting ofthe triatomic form of OXYGEN. It is foundin its highest concentrations in theSTRATOSPHERE, where it is constantly cre-ated and destroyed naturally by the inter-action between ultraviolet solar radiationand oxygen (see Chapman cycle of ozoneformation). In the stratosphere it provides

an important shield, protecting the Earthfrom the Sun’s harmful ultraviolet rays.Within the upper troposphere, ozone actsas a GREENHOUSE GAS but the contributionit makes to the enhanced GREENHOUSE EF-FECT has yet to be fully established. Ozoneoccurs also in polluted air within the lowerTROPOSPHERE as a chemical oxidant and amajor constituent of photochemical SMOG

above urban and industrial areas. This isparticularly apparent on sunny days be-cause solar radiation is required for its for-mation. As a pollutant it is an irritantaffecting the human respiratory system.See also ozone depletion; ozone layer.

ozone-depleting substance (ODS) Seeozone depletion.

ozone depletion The artificial destruc-tion of stratospheric OZONE above theAntarctic and surrounding regions by theinteraction of ultraviolet solar radiationwith ozone-depleting substances (ODSs).These compounds contain chlorine andbromine, such as the CHLOROFLUOROCAR-BONS (CFCs), that have ascended from theEarth’s surface. The destruction occurs an-nually, beginning in the spring of thesouthern hemisphere when solar radiationbegins to illuminate the highest latitudes.The intensely cold conditions in theAntarctic stratosphere generate POLAR

STRATOSPHERIC CLOUDS, the ice crystals ofwhich provide the platform on which thechemical reactions take place to destroythe ozone. The result is the formation ofthe ozone hole, a large area of the stratos-phere with extremely low levels of ozone.The size of the ozone hole is affected, in thelong term, by the extent of the emissions ofozone-depleting substances and by temper-ature variations in the upper atmosphere.Colder conditions contribute to the freez-ing of more ice crystals and hence morechemical activity. Ozone depletion is notrestricted solely to Antarctica; it also oc-curs to a lesser extent over parts of NorthAmerica, Europe, Asia, Africa, Australia,and South America.

The ozone destruction enables damag-ing ULTRAVIOLET RADIATION to reach thesurface of the Earth, where it is harmful to

oxygen-isotope analysis

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ozonosphere

living organisms. In humans, for example,exposure to increased levels of UVB radia-tion can cause skin cancers. Measurestaken to prevent ozone depletion includedthe MONTREAL PROTOCOL of 1987, whichled to decisions to phase out the use ofHALONS and CFCs.

ozone layer (ozonosphere) The layer inthe atmosphere at an altitude of about10–50 km (6–30 miles) above the Earth’ssurface in which ozone concentrations areat their greatest. Ozone is generated byphotochemical reactions between 20–60km (12–35 miles) above the Earth’s sur-face. There is a marked contrast betweenozone levels in the TROPOSPHERE andSTRATOSPHERE, which is the result of littlemixing between the two layers. Thestratosphere contains about 90% of the at-

mosphere’s ozone. At the Earth’s surface,ozone is destroyed through reactions withplants and water whereas in the stratos-phere it is very stable and may survive forseveral months.

ozonesonde /oh-zohn-sond/ An instru-ment attached to an air-borne balloon (seeballoon sounding) used to measure the ver-tical variations of OZONE concentration inthe atmosphere. It consists of an ozone sen-sor connected to a RADIOSONDE, whichtransfers data to a ground station and mea-sures humidity, pressure, temperature,wind speed and direction, and geopotentialheight simultaneously with ozone sam-pling. See also ozone layer.

ozonosphere /oh-zoh-nŏ-sfeer/ See ozonelayer.

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Pacific Ocean The largest and deepestof the Earth’s oceans. It covers approxi-mately 168 million sq km (65 million sqmiles) and occupies 33% of the Earth’s sur-face. This ocean extends from the Arctic tothe Antarctic regions and between Northand South America on the east and Aus-tralia and Asia on the west. Its maximumlength is about 14,500 km (9000 miles)and its maximum width about 17,500 km(11,000 miles) between the Isthmus ofPanama and the Malay Peninsula. Its aver-age depth is around 4000 m (13,000 ft)with its greatest depth in the MarianaTrench, approximately 11,000 m (36,000ft) deep.

paleoclimatic indicator /pal-ee-oh-klÿ-mat-ik/ Any one of a number of sourcesfrom which information and evidenceabout past climates in geologic history (pa-leoclimates) may be derived. These includemicrofossils (e.g. foraminifera), macrofos-sils (e.g. mollusks and corals), plants (e.g.tree rings and pollen), as well as glacial,periglacial (e.g. ice-wedge casts), and plu-vial deposits, which provide morphologicalevidence for past climates.

paleoclimatology /pal-ee-oh-klÿ-mă-tol-ŏ-jee/ The study of past CLIMATES. In-creasingly this study involves looking atcauses of the variations among climates ofpast geologic ages and relating them tochanges in more recent climate conditions.Methods to provide information and dataabout past climates include den-drochronology, palynology and pollenanalysis, ice-core analysis, deep-sea sedi-ment core analysis, as well as the study ofglacial deposits, fossils, and rock records.

Palmer Drought Severity Index (PDSI)

A meteorological drought severity indexthat was developed by American meteorol-ogist Wayne C. Palmer in 1965 and nowused by many US government agencies.The index shows long-term abnormal dry-ness or wetness, reflecting the supply-and-demand concept of the regional waterbalance equation. The PDSI is calculatedbased on precipitation and temperaturedata, potential and actual evapotranspira-tion, runoff, as well as the local availablewater content of the soil. Drought condi-tions are shown in the index by minusnumbers: 0 is normal, –2 is moderatedrought, –3 is severe drought, and –4 orless is extreme drought. The index is alsoused to indicate moist conditions: 2 is un-usually moist, 3 is very moist, and 4 ormore is extremely moist.

palynology /pal-ă-nol-ŏ-jee/ The studyof living and fossil pollen grains andspores, together with some other microfos-sils. It has applications in the study of pastclimates (paleoclimatology) and climatechange. See also pollen analysis.

pampero /pahm-pair-oh/ The local namein Argentina and Uruguay for a line squallwith a rapid increase in wind speed; it issometimes accompanied by rain, thunder,and lightning. It typically includes anarched cloud formation along the leadingedge and is followed by a cool southwest-erly wind.

pancake ice Small thin disks or cakes ofice, roughly circular in shape with raisedrims, that form on the surface during thefreezing of freshwater lakes and ocean salt-water. Pancake ice and ice rind may even-tually freeze together to form larger floesor an extensive, unbroken sheet of ice.

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pannus /pan-ŭs/ A type of ACCESSORY

CLOUD that occurs below another maincloud and is occasionally attached to it, inthe form of a more-or-less continuoussheet of ragged shreds. The main cloud canbe ALTOSTRATUS, NIMBOSTRATUS, CUMULUS,or CUMULONIMBUS. See also cloud classifi-cation.

papagayo /pah-pă0gÿ-oh/ A strong,cold, and dry northerly wind that blowsacross the Gulf of Papagayo into north-western districts of Costa Rica, CentralAmerica. It is also experienced on the Mex-ican Plateau and is similar to the NORTE

and NORTHER.

paraselene /pa-ră-sĕ-lee-nee/ (mockMoon) An optical phenomenon, a typeof HALO, in which a bright area of light ap-pears on a level with the Moon. It isformed in the same way as the solar PARHE-LION but is less bright.

parcel of air See air parcel.

parhelion /par-hee-lee-ŏn/ (mock Sun, Sundog) An optical phenomenon, a type ofHALO, in which a patch of bright light is vis-ible at the same level as the Sun. It isformed by the refraction of sunlight byhexagonal plate-like ice crystals with diam-eters less than 30 µm (0.001 in) that arepresent, for example, within cirrostratus orcirrus cloud. A parhelion generally occurson the main 22° halo around the Sun (thehalo itself is not always visible), and thephenomenon often occurs as a pair ofparhelia with one either side of the Sun.More rarely it occurs at other points on theparhelic circle (e.g. 46° or 120°). Compareparaselene.

partial drought In the UK, a period ofat least 29 consecutive days during whichmean daily rainfall over that period is lessthan 0.25 mm (0.01 in). This definition isinappropriate for dissimilar climates; itwas originally defined in 1887 by the for-mer British Rainfall Organisation but is no longer much used. See also absolutedrought.

particulates (particulate matter, PM)Tiny solid or liquid particles, organic andinorganic, that are suspended and carriedin the atmosphere. Natural sources of par-ticulates include dust storms and volcanicactivity, salt spray from the oceans, forestand bush fires, and pollens from vegeta-tion. Anthropogenic sources include thecombustion of fossil fuels in power genera-tion and vehicle engines, industrialprocesses, and agricultural practices. Par-ticulates with a size of less than 10 µm areof particular concern to human health asthey can enter the respiratory system andpenetrate the lung. Fine particulates, lessthan 2.5 µm in size, can remain suspendedin the atmosphere for long periods. Theyhave the property of scattering sunlightand reducing visibility, such as in haze.

pascal /pass-kăl/ (Pa) The SI unit of PRES-SURE defined as 1 newton per square meter.The multiple HECTOPASCAL is used in scien-tific meteorology.

passive remote sensing The means ofimaging atmospheric and other phenom-ena in which the sensing instrument mea-sures the radiation being emitted by thesource. Unlike ACTIVE REMOTE SENSING, itdoes not itself emit radiation to enable themeasurement to be made. INFRARED SATEL-LITE IMAGERY is an example of passive re-mote sensing in which the instrumentmeasures the INFRARED RADIATION radiatedup to the satellite from the Earth.

past weather Part of the synoptic reportfrom surface stations that summarizes notmore than two types of weather that haveoccurred in the past three or six hours de-pending on the observation hour. The ob-server can select from 10 basic weathertypes, including RAIN, DRIZZLE, SNOW,SHOWER, FOG, and THUNDERSTORM. Thepast weather symbols form part of the STA-TION MODEL.

PDSI See Palmer Drought Severity Index.

pearl-necklace lightning See beadedlightning.

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Penman formula In climatology, amethod of estimating POTENTIAL EVAPO-TRANSPIRATION devised by the Britishphysicist Howard Latimer Penman(1909–84). Evaporation loss is expressedin terms of the solar radiation, mean airtemperature, mean atmospheric relativehumidity, and mean wind speed. The for-mula and its modifications have beenwidely used by agricultural and soil scien-tists.

pentad /pen-tad/ In meteorology, a pe-riod of five successive days. It divides con-veniently into the number of days (365) ofa normal year and is used in preference tothe week in the analysis of meteorologicaldata.

perenially frozen ground See per-mafrost.

perfluorocarbon /per-floo-ŏ-roh-kar-bŏn/ (PFC) An artificial compound com-posed of carbon and fluorine, the two maincompounds being perfluoromethane (CF4)and perfluoroethane (C2F6). The PFCswere introduced, together with the hydro-fluorocarbons (HFCs) as alternatives toozone-depleting substances, such as theCHLOROFLUOROCARBONS (CFCs). They donot harm the ozone layer but as GREEN-HOUSE GASES possessing long atmosphericlifetimes (up to 50,000 years) they wereone of the six gases listed for limitation inthe KYOTO PROTOCOL. They are emitted asby-products in industrial processes (no-tably aluminum smelting).

perigee /pe-ră-jee/ In remote sensing, thepoint along a satellite’s orbit at which it isnearest to the Earth or the point that it isorbiting. More generally in astronomy anyEarth satellite, including the Moon, is saidto be in perigee when it is closest to theEarth. Compare apogee.

periglacial climate /pe-ră-glay-shăl/Originally defined as the climate that wasassociated with regions adjacent to and af-fected by the Quaternary ice sheets, theterm has also been expanded to cover con-temporary climates adjacent to ice sheets,

ice caps, and glaciers. It may also includeTUNDRA and cold climates in which frostsand permafrost processes occur even ifthey are not in close proximity to icesheets. Periglacial climates are arid andtemperatures may be below freezing for sixmonths or more.

perihelion /pe-ră-hee-lee-ŏn/ The pointin a planet’s orbit at which it is closest tothe Sun. For the Earth, this occurs aboutJanuary 3 when the Earth–Sun distance isabout 1.5% less than the mean Earth–Sundistance. The effect of this proximity to theSun on the Earth’s climate is to lessen theseverity of winters in the northern hemi-sphere.

perlucidus /per-loo-să-dŭs/ A variety ofcloud; a widespread sheet or patch of cloudthat has marked breaks between the cloudelements such that the Sun, Moon, clearsky, or overlying cloud can be seen. It is ap-plied to STRATOCUMULUS and ALTOCUMU-LUS. See also cloud classification.

permafrost /per-mă-frôst/ (pereniallyfrozen ground) Ground that remainsbelow 0°C (32°F) and persists for at leasttwo consecutive years. Around 25% of theEarth’s surface is underlain by permafrost,predominantly in Russia, Canada, andAlaska (where it occupies around 80% ofthe land area). It can extend to depths inexcess of 500 m (1640 ft) in Canada andAlaska and to over 700 m (2300 ft) inSiberia. Recent studies of permafrost inAlaska and Siberia have revealed warmingwithin some permafrost areas; for exam-ple, in Fairbanks (Alaska) and Yakutsk(Siberia) the permafrost warmed by 1.5°Cin the 30 years leading up to 2000. Thethawing of permafrost in eastern Siberiasaw the disappearance, during the 30 yearsup to 2005, of many lakes that were oncetrapped above the frozen ground but havesince drained underground. In westernSiberia, the thawing of the permafrostwithin the extensive peat bogs has led tothe formation of ponds and lakes and therelease of methane (a potent greenhousegas) previously trapped below the per-mafrost. Thawing permafrost has been

Penman formula

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identified as a source of feedbacks thatcould accelerate climate change by releas-ing greenhouse gases into the atmosphere.

permanent snow line See snow line.

perpetual frost climate One of the cli-mate subgroups in the KÖPPEN CLIMATE

CLASSIFICATION, designated as EF. This is anice-sheet climate with mean monthly tem-peratures below 0°C (32°F) throughout theyear.

persistence In meteorology, weatherconditions that continue for longer thanthe average or expected time.

perturbation In meteorology, a distur-bance in a field. An example is a travelingdisturbance in the pressure field, such as aLOW. At one site, a time series of pressurewill illustrate the passage of a low becausethe graph will be perturbed as the lowmoves across. Perturbations can also occurin other fields, such as temperature, wind,or humidity, and on a range of space scales.A THUNDERSTORM can be viewed as a per-turbation of atmospheric properties withina larger scale environment.

Peru Current (Humboldt Current) Acold current in the southeast Pacific Oceanthat flows north along the coasts of Chileand Peru. The EKMAN EFFECT tends to de-flect the flow in a westerly direction gener-ating upwelling of nutrient-rich cold water,which supports diverse ecosystems and im-portant fisheries, from deeper levels to re-place the flow. During an EL NIÑO event theupwelling is reduced resulting in a rise insea surface temperature and a decline infish stocks and other marine life.

PFC See perfluorocarbon.

PGF See pressure gradient force.

pH A quantitative measure of the degreeof acidity or alkalinity of a solution. It isdefined as the negative logarithm to thebase ten of the hydrogen-ion concentra-tion, expressed in gram ions per liter of so-lution:

pH = –log10 [H+] = log10 1/[H+].The pH scale is 0–14: a neutral medium(e.g. pure water) has a pH of 7 (i.e. a hy-drogen ion concentration of 10–7

mole/dm3, which equals the hydoxyl ionconcentration); acidic solutions have a pHbelow 7; alkaline solutions have a pHgreater than 7. Because the pH scale is log-arithmic, a decrease in pH from, for exam-ple, 4 to 3 indicates a ten-fold increase inthe acidity of the solution. Rainwater isslightly acidic (but see ACID RAIN, in whichthe pH is below 5). Water in saline envi-ronments may have a pH value of 9 orhigher; the pH of ocean water typicallyfalls in the range 8.1–8.3.

photochemical smog /foh-tŏ-kem-ă-kăl/ See smog.

photodisintegration /foh-toh-di-sin-tĕ-gray-shŏn/ The splitting of molecules byPHOTODISSOCIATION in such a way that therecombining of the molecular fragments isunlikely.

photodissociation /foh-toh-di-soh-see-ay-shŏn/ The splitting of a molecule intotwo or more smaller molecules or atoms byabsorption of radiation, particularly ofhigh-energy ultraviolet radiation. Thisprocess contributes to the production ofphotochemical SMOG and to the productionof OZONE in the atmosphere. The photodis-sociation rate is the number of moleculesdissociated by radiation per unit volumeper second.

photogrammetry /foh-tŏ-gram-ĕ-tree/The branch of remote sensing concernedwith the plotting and accurate measure-ment of features visible on aerial pho-tographs or satellite images.

photometeor /foh-toh-mee-tee-er/ Anyone of a number of luminous phenomenaproduced in the atmosphere as a result ofdiffraction, refraction, reflection, and/orinterference from the Sun or Moon, such asa halo, corona, glory, green flash, rainbow,fogbow, shimmer, scintillation, mirage,and crepuscular rays.

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photometry /phŏ-tom-ĕ-tree/ The branchof physics concerned with the measure-ment of the intensity of light.

photosphere /foh-tŏ-sfeer/ That part ofthe Sun’s gaseous atmosphere from whichcontinuous emission of radiation takesplace. Known as the surface or visible diskof the Sun, it consists of plasma about1000 km (621 miles) thick at a temperatureof about 6000 K.

photosynthesis /foh-tŏ-sin-th’ĕ-sis/ Thecomplex process by which organic com-pounds are synthesized by chlorophyll-containing cells within green plants andsome bacteria, from carbon dioxide and asource of hydrogen, such as water, in thepresence of radiant energy from the Sun.During the process oxygen is simultane-ously released. Photosynthesis is a vitalprocess for life on Earth as plants, either di-rectly or indirectly, provide the food sourcefor other forms of life. It is, in addition, theprimary source of oxygen in the atmos-phere and the absorption of carbon diox-ide is vital in the carbon cycle.

physical climatology The branch ofclimatology concerned with the physicalprocesses that are responsible for and in-fluence the climates of the world. Areas ofstudy include the global energy balance, ra-diation transfer, the greenhouse effect, theinteraction between the Earth’s atmos-phere and the surface, the hydrologic cycle,and climate change.

physical meteorology The branch ofmeteorology concerned with the structureand composition of the atmosphere. Areasof study include aerosols and cloudphysics, precipitation processes, atmos-pheric electricity, radiation, atmosphericthermodynamics, the boundary layer, tur-bulence, and gravity waves.

physiological drought 1. A temporarydaytime state of drought in plants due tothe losses of water by transpiration beingmore rapid than that taken up by roots al-though the soil may have an adequate sup-

ply; it is usually followed by the night-timerecovery of the plant.2. The condition of plants that suffer theloss of a physiologically suitable watersource and are unable to utilize the waterthat is present, for example frozen water orsalt water.

pibal /pÿ-bał/ See pilot balloon.

Piché evaporimeter /pee-shay/ Seeevaporimeter.

piezoresistance barometer See barom-eter.

pileus /pÿ-lee-ŭs, pil-ee-/ (cap cloud) Asmall ACCESSORY CLOUD that sometimesforms above CUMULUS or CUMULONIMBUS

clouds. It appears as a smooth shallow‘cap’, and is formed by air flowing over thetop of the deep cloud, which acts some-what like a hill, forcing the moist environ-mental air around and over it. This issimilar to the way in which a lenticularcloud is formed. Occasionally several lay-ers of pileus may be observed. [Latin: ‘cap’]See also cloud classification.

pilot balloon A small meteorologicalballoon, inflated with hydrogen or helium,and used for visually observing the upperwind speed and direction above a station.The balloon, which carries no instruments,ascends at a constant rate: the ascensionrate is approximately determined by care-ful inflation to a given total lift. A pilot bal-loon observation (pibal), the measurementand calculation of the wind speed and di-rection, is made by reading the elevationand azimuth angles of a theodolite whilevisually tracking the pilot balloon, or theballoon can be tracked by radar. See alsoballoon sounding; radiosonde.

pitot tube anemometer /pee-toh/ AnANEMOMETER that uses the build up of pres-sure in a Pitot tube to measure wind speed.The pitot tube, invented by Henri Pitot(1695–1771), consists of an L-shaped tubewith an opening at one end facing into theflow. The difference in pressure betweenthe inside and the outside of the tube is

photometry

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measured and converted to a wind speed.This type of anemometer is useful in strongsteady streams of air.

pixel /pik-sel/ In remote sensing, thesmallest element or building block of a re-motely sensed image. The best pixel di-mension for all AVHRR images is at theSUBSATELLITE POINT, where it is a 1.1 kmsquare. [Short for: picture element]

Planck’s law /planks, plahnks/ (Planck ra-diation formula) A law of radiation stat-ing that the wavelength of propagation ofradiation depends on the temperature ofthe emitting body. It is named for the Ger-man physicist Max Planck (1858–1947),who discovered it. Planck’s law describesthe distribution of energy across the wave-lengths of the electromagnetic spectrum fora given temperature of a perfect radiator(black body). It introduced the concept ofenergy quanta, upon which the quantumtheory is based.

planetary boundary layer (frictionlayer) The atmospheric boundary layerthat occupies approximately the lowestkilometer of the troposphere, but heightvaries considerably according to locationand time from around 500 m (1600 ft)above ocean surfaces, or on calm nightsabove land surfaces, to 1.5 km (1 mile)during daytime. It is the layer that is influ-enced by the frictional effects of the Earth’ssurface, and above the SURFACE BOUNDARY

LAYER is also known as the EKMAN LAYER

since the effects of friction decrease withheight. The top of the planetary boundarylayer is the level at which there is no longerany influence of friction on airflow, whichthen approximates to the GEOSTROPHIC

WIND. The top of the planetary boundarylayer also rises during the day as turbulenceincreases and falls at night when turbu-lence is dampened and a temperature in-version may be present.

planetary vorticity See vorticity.

planetary wind One of the major windsystems of the Earth that results from acombination of solar radiation and the ro-

tation of the Earth, including the TRADE

WINDS and WESTERLIES. Compare localwind.

plan position indicator (PPI) A stan-dard type of RADAR image display in whichthe antenna is at the center of a circularmap. The location of the phenomenonbeing sensed, such as precipitation, is dis-played in terms of its plan position, inwhich the variables are the radial distancefrom the antenna and the AZIMUTH ANGLE.

plate tectonics The theory that theEarth’s lithosphere is divided into mobileoceanic and continental plates, which arein motion relative to each other. The rela-tive positions and extent of the major landmasses and ocean areas have altered overgeologic time with consequent affects onclimate.

plume blight In the US, an identifiablecoherent pollution plume that is visuallyintrusive in an area in which good visibilityis regarded as an important characteristic.

plum rains See Mai-yu season.

pluvial /ploo-vee-ăl/ A period of greaterprecipitation and a wetter climate thanpreceding or succeeding periods. The evi-dence of past pluvials is provided in the ge-ologic record, especially in tropical andsubtropical latitudes. During full glaciationin polar regions a number of the semidesertareas of the tropics experienced pluvials, inresponse to the southward movement ofthe circulation belts. Compare interpluvial.

pluviometric coefficient /ploo-vee-ŏ-met-rik/ In climatology, a measure of thedegree of climatological seasonality in theRAINFALL receipt of a location. Pluviomet-ric coefficients are usually calculated on amonth-by-month basis and then averaged.For example, suppose the thirty-year meanannual rainfall at a location (X) is 1200mm and the thirty-year mean January rain-fall is 50 mm, then the pluviometric coeffi-cient for January is calculated as follows:

The amount of rainfall occurring inJanuary if the total is evenly distributed

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throughout the year is (31/365.25) × 1200= 101.85 mm. Then January’s pluviometriccoefficient (PC) is 50/101.85 = 0.49.

The more that the pluviometric coeffi-cient diverges from 1, the more that the dis-tribution of rainfall at X is becoming lessuniform in time, with marked drier andwetter periods evident throughout theyear. A line joining all points in space thathave identical pluviometric coefficients isan equipluve.

PM See particulates.

POES (Polar-orbiting Operational Envi-ronmental Satellite) The current series ofUS POLAR-ORBITING SATELLITES operated bythe National Oceanic and AtmosphericAdministration (NOAA), comprised at anyone time of two satellites that circle theEarth at right angles to each other. Thefirst series of polar-orbiting satellites toprovide global meteorological data beganwith TIROS (Television and Infrared Obser-vation Satellite), launched by NASA in1960. NOAA has been flying its own satel-lites since the launch of NOAA–6 in 1979;prior to this the NOAA series (TIROS 2 to5) was also operated by NASA. A new se-ries of five satellites was launched in 1998with NOAA–15, which carry more ad-vanced instruments. NOAA–16, came intooperation in 2001 to replace NOAA–14and NOAA–17, the most recent of the se-ries was launced in 2002. The POES satel-lites provide meteorological data that istransmitted globally and used by the Na-tional Weather Service (NWS) in 3–10 dayforecasts and 30–90 day climate outlooks.Images are provided of cloud cover, verti-cal temperature and humidity profiles, andsurface parameters, including sea surfacetemperature, snow, and ice cover. Thesatellites also carry ultraviolet sensors thatprovide ozone levels in the atmosphere.

point rainfall The amount of RAIN thatfalls at one location. It does not necessarilyprovide a good indicator of rainfall receiptover the surrounding area.

point source In air pollution studies,any industrial unit that discharges pollu-

tants into the atmosphere. A chimney stackof a factory that emits at least 100 tonnes ayear is subject to a special emissions inven-tory and regulation. In the US, such stacksare known as special emission sources.Compare area source; line source.

polar /poh-ler/ Denoting either of theEarth’s poles or the region within the ARC-TIC or ANTARCTIC Circles.

polar-air depression See polar low.

polar air mass An extensive region ofair (AIR MASS) that has a source in ANTICY-CLONES over wintertime higher- and mid-dle-latitude continents. CONTINENTAL

POLAR (cP) air is very cold, dry, and stablebut if it flows out across adjacent oceansthe air mass is significantly modifiedthrough being strongly warmed and moist-ened to become MARITIME POLAR (mP) air,which is unstable.

polar cell A weak cellular circulationthat occurs in the north–south verticalplane within the high-latitude TROPOS-PHERE. It consists of sinking motion overthe poles, easterlies in the lower tropos-phere that flow toward middle latitudes,ascent in frontal zones, and a return flow inthe upper troposphere back toward thepole. It is the weakest of the cells in theTHREE-CELL MODEL.

polar climate The climate of the regionsthat lie poleward of the Arctic or AntarcticCircles. Mean monthly temperatures arebelow 0°C (32°F) throughout the year withextremes of cold temperatures during thewinter; annual temperature ranges arelarge. During summer there are long peri-ods of continuous day (see midnight Sun),while during winter there are long periodsof continuous night. Precipitation is gener-ally low with most occurring on the coastalmargins. High winds occur around themargins of ice caps of Greenland andAntarctica. The climate is denoted by theletter E in the KÖPPEN CLIMATE CLASSIFICA-TION.

polar easterlies The relatively weak

PM

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east winds that flow out across the highestlatitudes from the ANTICYCLONES (highs)that sometimes occur over polar areas.

polar front The gently sloping semi-per-manent zones that separates warm moistMARITIME TROPICAL air from cold MARITIME

POLAR air along the middle latitudes ofeach hemisphere. Over the North Atlanticand North Pacific Oceans it can often betraced as a continuous line over severalthousand kilometers, generally alignedsouthwest to northeast. It is the front alongwhich cyclone waves (see frontal wave)form and run, and from which most CY-CLONES evolve. The polar front is inti-mately linked to the POLAR FRONT

JETSTREAM. See also Atlantic polar front.

Polar Front Jet Stream The markedupper tropospheric core of very fast flow-ing air associated with the POLAR FRONT. Itis usually many thousands of kilometerslong, perhaps a hundred wide, and a kilo-meter or two deep. It is roughly twice asstrong in the winter than in the summer be-cause of the increased frontal thermal con-trasts; wind speeds in the winter can reach200 knots (230 mph). See also jet stream.

polar high (polar anticyclone) An ex-tensive region of high pressure across thehighest latitudes, the polar regions; asource of very cold and generally dry air.The anticyclone over Antarctica (theAntarctic high) is semi-permanent; thatover the Arctic (the Arctic high) is generallyseasonal.

polar low (polar-air depression) A rela-tively small-scale non-frontal LOW or low-pressure system that characterizes thehighest latitudes; it is a SECONDARY DEPRES-SION that forms over oceans poleward ofthe polar front and can produce blusterysnowy conditions, particularly in the win-tertime, when they are most common.

Polar-Night Jet Stream The westerlywind speed maximum that occurs in thelower STRATOSPHERE during the prolongedperiods of darkness at the highest of lati-tudes. The darkness leads to deeply cold air

over the poles and, consequently, a west-erly jet. The situation is reversed in the longdaylight summer in high latitudes when thepoles are warmer (in the lower stratos-phere) than further toward the Equator,and as a result the flow evolves into aneasterly current. See also jet stream.

polar orbiter See polar-orbiting satel-lite.

Polar-orbiting Operational Environ-mental Satellite See POES.

polar-orbiting satellite A satellite witha near-circular north–south trajectory,such as that of the POES series of satellitesoperated by the US National Oceanic andAtmospheric Administration (NOAA),which crosses over or near the polar re-gions on every orbit. With the POES serieselevation, the orbital period is about 100minutes, so the satellite crosses near a poleapproximately every 50 minutes.

polar outbreak The rapid incursion ofmuch colder and drier polar air, precededby a cold front, over a high- or middle-lat-itude site. In the winter, such outbreaks cansometimes reach the subtropics, ending upoccasionally as CUT-OFF LOWS, and bringstorms followed by cold clear weather.They are not normally seen as a summerphenomenon.

polar stratospheric cloud (PSC) Icecrystal clouds found within the lowerSTRATOSPHERE when temperatures fallbelow –85°C (–121°F). Such a depth ofcold is found usually only above theAntarctic during the long polar winter. Theice crystals that compose these polarstratographic clouds provide the crucialplatform on which the chemical reactionsresponsible for OZONE DEPLETION occurduring the late winter and early spring.

polar vortex See circumpolar vortex.

polar zone See frigid zone.

pole of cold See cold pole.

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pollen analysis The study of fossil pol-lens and spores in sediments, such as peatbogs and lake sediments. As pollen is par-ticularly resistant to decay, these studiesenable the plant species to be ascertained,which in turn indicates the temperatureand moisture conditions of past geologictimes for an area. Pollen analysis can yieldimportant evidence for past climates as dif-ferent species of plants vary in their grow-ing requirements. See also palynology.

pollutant A natural or synthetic sub-stance or effect that causes deterioration inthe biosphere by changing environmentalconditions, such as altering growth rates ofspecies, interfering with the food chain,causing problems with human health, well-being, and property.

pollution The introduction into the en-vironment of substances (solid, liquid, orgas), that are likely to have an adverse ef-fect on susceptible components of the bios-phere, including soil and water habitats,ambient air, and all or any forms of life.See also air pollution.

pollution dome The accumulation ofpollutants in an urban or industrial zoneover a period of at least a few days due toa lack of ventilation in the PLANETARY

BOUNDARY LAYER caused by persistent highpressure. Pollutants accumulate just be-neath the temperature INVERSION level.

pollution plume An elevated point-source emission from a chimney stack withpollutants capable of being transportedmany kilometers downwind. Characteris-tics include initial plume rise, Gaussian dis-tribution of pollutant concentrations in thehorizontal, and vertical and dry depositionin a zone 10 to 100 km (6–60 miles) down-stream.

Pollution Standards Index See AirQuality Index.

ponente /po-nen-tay/ A westerly windthat blows across the Mediterranean Basin.The word is most commonly applied to a

SEA BREEZE felt along the western coast ofItaly.

potential energy The energy possessedby a body in the gravitational field of theEarth. It can be expressed as the energy re-quired to bring the body (e.g. a parcel ofair) from a fixed datum, usually mean sealevel, to a given position above sea level.

potential evapotranspiration Themaximum amount of WATER VAPOR that iscapable of being released to the atmos-phere by the processes of evaporation andtranspiration in a given weather situationfrom a surface covered by green vegetationthat has a continuous supply of water anddoes not completely shade the ground. It isa measure of the ability of the atmosphereto remove water from the surface throughthe processes of EVAPORATION and TRANSPI-RATION assuming no control on water sup-ply. Three conditions are important in thisprocess: energy (often through sunlight),eddy diffusion through wind, and thewater-vapor gradient between the groundsurface and atmosphere. Formula for theestimation of potential evapotranspirationinclude the PENMAN FORMULA and THORN-THWAITE’S INDEX OF POTENTIAL EVAPOTRAN-SPIRATION.

potential instability of an air mass See convective instability.

potential temperature The tempera-ture that an air parcel would have ifbrought from its initial state to the stan-dard pressure of 1000 hPa along the DRY

ADIABATIC LAPSE RATE.

powder snow A type of SNOW in whichthe crystals are dry, loose, and unconsoli-dated. It occurs at very low temperatures,being common in polar regions and moun-tain areas. Powder snow is ideal for skiing.

PPI See plan position indicator.

praecipitatio /pri-sip-ă-tay-shee-oh/ Asupplementary cloud feature in CLOUD

CLASSIFICATION that includes all PRECIPITA-TION that falls from the cloud base to reach

pollen analysis

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the ground (for example, rain, snow, sleet,hail, graupel, and drizzle); it excludesVIRGA. It is associated with ALTOSTRATUS,CUMULONIMBUS, NIMBOSTRATUS, STRATOCU-MULUS, CUMULUS, and STRATUS.

precession /pri-sesh-ŏn/ The motion ofthe Earth’s rotation about its axis, which iscontinually changing its direction in space.The precession of the equinoxes refers tothe westward drift of the location of thePERIHELION such that the orbital ellipsetraced out by the Earth slowly movesaround the Sun with periodicities of23,000 years and 18,800 years. It is one ofthe factors contributing to variations in theamount of solar energy received by theEarth over long periods.

precipitable water /pri-sip-ă-tă-băl/ (totalcolumn moisture) The amount of RAIN-FALL that would result if all of the watervapor present in a standard column of airabove a specified pressure level was con-densed and precipitated on to a horizontalsurface of unit area. The equation for pre-cipitable water (Mw), expressed in grams,is:

Mw = (1/g).pp

2

1rdp

where g is the acceleration of free fall (9.8m s–2), p1 is the pressure (hPa or mb) at thetop of the column and p2 is the pressure atthe bottom, r is the humidity mixing ratiosummed over a number of layers. As pre-cipitation-generation processes are notcompletely efficient, the amount of rainfalldeposited in reality is usually significantlylower than the theoretical maximum Mwvalue.

precipitation /pri-sip-ă-tay-shŏn/ Thedeposition of water, in either liquid or solidform, derived from atmospheric sourcesthat falls onto the Earth’s surface. Precipi-tation encompasses RAIN, DRIZZLE, FREEZ-ING RAIN, FREEZING DRIZZLE, SNOW, SLEET,HAIL, GRAUPEL, ICE PELLETS, and otherforms. It is collected and measured primar-ily in a RAIN GAUGE. Any solid precipitation(e.g. snow) is melted before the total depthof water collected is measured in millime-ters or inches. As rain is usually the main

component of precipitation, the termsRAINFALL and precipitation are used inter-changeably.

Precipitation is initiated within cloudsby the BERGERON–FINDEISEN PROCESS, by theCOLLISION–COALESCENCE PROCESS, or by acombination of these processes. Threetypes of precipitation are recognized ac-cording to the mechanism of uplift in theirformation: CONVECTIVE PRECIPITATION,FRONTAL PRECIPITATION, and OROGRAPHIC

PRECIPITATION.

precipitation-efficiency index (precipi-tation-effectiveness index, precipitation-evaporation ratio) A measure of theusefulness of rainfall input that remainsfollowing evaporation losses (EFFECTIVE

PRECIPITATION) for human activities (e.g.water supplies and agriculture). A precipi-tation-efficiency index (P/E) was devised in1931 by the American climatologistCharles Warren Thornthwaite (1899–1963) to define objectively the Earth’smajor climatic regions. His formula is:

P/E = 11.5[p/(T – 10)]10/9

where p is the mean monthly precipitation(inches) and T is the mean monthly tem-perature (°F). As T (and so evaporation) in-creases, the denominator of the aboveexpression increases and hence P/E falls.

precipitation gauge See rain gauge.

precipitation radar A RADAR designedspecifically to sense the location and inten-sity of RAINDROPS. See also Doppler radar.

precipitation variability The varia-tions in precipitation totals through timeor over space. In climates where CONVEC-TIVE PRECIPITATION is dominant, for in-stance, rainfall totals can vary significantlybetween two places that are only 3 miles (5km) apart. Over time, precipitation inputcan vary at hourly, daily, weekly, monthly,seasonal, yearly and decadal timescales.The degree of precipitation variability canbe quantified by means of any standardstatistical measure such as the standard de-viation or the coefficient of variation.

present weather Part of the synoptic re-

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port that relates to the presence of weatherfeatures, such as fog, rain, drizzle, snow, orshowers, at the time that the observation isbeing made. There is also the possibility ofreporting one of these phenomena that didnot occur at the time of observation, butdid so in the past hour, since the previousobservation. There are 100 possibilities(00–99 in the SYNOPTIC CODE) for presentweather, of which only one must be re-ported. In general, low numbers relate toless important features, such as increase incloud or decrease in cloud, while the high-est numbers refer to THUNDERSTORMS ofvarious intensities. The present weathersymbol is plotted as a part of the STATION

MODEL.

pressure The force per unit area. ByNewton’s 2nd law, force is equal to theproduct of mass (m) and acceleration.Weight (W) is a force, defined as the prod-uct of an object’s mass and the accelerationof free fall: W = mg. The SI unit of force isthe newton (N); 1N being the magnitude ofthe force that gives an acceleration of 1m s–2 to a body of mass 1 kg. Pressure isthen defined as the number of newtons (N)acting per square meter (m2). The SI unit ofpressure is called the PASCAL (Pa), which isdefined as 1 N m–2.

pressure altimeter See altimeter.

pressure gradient The horizontal orvertical rate of change of barometric pres-sure. It is expressed in hPa per meter, forexample; the horizontal wind speed is di-rectly related to the magnitude of the hori-zontal GRADIENT of pressure. The pressuregradient in the vertical is very steep indeed,but the PRESSURE GRADIENT FORCE associ-ated with it is balanced by the force ofgravity.

pressure gradient force (PGF) The ac-celeration experienced by a unit mass of airin response to a change of barometric pres-sure in either the horizontal or verticalplane, i.e. a gradient of pressure. A strongpressure gradient force leads to strongwinds, while a weak gradient is associatedwith weak flow in the atmosphere. For

horizontal motion, this force is equated tothe Coriolis force in the geostrophic bal-ance (see Coriolis effect). In the case of ver-tical (up or down) motion, it is equated tothe product of the acceleration of free falland the air density in the hydrostatic bal-ance.

pressure gradient wind See gradientwind.

pressure-plate anemometer An AN-EMOMETER that uses the build up of pres-sure on a flat plate to determine windspeed. In this case the depression of a flatplate placed into the wind is measured.This type of anemometer is used to mea-sure atmospheric turbulence.

pressure system A synoptic-scale fea-ture on mean-sea-level charts. A LOW,CYCLONE, RIDGE, TROUGH, and an ANTICY-CLONE (high) are all pressure systems.

pressure tendency (barometric tendency)In synoptic meteorology, the change in AT-MOSPHERIC PRESSURE that has occurred at agiven location on the Earth’s surface, usu-ally over a period of three hours. For ex-ample, suppose at point X the atmosphericpressure at 3.00 pm is 1021 hPa, and at6.00 pm it is 1018 hPa, then the pressuretendency over this 3-hour period is –3 hPa.An ISALLOBAR joins locations with the samepressure tendency over a given period. Isal-lobar maps are often used to deduce the lo-cation of FRONTS. Pressure tendencies arereported for the last 24 hours rather thanthe last three in tropical latitudes. This isbecause there is a marked atmospherictidal change of pressure that dominatespressure variations through the day andswamps the normally subtle pressurechanges that occur in relation to weatherdisturbances.

pressure-tube anemometer An AN-EMOMETER similar to the PITOT TUBE

ANEMOMETER but using a tube which isopen at both ends and has a larger diame-ter at the ends than at the middle. Windspeed is calculated by measuring the pres-

pressure

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sure in the middle of the tube, at the con-striction.

prevailing wind The geostrophic-scalewind direction that occurs most frequentlyin a particular area; for example, in south-ern Chile the prevailing wind is from thewest, i.e. westerly. The prevailing wind isthus distinct from the DOMINANT WIND. Theprevailing wind depends on the site’s posi-tion in the global atmospheric circulationsystem (see general circulation). In manyregions (e.g. India) the prevailing windvaries on a seasonal basis, being affected asthe world’s pressure belts change their lat-itudinal positions.

Prevention of Significant Deteriora-tion In the US, a pollution control pro-gram inserted into the 1977 Clean AirAmendments. It seeks to identify threeclasses of areas: (1) to afford a level of pro-tection to non-polluted areas beyond theprovisions of the National Ambient AirQuality Standards (NAAQS); (2) to allowareas of moderate economic growth by re-laxing air-quality constraints; and (3) to re-define some areas from the second class ofarea to permit pollution levels to rise tofederal standards.

Primary Ambient Air Quality Stan-dards See National Ambient Air QualityStandards.

profiler An instrument that can be ei-ther satellite- or surface-based to provideinstantaneous vertical sections of variables,such as dry-bulb temperature, humidity,and wind speed. A wind profiler is a sur-face-based DOPPLER RADAR instrument thatcan take instantaneous high-resolutionmeasurements of the wind, including ve-locity, direction, and turbulence, in the ver-tical column above the radar site. Somewind profilers also measure temperatureusing a radio acoustic sounding system(RASS).

propeller anemometer An ANEMOME-TER consisting of propellers (frequently he-licoidal in shape) mounted on a horizontalpivotal axis. The propellers rotate in the

wind generating an electric current propor-tional to the wind speed, which is readfrom an appropriately calibrated meter.The vane-oriented propeller anemometerconsists of a two, three, or four bladed pro-peller that is turned into the wind by an at-tached vane. See also cup anemometer.

PSC See polar stratospheric cloud.

pseudoadiabatic chart /sew-doh-ad-ee-ă-bat-ik/ See Stüve diagram.

psychrometer /sÿ-krom-ĕ-ter/ A type ofHYGROMETER used to determine relativehumidity of the atmosphere. It consists oftwo THERMOMETERS, one a WET-BULB THER-MOMETER in which the bulb is kept wet bya surrounding wet-cloth wick and theother a DRY-BULB THERMOMETER. The wet-bulb temperature is lower than that of thedry-bulb in unsaturated air at a specifiedrelative humidity. This lower temperatureis a result of the release of latent heat,which is needed for the evaporation ofwater from the muslin, from the air sur-rounding the wet bulb. The psychrometerworks on the basis that the difference be-tween the two thermometers will be greaterin drier air. Simultaneous readings aretaken from the thermometers and relativehumidity and dew-point determined withthe use of psychometric tables. See alsosling psychrometer.

pulse length The duration of the ex-tremely short burst of radiation that isemitted by some ACTIVE REMOTE SENSING

instruments, such as RADAR and LIDAR.

pulse storm A THUNDERSTORM that pro-duces brief but strong UPDRAFTS. These arecommon across the Deep South of the USduring the summer where conditions arevery humid. The main threat of damagecomes from sudden down rushes of colderdenser air from within the storm, and fromvery large, and continually growing, hail-stones, which are repeatedly moved up anddown through the storm. Flash flooding isalso a threat, as these storms can remainstationary for long periods of time.

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pyranometer

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pyranometer /pi-ră-nom-ĕ-ter/ An in-strument with a hemispherical field of viewused to measure diffuse radiation and/ortotal (beam plus diffuse) radiation incidenton a horizontal surface. It generally con-sists of a THERMOPILE sensor connected to arecorder. A shading disk on a tracking armor a shadow band may be used to block di-rect radiation to investigate diffuse radia-tion only. See also pyrheliometer.

pyrheliometer /pÿr-hee-lee-om-ĕ-ter/ Aninstrument with a narrow field of viewused for measuring direct solar radiationincident on the collection unit. It is set tocollect radiation in a direction perpendicu-lar to the Sun’s rays. In order to obtaincontinuous readings the pyrheliometer ismounted on a power-driven tracker so thatit constantly follows the position of theSun. See also pyranometer.

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quasi-biennial oscillation (QBO) Aregularly occurring marked reversal of thedirection of the winds in the lower part ofthe tropical STRATOSPHERE, in which the di-rection changes gradually from westerly toeasterly and back to westerly with an aver-age period of 26 months, or roughly twoyears.

quasi-stationary front See stationaryfront.

quickflow The most rapidly respondinghydrologic processes and parts of thecatchment area. An arbitrary cut-off line isdrawn on a stream hydrograph, anythingabove this level represents the quickflow.

Q

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radar A REMOTE-SENSING instrument, de-veloped before World War II, used to de-tect distant objects as a result of theirability to reflect or scatter a beam of mi-crowave electromagnetic radiation. In me-teorology, radar is used in weatherforecasting to detect clouds and precipita-tion, and severe weather phenomena, suchas thunderstorms and tornadoes; the latterespecially with the use of DOPPLER RADAR.Radar is also used in the analysis and studyof such meteorological phenomena. SeeNEXRAD. See also lidar. [From: Radio de-tection and ranging]

radar altimetry A method, usingRADAR, of monitoring the distance from asatellite or an aircraft to a surface (e.g. asea or ice surface) by sending a pulse of mi-crowave radiation from the altimeter downto the surface – and measuring its traveltime to cover that distance and return tothe receiver on the satellite. The instrumentcan estimate the distance to within a fewcentimeters, permitting the routine map-ping of the height of wind-driven waves.

radar echo The signal returned to aradar antenna from specific objects, suchas PRECIPITATION particles. The BACKSCAT-TER produced by the objects is also knownas the echo.

radar imaging A method, using RADAR,of mapping the location and characteristicsof selected environmental phenomena byemitting a pulse of microwave radiationand analyzing the image partly scatteredback by them. Images of the intensity anddistribution of rain, or the height and ori-entation of wind-driven ocean waves areexamples of radar imaging.

radar meteorology The branch of me-teorology concerned with the use of (pri-marily) ground-based RADAR in both theanalysis and prediction of atmosphericphenomena over a wide variety of dis-tances. It includes research into the struc-ture and evolution of TORNADOES, TROPICAL

CYCLONES, and EXTRATROPICAL CYCLONES.This work enables the occurrence of thesephenomena to be predicted.

radar scatterometer An instrumentthat illuminates the Earth’s surface from asatellite or aircraft in order to deduce envi-ronmental variables from the BACKSCAT-TER, which varies with the roughness of theland, ice, and sea surfaces.

radar winds Atmospheric motionsensed by tracking a radar target attachedto a RADIOSONDE or the motion monitoredby a DOPPLER RADAR.

radiance The radiant flux density perunit solid angle, of a source of radiation. Itis measured by satellites, for example, andis expressed as a flow of energy in watts persquare meter (per unit solid angle).

radiation The transmission of energyprincipally by electromagnetic waves butalso by particulate cosmic radiationthrough space or a material medium. Elec-tromagnetic waves do not need air as amedium through which to propagate.

radiation balance The resultant flux ofincoming solar and outgoing terrestrial ra-diation through a horizontal surface. It isregarded as positive if the incoming fluxexceeds the outgoing flux. See net radia-tion.

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radiative-convective model

radiation budget The flux of individualcomponents of the incoming solar and out-going longwave (terrestrial) radiation; itincludes ABSORPTION and REFLECTANCE.

radiation densimeter /den-sim-ĕ-ter/ Aninstrument used to measure the density(per unit area or per unit volume) of radia-tion at a specific location.

radiation fog (ground fog) FOG formedover land, generally at night under clearskies when conditions are calm or verynearly calm in moist air. It relates to thecooling mechanism being provided by RA-DIATIVE COOLING of the Earth’s surface andlowest layers of the atmosphere in whichthe temperature of the air near the groundis reduced to below its dew-point. The fogis often deepest at sunrise but usually dis-perses soon after dawn when heated bysolar radiation; it may persist for longerover cold wet surfaces in winter. Radiationfog occurs most commonly in the autumnand winter when cooling is strong and thehours of darkness are more extensive.

Low-lying river valleys are particularly sus-ceptible to radiation fog as a result of thehigh moisture levels and cooler air sinkinginto the valley.

radiation inversion A temperature IN-VERSION in the layer of the atmosphere incontact with the ground surface caused bythe dominance of outgoing terrestrial radi-ation at night.

When NET RADIATION is negative, as oc-curs during the night, the air in contactwith the ground is cooled so that the nor-mal decline in temperature with height isreversed or inverted.

radiation night A night on which thelowest layers of the atmosphere are with-out cloud or wind allowing strong radia-tional cooling of the ground and adjacentair to take place. Such nights occur mostfrequently in winter in air masses with lowabsolute humidity.

radiative-convective model (RCM) Asimple thermodynamic MODEL used to sim-

carried byrising air(24)

reflected by clouds andatmosphere(77)

radiateddirectly tospacefrom Earth(40)

absorbed byatmosphere (67)

carried toclouds andatmosphereby latentheat (78)

radiated fromEarth’s surface(390)

back radiationfrom atmosphere(324)

outgoing long-waveradiation (235)

Note: numbers in bracketsare watts per square meter

emitted byatmosphere andclouds (195)

radiated toatmosphereand clouds(350)

reflectedby Earth’ssurface(30)

absorbed atsurface(168)

total reflectedsolar radiation (107)

incident solarradiation (342)

Earth’s radiation and energy budget

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ulate the equilibrium temperature profileof the atmosphere and surface usingknown solar radiation at the top of theatmosphere and information about the at-mospheric composition and surface al-bedo. Composition of the atmosphere mayinclude information about cloud heights,depth, and cloud water amount, as well aswater vapor and other gas concentrations.

radiative cooling A negative flux of ra-diation from the Earth’s surface leading tocooling in the adjacent layer of air duringnight under clear sky conditions. See radi-ation night.

radiative forcing A change in the RADI-ATION BALANCE at the TROPOPAUSE in re-sponse to some externally appliedalteration in radiation to the climate sys-tem, such as an increase in GREENHOUSE GAS

concentrations (excluding water vapor),forcing an adjustment in the RADIATION

BUDGET. It is expressed in Watts per squaremeter (W m-2).

radiative warming See global warm-ing.

radiatus /ray-dee-ay-tŭs/ A variety ofcloud that occurs in parallel bands, alignedsuch that the lines of cloud appear to con-verge toward a point on the horizon (theradiation point) as a result of the effect ofperspective. It is applied to CUMULUS, STRA-TOCUMULUS, ALTOSTRATUS, ALTOCUMULUS,and CIRRUS. See also cloud classification.

radio altimeter See altimeter.

radiocarbon dating /ray-dee-oh-kar-bŏn/See carbon dating.

radiometer /ray-dee-om-ĕ-ter/ An in-strument used for detecting and measuringelectromagnetic radiation. Radiometerscan respond to a wide band of wavelengths(e.g. solar radiation) or to very narrowbands (e.g. infrared radiation). See alsopyranometer; pyrheliometer.

radiometry /ray-dee-om-ĕ-tree/ The sci-

ence of measuring radiation, especially thatassociated with infrared radiation.

radiosonde /ray-dee-oh-sond/ (radio-sounding device) An instrument pack-age, attached to a meteorological balloonfilled with a lighter-than-air gas such as he-lium or hydrogen, and carried aloftthrough the atmosphere to send back me-teorological readings including pressure,temperature, and humidity via a radiotransmitter. The data collected is transmit-ted to a ground station. The balloon istracked by a radiotheodolite or by radar todetermine wind speed and direction. Ra-diotheodolites used to track the balloonand receive the radio signals are beingreplaced by the use of the GLOBAL POSITION-ING SYSTEM (GPS). Balloon-borne radioson-des can reach altitudes as high as 30,000km (100,000 ft) before the balloon burstsand the radiosonde is returned to theground carried by parachute. See also bal-loon sounding; ozonesonde; pilot balloon.

rain PRECIPITATION in the form of liquidwater droplets, with diameters greater than0.5 mm (0.02 in). It is formed by conden-sation of water vapor in the atmosphereand falls under the influence of gravityfrom clouds to the Earth’s surface.

rainbow A well-known optical phe-nomenon that takes the form of an arc ofthe colors of the spectrum, which occurswhen the Sun’s rays are refracted and re-flected by water droplets in the atmos-phere. It appears when the Sun is shiningand it is raining in the opposite direction tothe Sun; it is seen when the observer is be-tween the Sun and the rain. The angle of re-flection determines the size of the rainbow.A primary bow appears with an angulardistance of about 42° centered on the anti-solar point; a secondary bow is often alsovisible with an angular distance of 51°. Thebow is produced when sunlight is refractedas it passes through the individual waterdroplets and is reflected from the rear ofthe droplets. In the primary bow the colorseparation resulting from the refractionproduces a spectrum with red on the out-side of the bow going through to violet on

radiative cooling

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the inner edge. The secondary bow, if vis-ible, is dimmer and the colors appear in thebow in reverse order (i.e. red is on the in-side and violet on the outside) as the lightis reflected twice within the raindrop be-fore it exits.

rain day 1. In the US, any day with mea-surable rainfall.2. In the UK, an obsolete term for a periodof 24 hours from 0900 UTC during whichat least 0.2 mm (0.008 in) of rain isrecorded. In other countries, the thresholdof 1 mm (0.04 in) is sometimes used.

raindrop A droplet of water with a di-ameter of at least 0.5 mm (0.02 in) andusually in the range 1–2 mm (0.04–0.08 in)that falls to the Earth’s surface from the at-mosphere; DRIZZLE droplets have diametersless than 0.5 mm (0.02 in). Very smalldroplets evaporate in the unsaturated airbelow the cloud base; large droplets, withdiameters around 5 mm (0.2 in), frequentlydisintegrate under aerodynamic forces be-fore reaching the ground.

raindrop size spectrum The size distri-bution of RAINDROPS in a PRECIPITATION

event. In an intense rainfall event, dropswith diameters over 5 mm (0.2 in) canoccur; in an event of moderate intensity thedroplets are typically less than 0.75 mm(0.03 in) in diameter.

rain factor In climatology, an index thatquantifies the degree of aridity of a loca-tion. Devised in 1920 by R. Lang, the rainfactor (rf) is expressed as mean annual pre-cipitation (mm)/mean annual temperature(°C). As precipitation decreases and tem-perature increases, rf decreases and the de-gree of aridity increases.

rainfall The total amount of PRECIPITA-TION of all forms. It is usually the totaldepth of water (including any melted solidforms of precipitation that have occurred)collected in a rain gauge at a particular lo-cation over a period of time, and is usuallymeasured once every 24 hours. Rainfalland precipitation are also used inter-changeably.

rainfall intensity (precipitation intensity)The amount of RAINFALL or PRECIPITATION

that occurs over a particular period; a mea-sure of the rate of rainfall receipt. Rainfallintensity (rainfall amount/duration) is typ-ically expressed in millimeters or inches perunit time, usually per hour (mm/h or in/h).High rainfall intensities often lead to local-ized flooding, especially when they occurfollowing periods of heavy rainfall.

rain-free base The low dark cloud basefrom which no rain is seen to be falling. Itusually extends away from the core, whereair is rising in the updraft, in a THUNDER-STORM.

rain gauge (precipitation gauge) An in-strument used for collecting and measuringprecipitation (rainfall and solid precipita-tion, e.g. snow). The standard rain gaugeconsists of a 50 cm (20 in) high cylinderwith a 20 cm (8 in) diameter funnel and anarrow tube to collect the rain and mini-mize evaporation losses. The depth ofwater is usually measured once a day by anobserver; either directly from the gradu-ated tube, or with a standard ruler, or thecollected water is poured into a graduatedmeasuring vessel. Any water that has over-flowed from the inner tube into the outercylinder is also measured. Rainfallamounts are determined to the nearest 0.1mm or 0.01 in. Some rain gauges alsorecord rainfall intensity. Self-recordingrain gauges include the HYETOGRAPH, TIP-PING-BUCKET RAIN GAUGE, and the WEIGHING

RAIN GAUGE.

rain making Attempts to generate rainby artificial means, primarily throughCLOUD-SEEDING experiments. See artificialrain.

rain shadow An area to the lee of amountain or range of mountains or hillsthat is sheltered from the prevailing rain-bearing winds and so receives loweramounts of rainfall than the windwardside. A rain shadow is a product of theOROGRAPHIC PRECIPITATION mechanism:forced ascent of moist air results in heavyrainfall on the windward slope, but this air

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stream is compressed, and so warms anddries out, as it descends the lee slope. Forexample, the desert areas of eastern Cali-fornia and Nevada form a rain shadow,being sheltered from the prevailing wester-lies by the Coastal Range and SierraNevada.

range The straight-line distance from aRADAR’s antenna to the target. It is not thedistance projected vertically onto the sur-face, but the true value along the inclinedradar beam.

Rankine scale /rank-in/ An absolutetemperature scale that is sometimes used inthe US. The zero point of the scale is at ab-solute zero but it differs from the Kelvinscale (see Kelvin temperature) in that it isbased on the units of the FAHRENHEIT

SCALE, so one Rankine degree (1°R) is 5/9of one kelvin degree (1 K). The freezingpoint of water on the Rankine scale is491.67°R. It is named for the Scottishphysicist William J. M. Rankine (1820–70).

Raoult’s law /rah-oolz/ A physicalchemistry law that describes the relation-ship between the VAPOR PRESSURE of a solu-tion and the SATURATION VAPOR PRESSURE ofa component over a solution, and the molefraction of the component in the solution.The law states that the partial pressure of asolute in equilibrium with a solution isequal to the mole fraction of the solutetimes the vapor pressure of the pure solute.The presence of a solute will lower the sat-uration vapor pressure. The law was dis-covered by the French scientist FrançoisRaoult (1830–1901).

rapid deepener See explosive cyclogen-esis.

ravine wind A wind generated as a re-sult of a pressure difference (i.e. pressuregradient) between two ends of a narrowvalley. The ravine wind blows from higherto lower pressure, its velocity being in-creased by the FUNNELING effect created bythe presence of the ravine itself.

rawinsonde /ray-win-sond/ A RA-DIOSONDE, tracked by radar or radio-theodolite to measure wind speed anddirection aloft.

Rayleigh scattering /ray-lee/ The SCAT-TERING of ELECTROMAGNETIC RADIATION

produced in the atmosphere when the scat-tering particles, such as air molecules, aresmall compared to the wavelength of theincident RADIATION. The amount scatteredis inversely proportional to the fourthpower of the wavelength: the shorter thewavelength of the incident light, the morelight is scattered. The Rayleigh scatteringdominates the shorter end of the visiblespectrum, hence the blue color of the day-time sky. Sunsets and sunrises appear red-dish because the light travels a longer paththrough the atmosphere than at middayand the scattering of the shorter blue wave-length is more complete leaving the re-maining reddish colors of the spectrum. Itis named for the British physicist BaronRayleigh (1842–1919). Compare Mie scat-tering; non-selective scattering.

RCM See radiative-convective model.

Réaumur scale /ray-ŏ-myoor/ A tem-perature scale developed in 1731 by theFrench physicist and inventor, René-An-toine Ferchault de Réaumur (1683–1757).The freezing point of water is representedby 0 degrees and the boiling point by 80degrees, at normal atmospheric pressure.The scale is now little used.

recurrence interval The average num-ber of years that separate a RAINFALL IN-TENSITY of a given magnitude at aparticular location. The recurrence intervalis used to define the event’s return period.For example, suppose that there have onlybeen two occasions over the last 100 yearswhen the rainfall intensity at a location Xhas equaled or exceeded 75 mm/h (3 in/h).The return period of such an event is thusonce in every 50 years. (This does not meanthat events of this magnitude occur at pre-scribed 50-year intervals, e.g. 1900, 1950,2000, but that 50 years is statistically themost likely interval between such events.)

range

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An analogy can be drawn with a bag con-taining 98 black balls and 2 white balls.While the most likely outcome is for 1 inevery 50 balls drawn from the bag to bewhite, it is possible that the 2 white ballscould be drawn in rapid succession, or evenconsecutively. The occurrence of two 75mm/h (3 in/h) events in the course of onlytwo years is thus not necessarily inconsis-tent with such events having a return pe-riod of 1 in 50 years. Recurrence interval/return period estimation is most reliablewhen based on long time series (<100years); estimates made using short recordlengths are frequently unreliable. Returnperiods are often given following majorflood events in order to quantify the rarityof such events.

red rain See blood rain.

reflectance (reflection, reflectivity) Thereturn into space of solar radiation fallingon a surface that is the boundary betweentwo media (such as between clear air and acloud). Total reflection occurs when all theincident radiation is returned. About 4%of the incoming solar radiation reachingthe Earth’s surface is directly reflected backto space, with no change to the wave-length. See also albedo.

refraction The deflection in the direc-tion of propagation of a wave (e.g. electro-magnetic radiation) when it passesobliquely from one medium to another inwhich the wave velocity is different. This isdescribed by SNELL’S LAW. In the atmos-phere the different mediums may be, forexample, air layers of different density ortemperature, or the air and water droplets.Refraction is responsible for a number ofphenomena in the atmosphere, includingHALO phenomena, RAINBOWS, and MIRAGES.

regime A recurring pattern, which canoccur with climate.

relative humidity The amount ofWATER VAPOR in the atmosphere at a giventemperature usually expressed as a per-centage of the amount of moisture that canexist in the same volume of air if it is satu-

rated at the same temperature. Warm aircan contain more water vapor than coldair, so the same amount of ABSOLUTE HU-MIDITY or SPECIFIC HUMIDITY in a parcel ofair will have a higher relative humidity ifthe air is cooler and a lower relative hu-midity if the air is warmer. See also dew-point.

relative vorticity See vorticity.

relief rain (relief rainfall) See oro-graphic precipitation.

remote sensing The range of methodsused in measuring environmental variablesby monitoring, quantifying, and processingsignals either emitted by natural features(PASSIVE REMOTE SENSING) or scattered backto an artificial signal source (ACTIVE RE-MOTE SENSING). The method is described asremote as the measuring instruments usedare never in direct physical contact with themedium being measured. SATELLITE im-agery, RADAR and LIDAR, and aerial photog-raphy are all examples of remote-sensingmethods that have extensive applicationsin meteorology and climatology.

reshabar /resh-ă-bar, ri-shah-bar/ 1. Astrong northwesterly wind that blowsacross the Caucasus Mountains from theBlack Sea in the west to the Caspian Sea inthe east.2. A LOCAL WIND that affects northernSyria, northern Iraq, western Iran, andsoutheastern Turkey, correspondingroughly to the Kurdish homeland. It is coldin winter and hot in summer.

residence time The length of time forwhich a particular constituent persistswithin a reservoir, such as the atmosphere.Residence times of gases in the atmospherevary greatly; for ammonia it is around 10days, for nitrous oxide approximately 20years, while for nitrogen it is about 16 mil-lion years. See also atmospheric lifetime.

resolution In remote sensing, the size ofthe smallest element in the satellite-viewedscene that can be resolved by an instru-ment, which determines the sharpness of

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the image received. The theoretical limit ofwhat can be resolved by an optical systemdepends on the wavelength of the radiationbeing observed (the smaller the better), thesize of the sensor’s instantaneous field ofview (IFOV; the smaller the better), and thedistance from the Earth (the smaller thebetter).

retrogression (retrograde motion) Inmeteorology, motion that goes against thenormal flow. For example, a situation inwhich large-scale ROSSBY WAVES (longwaves) move westward, contrary to thegenerally westerly winds flowing throughthe pattern. More generally, it is the condi-tion in which such waves move in the op-posite direction to the air flowing throughthem. It can also refer to a complete changein direction of a weather system, such as aLOW.

return flow 1. A part of an eddy that oc-casionally forms to the lee of a hill top ormountain peak. Under certain conditions arotor or roll-like motion about a horizon-tal axis can form to the lee; the return flowis that component of the rotor that flowsback toward the top or peak, in oppositionto the general air flow across the barrier.2. Winds that flow on the west side of aneastward-moving anticyclone (high); fromthe south in the northern hemisphere andfrom the north in the southern hemisphere.

return period See recurrence interval.

return stroke (main stroke, returnstreamer) The final phase of a CLOUD-TO-GROUND DISCHARGE stroke in which intenseluminosity propagates upward from theEarth’s surface to the base of a thunder-cloud, and along which the major chargetransfer (∼10 C at ∼30 kA) occurs. It takesless than 100 microseconds. The returnstroke follows the formation of a suitablelightning channel by a STEPPED LEADER andascending streamers.

revolving storm See tropical cyclone.

ridge (wedge) An elongated area of rela-tively high pressure, characterized by a

generally anticyclonic type of weather (i.e.generally settled fair weather), that may bean extension of an ANTICYCLONE or high-pressure region and is bounded by lowpressure. It is similar in the pattern ofmean-sea-level isobars to a ridge on a land-surface topography map. Compare trough.

rime (rime ice) A deposit of white,opaque, rough-textured ice crystals thatforms when a cloud or fog of supercooledwater droplets comes into contact with asolid surface in which the temperature isbelow 0°C (32°F), the freezing point ofwater. Soft rime forms from the waterdroplets in fog, hard rime from the freezingof water drops in DRIZZLE. Rime ice accu-mulates on the windward side of objects(e.g. a fence) to form frost feathers. It oc-curs mainly in upland areas in winter andis uncommon at low elevations where su-percooled fogs are rare. See also freezingfog.

roaring forties The strong westerlywinds of the temperate belt of the southernhemisphere, usually defined as being be-tween 40° and 50°S and thus encompass-ing part of the desolate Southern Ocean,where there are few landmasses to disruptthe atmospheric flow. Consequently, thisregion is affected by strong and persistentwesterly winds of over 40 knots (46 mph),with depressions tracking from west to eastwith great regularity; they bring rain butalso relatively mild conditions. Its stormyconditions and isolation make this a haz-ardous zone, which is avoided by mostshipping. See also brave west winds.

rocketsonde /rok-it-sond/ A package ofinstruments transported into the thermos-phere of the upper atmosphere by a meteo-rological rocket before being ejected anddescending to the Earth’s surface borne byparachute. It provides instantaneous verti-cal profiles of a number of variables, suchas temperature, pressure, ozone levels, andwind speed and direction as it descends.For example, temperature is measured by athermistor, the results being transmitted toa ground station, and the rocketsonde maybe tracked by radar or carry a transponder

retrogression

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rotor streaming

that can be tracked by a radio-directionfinder and ranging system to determine al-titude and wind speed. See also ra-diosonde.

roll cloud An elongated low-level AC-CESSORY CLOUD in the shape of a horizontaltube that appears to spin slowly about itshorizontal axis and is completely detachedfrom the cumulonimbus base. A relativelyrare cloud, it occurs along the GUST FRONT

that flows out across the surface from athunderstorm and typically is observedalong the leading edge of thunderstorms orsquall lines. It is sometimes associated witha COLD FRONT. The formation of roll cloudsis not fully understood but may result fromthe motions of warm air riding up and overthe cool air from the downdraft that ismoving down and under creating aswirling motion or eddy.

rope cloud Long thin lines of low cloudthat have been observed on satellite imagesto be associated with rapidly deepeningFRONTAL CYCLONES.

rope stage The decaying stage of a TOR-NADO indicated by a narrowing and shrink-ing of its condensation funnel.

Rossby wave /ross-bee/ See long wave.

rotating cups anemometer See cupanemometer.

rotation of the Earth The rate at whichthe Earth rotates, an important factor inmany of the equations that are used to ex-

plain the motions of the atmosphere. It is2π radians per day (or in 86,400 seconds),i.e. 7.27 × 10–5 radians per second.

rotor cloud A small ragged cloud thatsometimes occurs to the lee of hill tops ormountain peaks in association with aneddy that forms just downwind of such fea-tures, or underneath an outbreak of len-ticular cloud (see lenticularis). In theorographic case, the eddy rotates about ahorizontal axis so that the upper part of therotor blows toward the top or peak, con-trary to the general flow across the barrier.Rotors can be cloud free, but if the humid-ity conditions are right, they can contain aslowly rotating cloud. Underneath lenticu-lar clouds to the lee of a barrier, the rotorclouds mark the presence of an overturningcell of air that is often very turbulent.

rotor streaming Air motion in a verti-cal plane that is a product of streamline di-fluence to the lee of a topographic barrier.It occurs downwind of the highest veloci-ties, which occur in the immediate lee ofthe mountain range. The mesoscale circu-lation cell established can often lead to thelocal wind direction (the DOMINANT WIND)being diametrically opposed to thegeostrophic-flow direction (the PREVAILING

WIND). The phenomenon is chiefly associ-ated with LEE WAVES. The ascent of moistair in the eddy circulation often producesROTOR CLOUD, such as that seen to the eastof the New Zealand Alps.

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Saffir–Simpson Hurricane Scale /sa-feer simp-sŏn/ A reference scale to clas-sify the strength of hurricanes, based oncentral pressure, wind speed, and the mag-nitude of an associated STORM SURGE. It isused by the US National Weather Serviceto assess the potential damage to propertyand flooding from a hurricane landfall.The scale is also now used elsewhere in theworld to assess the strength of all TROPICAL

CYCLONES with wind speeds greater than64 knots (75 mph). All winds in the scaleuse the US one-minute average. The scalewas developed in the early 1970s by Her-bert Saffir (1917– ), a consulting engi-neer, and Robert Simpson (1912– ), thendirector of the US National HurricaneCenter.

Saint Elmo’s fire (corposant) The vis-ible luminous glow of an electrical dis-charge that may be seen, for example, on the masts of ships, aircraft propellersand wing tips, lightning conductors, andmetal towers; it can also often be heard. Itoccurs during thunderstorms when there ishigh voltage within the atmosphere be-tween the cloud and the ground and the

electrical charge affects the air molecules,tearing them apart, and causing them toglow.

Saint Swithin’s day /swith-’nz/ In UKweather folklore, it is said that if it rains onSt Swithin’s day (July 15), rain will also fallon the 40 days following. There are nogrounds for this in reality.

SALR See saturated adiabatic lapse rate.

saltation /sal-tay-shŏn/ An EOLIAN

transport process, in which a turbulent air-flow lifts particles steeply from the surfaceinto a highly localized zone possessinghigher velocities. The particles are thentransported a short distance downstream,before being gently dropped to the surfaceunder the force of gravity.

samoon /să-moon/ (samun) A character-istically hot and dry FÖHN-type wind inIran; it results from the ADIABATIC com-pression of an airflow descending from themountains of Kurdistan.

samuel See simoom.

S

SAFFIR–SIMPSON HURRICANE SCALE

strength central pressure wind speed storm surge magnitude

inches Hg hPa knots mph m ft

1 28.91 980 64–82 75–95 1.2–1.5 4–52 28.50–28.91 965–979 83–95 96–110 1.8–2.4 6–83 27.91–28.49 945–964 96–113 111–130 2.7–3.7 9–124 27.17–27.90 920–944 114–135 131–155 4–5.5 13–185 <27.17 <920 >135 >155 >5.5 >18

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sand pillar See dust devil.

sandstorm A desert storm in which fineor coarse sand is levitated by strong winds.Visibility is reduced, sometimes to only afew meters. Sandstorms are usually verti-cally shallow, rarely exceeding 30 m (100ft) in depth. They may last from minutes tohours and can progress across desert areas.See also dust storm.

Santa Ana (Santa Anna) A LOCAL WIND

with FÖHN characteristics experienced insouthern California when a north to eastwind blows during the passage of a low-pressure system off the Californian coast inlate fall, winter, and early spring. Thename derives from the town of Santa Ana,located to the southeast of Los Angeles,California. The wind blows from the GreatBasin high, which lies usually over S Idaho,Utah, Nevada, and eastern California, thencrosses the coastal ranges, descendingthrough narrow passes, such as the Cajonand Santa Ana, to reach the southwest Cal-ifornian coast as a hot, dry (with a conse-quent lowering of humidity), andsometimes dusty wind. The desiccativepower of the Santa Ana can cause muchdamage to crops, particularly when it oc-curs during the spring growing season, andcan also create dangerous wildfire condi-tions, especially in late fall.

SAR See synthetic aperture radar.

sastruga (plural sastrugi) A wave withsharp ridges formed in hard surface snowin polar regions. Sastrugi may be up to 2 m(6 ft) high; they are formed through ero-sion and deposition by the wind and areusually aligned parallel to the direction ofthe prevailing wind. [Russian: ‘groove’]

satellite 1. (natural satellite) A body thatis constrained to orbit a larger celestialbody; for example, the Moon is a satelliteof the Earth, which in turn is a satellite ofthe Sun.2. (artificial satellite) A manufacturedspacecraft launched into orbit around theEarth or another celestial body. METEORO-LOGICAL SATELLITES provide considerable

data for weather forecasting and research.See also geostationary satellite; polar-or-biting satellite.

satellite sounding A SOUNDING, such asan instantaneous vertical profile of temper-ature and humidity, obtained from instru-ments on a meteorological satellite. Severalthousand soundings are produced everyday by meteorological satellites, which arefed into operational NUMERICAL WEATHER

PREDICTION models.

SATEM A specific SYNOPTIC CODE usedto relay degraded quality vertical tempera-ture and humidity profiles from the NOAApolar-orbiting satellites. These messagesare relayed with some delay over theGlobal Telecommunications System.

SATOB A specific SYNOPTIC CODE usedto relay cloud motion vector values derivedautomatically from the GEOSTATIONARY

SATELLITES, such as METEOSAT. They aretransmitted with some delay over theGlobal Telecommunications System.

saturated adiabat A curved line drawnon a THERMODYNAMIC DIAGRAM that tracesthe path of a saturated air parcel movingthrough the atmosphere adiabatically. Seedry adiabat.

saturated adiabatic lapse rate (SALR,moist adiabatic lapse rate, wet adiabaticlapse rate) The change of temperaturewith height of a saturated air parcel that iseither ascending or descending adiabati-cally. In a rising parcel of air the saturatedadiabatic lapse rate is lower than the DRY

ADIABATIC LAPSE RATE as a result of the re-lease of latent heat that takes place whensome of the moisture held by the parcelcondenses. The saturated adiabatic lapserate varies depending on the amount ofmoisture contained by the air: the greaterthe amount, the smaller the adiabatic lapserate. It is generally in the range 4–9°Ckm–1. As the air parcel rises and cools andmoisture is lost via condensation the lapserate increases and in the upper tropospherethe magnitude of the saturated adiabaticlapse rate approaches that of the dry adia-

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batic lapse rate. The saturated adiabaticlapse rate is sometimes called a processlapse rate, as it refers to the thermal behav-ior of an air parcel as opposed to that ofthe surrounding environmental air.

saturated air (saturation) A conditionin which the level of WATER VAPOR in a par-cel of air is the maximum possible at theexisting temperature and pressure withoutbringing about condensation. In saturatedair the rate at which water molecules enterthe air by evaporation exactly balances therate at which molecules leave by condensa-tion. For practical purposes the capacity ofthe air to hold water vapor predominantlydepends on temperature. The addition ofmore water vapor or a decrease in temper-ature will lead to condensation.

saturation See saturated air.

saturation deficit The difference be-tween the actual VAPOR PRESSURE of a moistsample of air at a given temperature andthe SATURATION VAPOR PRESSURE corre-sponding to that sample. It is also referredto as the Vapor Pressure Deficit.

saturation mixing ratio A measure-ment of the maximum capacity of air tocontain water vapor. It is usually expressedas the grams of water vapor that would beneeded to saturate the air in each kilogramof air.

saturation vapor pressure The maxi-mum possible partial pressure of watervapor in the atmosphere at a given temper-ature. The higher the air temperature, thegreater the saturation vapor pressure, asexpressed by the CLAUSIUS–CLAPEYRON RE-LATION; saturation vapor pressure in-creases rapidly with temperature and at32°C (90°F) is around double the value at21°C (70°F). Saturation vapor pressure isgreater over a curved surface (e.g. a waterdroplet) than over a plane surface, and isgreater over pure water than over waterwith a dissolved solute.

savanna (savannah) Tropical or sub-tropical open grassland with widely spaced

drought-resistant trees and shrubs. It has apronounced dry season during whichbrush fires are periodically a natural occur-rence. Savanna extends over much of trop-ical Africa and also occurs in SouthAmerica, India, Madagascar, and Aus-tralia. In the Köppen climate classificationsavanna is type A.

savanna climate See tropical wet-dryclimate.

scattering The process by which solarradiation incident on particles (e.g. airmolecules, water droplets, pollutants) isabsorbed and rapidly emitted in anotherdirection; it may be dispersed in all direc-tions, though not necessarily in equalamounts. The particles have an index of re-fraction that differs to that of the mediumwithin which they are held. The scatteredradiation does not undergo a change of fre-quency but its phase and polarization maychange considerably.The type of scatteringdepends on the wavelength of radiationand the size of the particles. See backscat-ter; Mie scattering; non-selective scatter-ing; Rayleigh scattering.

saturated air

192

supercooledwater

ice

temperature/°C

–20 –10 0 10 20 30| | | | | ||

|

|

|

|

|

|

|

| 40

30

20

10

0

vap

or

pre

ssu

re/m

b

Saturation vapor pressure curve

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scatterometer A radar instrument thatis borne by aircraft and satellites. It trans-mits radar pulses which are reflected asbackscattered energy, the intensity depend-ing on the surface roughness of the targetarea. The instrument was originally devel-oped for measuring near-surface windspeeds and direction over the ocean but hassince been used in a wide range of ice andland applications and is a useful tool inlong-term climate studies. For example, ithas been used for observations of polar ice,ice sheets, sea ice, and the tracking of ice-bergs; on land it has been used for soilmoisture and vegetation studies.

scavenging The process by which par-ticulate matter in the atmosphere is cap-tured and removed by precipitation.

scintillation /sin-tă-lay-shŏn/ The rapidchanges in brightness giving a twinkling ef-fect in the appearance of stars and other ce-lestial bodies. It results from refractionproduced by variations of density of airwithin the atmosphere through which thelight rays pass. Compare shimmer.

scirocco /să-rok-oh/ (sirocco) A local,often dusty, wind that affects the Mediter-ranean Basin, especially during the spring;it rarely occurs in summer. Associated withthe leading edge of a low-pressure system,the scirocco sweeps hot and dry air fromthe Arabian or Sahara deserts northward.While the northern African scirocco is dry,the wind’s passage across the Mediter-ranean Sea results in the acquisition ofmoisture in its lowest layers. The sciroccotypically arrives on the northern shorelineof the Mediterranean as a warm but dampand oppressive wind. A number of region-ally specific names exist for the scirocco,for example KHAMSIN in Egypt, LEVECHE insoutheastern Spain, GHIBLI in Libya, andchili in Tunisia. The scirocco is thus thegeneric name for a collection of warmsouth to southeasterly LOCAL WINDS.

Scotch mist (mizzle) A type of PRECIPI-TATION that consists of a mass of minutewater droplets in a combination of bothMIST (or FOG) and DRIZZLE. It usually occurs

in upland or mountainous areas where thecloud base is close to ground level and theair mass is saturated but stable.

screen See instrument shelter.

scud A type of rapidly moving raggedgray or dark gray low cloud, usually FRAC-TOSTRATUS, that occurs beneath higher raincloud (NIMBOSTRATUS or CUMULONIMBUS).Scud clouds frequently occur in associationwith, or behind, the gust front of a thun-derstorm.

SDM See statistical-dynamical model.

sea breeze A thermally induced diurnalwind that blows onshore, and is most com-mon in low and mid-latitude locations. Itdevelops as a result of the land warmingmore rapidly than the sea during the day,which creates a PRESSURE GRADIENT directedfrom higher pressure over the sea to lowerpressure over the land. A sea breeze canreach speeds in excess of 25 knots (29mph), although values are typically 8–14knots (9–16 mph). The sea breeze’s verticaldepth increases throughout the day, some-times reaching 1.2 km (4000 ft). Its coolingeffect also penetrates inland, occasionallybeing felt at distances of more than 50 km(30 miles) from the coast by 9 pm. A con-vergence line called the sea breeze front,marks the boundary between the incomingsea air and the air over the land; a zone ofenhanced CONVECTION. The developmentof a sea breeze is particularly favored whenconditions are calm and a weak offshoreGEOSTROPHIC WIND component exists.Compare land breeze.

sea fog See advection fog.

sea fret See fret.

sea ice Ice that forms on the surface ofthe sea from sea water when temperaturesfall below approximately –2°C (28°F).During the process of freezing the salt inthe sea water is released and returned tothe sea leaving ice that is virtually purewater. Vast areas around Antarctica andover the central Arctic are covered by sea

193

sea iceFREEDOM PALESTINE


ice, which forms during the winter season.The extent of sea ice has a bearing on cli-mate; it thermally insulates the ocean anddecouples it from the surface atmosphericwinds, but also reflects solar radiationback to space. The extent of sea ice islargely dependent on the local currents.Fast ice is a type of sea ice that is attachedto land.

Seasat 1 /see-sat/ A short-lived but pio-neering satellite, specifically designed toobserve the oceans, which was launched inJune 1978. It orbited at a height of 800 km(500 miles) and had an on-board SYN-THETIC APERTURE RADAR, MULTISPECTRAL

SCANNER, radar altimeter (see radar altime-try), and scatterometer. It gathered signifi-cant data but unfortunately lasted only105 days before malfunctioning.

season A number of months of the yearwith pronounced climatic characteristics.The methods to determine the limits ofeach season are not clearly defined. Inmany middle-latitude countries of thenorthern hemisphere the year is conven-tionally divided into four seasons (WINTER,SPRING, SUMMER, and AUTUMN), even ifthere is little climatic justification for this.The four seasons are often taken to beginon the winter solstice (December 22/23),the vernal equinox (March 20/21), thesummer solstice (June 21/22), and the au-tumn equinox (September 22/23). In polarlatitudes there is a short summer seasonand long winter with no spring or autumnclearly identifiable. In the tropics, the divi-sion is usually based on receipt of precipi-tation, between the wet and dry seasons, orthe MONSOONS. Poleward of the tropics,seasonal variations in temperature producehot and cold (or cool) seasons.

seasonal prediction Longer-term andmainly experimental forecasts undertakenby some weather services. Such computer-based predictions normally take the formof a global assessment of the anomaly pat-tern of PRECIPITATION and/or TEMPERATURE

for a seasonal period (e.g. June to August).

sea-surface temperature (SST) The

temperature of the surface layer of the seaor ocean (approximately 0.5 m deep).

Secondary Ambient Air Quality Stan-dards See National Ambient Air QualityStandards.

secondary cold front (secondaryfront) The leading edge of markedlycolder air within a cold AIR MASS behind aprimary COLD FRONT. It is usually associ-ated with a TROUGH within the polar air.

secondary depression Generally, a rel-atively small-scale low-pressure area thatforms on the flank of a much larger ‘par-ent’ LOW, and is embedded within thelarge-scale winds that circulate around thelarger feature. See polar low.

secondary front See secondary coldfront.

secondary pollutant A pollutant that isnot emitted directly into the atmospherebut is formed as a result of chemicalchanges in the atmosphere, such as photo-chemical action (see photodissociation)and other reactions. Examples are ozoneand nitrogen dioxide. See also smog.

secular trend In climatology, the slowchange, either an increase or a decrease, inthe values of a climatic element (e.g. tem-perature) that takes place over a long pe-riod, after fluctuations that occur over acomparatively short period have been elim-inated. For example, the trend of increasedcarbon dioxide concentration in the atmos-phere produced by the combustion of fos-sil fuels.

seiche /saysh/ A stationary wave (i.e. awave that oscillates without progressing)that occurs in an enclosed or semi-enclosedbody of water, such as a lake, bay, or estu-ary. It is caused, for example, by strongwinds, earthquakes, landslides, or changesin atmospheric pressure. The oscillationcontinues for some time after the force ini-tiating its formation has ceased, occasion-ally lasting for several days.

Seasat 1

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sensible heat Energy held by a body asa form of heat that can be monitored orsensed, for example by means of ther-mometers; it is transferred by CONDUCTION

and/or CONVECTION without a change ofphase. It is a component of the BOWEN

RATIO. Compare latent heat.

sensible temperature The temperatureas it is felt or experienced by an individual.This can feel different to the temperatureactually recorded as a result, for example,of humidity levels, which can make condi-tions feel hotter (see heat index; tempera-ture–humidity index) or wind speed, whichcan make conditions feel colder (see windchill).

sensor The part of a scanner (e.g. on asatellite) that is sensitive to light or otherelectromagnetic radiation and measuresthe RADIANCE from natural objects. A scan-ner may have multiple sensors to eithersense different bands or increase the spatialresolution.

serein /se-ran, -rang/ The phenomenonof fine rain that falls from an apparentlycloudless sky overhead. It can be explainedby either the movement of the cloud systemaway from the observer’s overhead posi-tion at the time the rain reaches the groundsurface, or by dissipation of the cloud fol-lowing the formation of raindrops.

severe thunderstorm An intense THUN-DERSTORM with downdraft winds exceed-ing 50 knots (58 mph) and/or hailstoneswith a diameter of at least 20 mm (0.75 in).These thunderstorms are capable of pro-ducing heavy showers, flash floods, andtornadoes.

severe weather threat index (SWEAT)A numerical index developed by the US AirForce to indicate the probability of severeweather; it utilizes wind shear, wind speed,and instability.

shade temperature The temperature ofair measured by a thermometer mounted inan INSTRUMENT SHELTER. In this location,air circulates freely around the thermome-

ter while it is sheltered from the effects ofprecipitation, direct solar radiation, andenergy emitted from the ground and sur-rounding objects.

shamal /shă-mahl/ A northwesterlywind that blows across the Tigris and Eu-phrates plains in Iraq and down into thePersian Gulf. It occurs most often on sum-mer days and is associated with dry andhot conditions, its force sometimes initiat-ing dust storms. It decreases in intensity atnight.

sharav /shă-rahv/ A shallow low-pres-sure area that forms to the LEEWARD of theAtlas Mountains in North Africa; a form oflee cyclogenesis. The controlling upper-level trough intensifies slightly when cross-ing the mountain range and then induces alow-level cyclone in the lee. Sharav eventsbring hot and dry weather to the region.

shear See wind shear.

sheet lightning A diffuse illuminationof a cloud by a LIGHTNING discharge inwhich the forked discharge form is not vis-ible because of the presence of obfuscatingcloud.

shelf cloud See arcus.

shelterbelt See windbreak.

shimmer The wavering or tremulous ef-fect in images of features produced in thelowest layers of the atmosphere, above aheated surface. It results from the varyingatmospheric refraction in the line of sightproduced by variations in the temperature,and hence density, of the air. Comparescintillation.

ship surface observations Meteoro-logical observations made aboard aweather ship.

short-fuse warning A warning ofshort-lived severe weather issued by the USNational Weather Service (NWS). Theseinclude warnings for tornado events, thun-derstorms, and flash floods.

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short-range weather forecasting (short-term weather forecasting) The predictionof the weather, normally for 48 hoursahead.

shortwave radiation The ELECTRO-MAGNETIC RADIATION, with a peak in inten-sity at a wavelength of 0.5 µm, within arange of 0.3 µm to 4 µm, that is received ator near the Earth’s surface. Compare long-wave radiation.

short-wave trough A smaller scale fea-ture that is part of a LONG-WAVE trough.Such smaller troughs often move rapidlythrough the long-wave pattern, and are fre-quently associated with major cyclonic de-velopments.

shower A short-lived precipitation (e.g.rain, hail, snow) event that often has a du-ration of less than 10 minutes and startsand ends abruptly. Showers range in theirintensity between light, moderate, heavy,and violent and often change intensityabruptly. They are typically associatedwith deep convective clouds (e.g. cumu-lonimbus), which do not usually com-pletely cover the sky; brightness isfrequently evident during shower events.Showers can occur at any time of year, es-pecially in the tropics; in mid-latitude envi-ronments they occur with greatestfrequency on hot afternoons during sum-mer.

Shurin season /shoor-in/ A warm sea-son peak in rainfall beginning in late Au-gust and lasting through into October inthe vicinity of Japan. Much of the rainfalloccurs in association with tropical stormsthat recurve north in the western North Pa-cific.

Siberian high (Siberian anticyclone, Asianhigh, Asian anticyclone) An extensiveANTICYCLONE (high) that covers most ofmiddle- and high-latitude Asia during thewinter months and disappears in summer.It is a thermal high formed by RADIATIVE

COOLING, is characterized by extremely lowtemperatures, and extends up vertically tosome 2–3 km (1–2 miles). The air circulat-

ing out of it to the south forms the north-east MONSOON across the Indian subconti-nent and adjacent regions.

side-looking airborne radar (SLAR) An airborne radar system used to mapmorphological features on the Earth’s sur-face in such a way that the images exagger-ate the relief, somewhat analogous tophotography at low solar illumination an-gles.

silver thaw See glaze.

simoom /să-moom/ (simmoom) A localshort-lived SCIROCCO-type southerly windexperienced in the deserts of North Africaand Arabia, particularly in spring and sum-mer, and also beyond in central and south-ern parts of the Mediterranean. In desertregions, the simoom is extremely hot ataround 50°C (122°F) and dry, and carriesmuch dust and sand, sometimes reducingvisibility to practically zero. It is similar inappearance to a DUST DEVIL or WHIRLWIND.In Turkey, this wind is referred to locally asthe samuel.

singularity A period of distinctiveweather or a weather phenomenon thathas a tendency to recur in the majority ofyears on or about the same time. For ex-ample, the January thaw of New England,which occurs on or around January 20–23in most years.

In a development of the BUCHAN SPELL,singularities were formerly used to seek toidentify some regularity or pattern in theclimate of Western Europe. Verification ofthe objectivity in such a scheme is difficultand controversial.

sink In air pollution, the surface or veg-etation receiving pollutants removed fromthe atmosphere. It is the final phase in theatmospheric process of source, transport,transformation, and sink. For example,soil and trees act as natural sinks for car-bon from the atmosphere.

sinking See downwelling.

sirocco /să-rok-oh/ See scirocco.

short-range weather forecasting

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SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. See Ta-bles overleaf.

skew-T log-P diagram (modified ema-gram) A type of THERMODYNAMIC DIA-GRAM widely used in the US, in which thedry adiabats make an angle of about 45°with the ISOBARS and isopleths of SATURA-TION MIXING RATIO are almost vertical andstraight. Modified in 1947 from the origi-nal emagram, in order to create straighthorizontal isobars, the skew-T log-P dia-gram is similar in appearance but not iden-tical to a TEPHIGRAM. See also Stüvediagram.

sky cover The amount, in OKTAS, of thesky that is covered by different types ofcloud.

sky radiation See diffuse radiation.

sky-view factor The proportion of thetotal sky hemisphere that is visible to anobserver. It is particularly relevant as a pa-rameter in climate studies of urban areas,where tall buildings may obstruct the viewof much of the sky, and has important im-plications for surface temperatures.

SLAR See side-looking airborne radar.

slash and burn The practice of clearingland, typically in tropical rainforest, bycutting down the vegetation and clearing itby burning. The cleared plots of land arecultivated for a number of seasons beforebeing abandoned and a new site cleared.

sleet 1. In the US, PRECIPITATION in theform of ice pellets of less than 5 mm (0.2in) diameter. These grains are either frozenRAINDROPS or SNOWFLAKES that have par-tially melted and then refrozen after pas-sage through a cold layer of air at belowfreezing temperature close to the Earth’ssurface.2. In the UK, precipitation that takes theform of a mixture of rain and snow, or par-tially melted falling snow.

sling psychrometer /sÿ-krom-ĕ-ter/(whirling psychrometer) A type of PSY-CHROMETER used to measure relative hu-midity that is designed to ventilate thewet-bulb thermometer and thus speed upevaporation; the dry-bulb and wet-bulbthermometers are both mounted on a typeof frame. The wick of the wet-bulb ther-mometer is dipped in distilled water andthe psychrometer is then whirled aroundby the handle before readings are taken.

slope convection Generally large-scalemotion that is transporting heat, for exam-ple, in a shallow sloping manner ratherthan the more localized upright CONVEC-TION made visible by CUMULIFORM clouds.FRONTAL CYCLONES are examples of slopingconvection in the sense that the warm,moist air that streams poleward throughsuch a feature is acting as a massive gentlyinclined convective current.

slope wind See along-slope wind sys-tem; anabatic wind; katabatic wind.

slush A colloquial name for soiled snowand ice that has become softened and satu-rated with water from rain and/or warmertemperatures. It often clogs roads and side-walks during the thaw that follows a snow-fall.

small hail See graupel.

smog A type of polluted atmospherecontaining smoke and/or a high concentra-tion of aerosols and primary pollutants dueto adverse weather conditions. The termwas coined by H. A. Des Voex in 1905 todescribe ‘smoke-impregnated fog’ but isnow also used for concentrations of pollu-tants within the atmosphere above urbanareas where fog may not be present. Indus-trial smog results from fog in combinationwith smoke and sulfur dioxide released inthe burning of coal. This was responsiblefor the severe ‘killer smogs’ that occurredin the UK, during the mid-20th century,such as the London fog of 1952, which ledto the passing of the UK’s Clean Air Act of1956. Photochemical smog is a hazy at-mosphere resulting from a cocktail of

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198

TABLE 1: BASE AND SUPPLEMENTARY SI UNITS

Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr

*supplementary units

TABLE 2: DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

TABLE 3: DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

FREEDOM PALESTINE


mostly gaseous pollutants that have an ox-idizing action, occurring especially inurban areas experiencing high solar-radia-tion values (originally renowned as a fea-ture in Los Angeles, but now commonthroughout the urbanized world). Thecommonest oxidant is OZONE formed in theenvirons of the BOUNDARY LAYER by solarradiation dissociating molecules of oxidesof nitrogen (NOx) and volatile organiccompounds (VOCs), emitted chiefly by ve-hicle exhausts but also by industrial powerplants. Ozone is an irritant to humans andalso causes harm to plant tissue. In addi-tion to ozone, other secondary pollutantsinclude many hazardous chemicals.

smoke management In the UK, as a re-sult of ‘killer smogs’, the Clean Air Act of1956 was passed. Local authorities weregranted powers to authorize the use ofsmokeless combustion fuels in order topromote smoke management. These pow-ers were increased in 1968, and again in1993 with a new Clean Air Act. More effi-cient fuel burners were promoted until useof North Sea gas eased the problem. Seesmog.

Snell’s law The law of refraction statingthat the ratio of the sine of the angle of re-fraction (r) to the sine of the angle of inci-dence (i) is equal to the ratio of therefractive index (ni) of the material throughwhich the incident ray is passing to that ofthe medium through which the refractedray passes (nr), i.e.

sinr/sini = ni/nr.The law is named for the Dutch physicistWillebrord Snell (1591–1626).

snow A solid form of PRECIPITATION

comprised of ice crystals formed fromwater vapor in the atmosphere when thetemperature is below freezing-point. Whenthe temperature is well below 0°C (32°F),the snow may fall in the form of individualcrystals or minute ice spicules. As the tem-perature approaches 0°C (32°F), the icespicules aggregate to form SNOWFLAKES.The properties of snow (e.g. density) thusvary according to the temperature: dry andpowdery at very low temperatures, becom-

ing increasing wet, compact, and structure-less as the temperature approaches 0°C(32°F).

The variable density of snowfall makesit difficult to measure accurately; for in-stance, a normal rain gauge is blocked byfalling snow. Conventionally, snowfall ismeasured by inserting a ruler into a hori-zontal layer of undrifted snow. A depth of100–150 mm (4–6 in) of wet compactsnow typically produces 25 mm (1 in) ofwater; a much greater depth of 500–700mm (20–28 in) of dry powdery snow isneeded to produce the same quantity ofwater. Snow samplers can also be used toprovide more accurate measurements ofthe water content of a snow pack.

snow crystal A minute ice spicule thataggregates with other snow crystals toform a SNOWFLAKE. The shape of a snowcrystal is influenced by the temperatureand humidity at which it forms. At least6000 different shapes of crystal have beenrecognized: these range from hollowcolumnar prisms that form only in dry airbetween –25°C (–13°F) and –50°C(–45°F); dendritic crystals that develop inquite moist air at temperatures of –12°C to–16°C (10°F to 3°F); and thin hexagonalneedle crystals, which are most likely toform in moist air between 0°C (32°F) and–5°C (23°F).

snowflake An aggregation of SNOW

CRYSTALS. The size of a snowflake variesaccording to the temperature of formation,being generally larger as the temperatureapproaches freezing-point. See also snow.

snow gauge An instrument used for col-lecting and measuring the depth of snow.Snow gauges collect the solid precipitationand melt it before taking a reading.

snow grains A form of PRECIPITATION

comprising small opaque particles of ice.Snow grains are usually flattened and elon-gated in shape, having diameters of lessthan 1 mm (0.04 in). Such grains often fallfrom stratus clouds or from banks of fogthat occur on days with subfreezing tem-peratures.

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snow line The permanent snow line isthe altitude on a hillslope or mountain thatmarks the lower limit of permanent snowcover. Above the snow line, ACCUMULATION

of snow during the winter always equals orexceeds the amount lost by ablation (melt-ing and evaporation) in the summer. Theheight of the snow line, which is irregularand not always sharply defined, is heavilyaffected by summer temperatures andtherefore by latitude. On tropical moun-tains it may be as high as 5000 m (16,000ft), in the European Alps it typically occursbetween 2500 m (8000 ft) and 3000 m(10,000 ft), and in S Greenland it occurs ataround 600 m (2000 ft), before falling tosea level near the poles. The degree of ex-posure or shelter, slope angle, and theamount and seasonal distribution of PRE-CIPITATION also influence the altitude of thesnow line. These factors can result inmarked variations in the altitude of thesnow line over short distances. The sea-sonal (ephemeral) snow line marks thelower limit of seasonal snow cover duringwinter.

snow pellet See graupel.

snow sampler An instrument used tocollect samples of snow. It generally con-sists of a long metal cylinder, closed at oneend, which is driven into the snow pack.The water from the melted sample is mea-sured and analyzed.

soft hail See graupel.

SOI See Southern Oscillation.

soil moisture storage capacity Seefield capacity.

soil temperature The temperature ofsoil measured at a variety of differentdepths (e.g. 5, 10, 20, 30, and 100 mmdepths). These measurements are recordedat many weather stations.

solar constant The amount of energypassing in unit time through a unit surfaceperpendicular to the Sun’s rays at the topof the atmosphere. Current satellite mea-

surements indicate a value of 1367 W m–2.Lower values are recorded by instrumentson the surface of the Earth, as a result of at-mospheric ATTENUATION.

solar day The interval of time duringwhich the Earth makes one complete revo-lution on its axis in relation to the Sun. Theaverage length of the solar day is 86,400seconds.

solar energy See solar radiation.

solar flare A solar protuberance of in-candescent gas emanating from the Sun’schromosphere, usually above certain typesof sunspot, and resulting from an explosiverelease of electromagnetic radiation andenergetic particles. Flares last for a fewhours and may be classified on an ascend-ing scale of one to three plus based on theintensity of the emitted light. The radiationand particles affect the Earth’s geomag-netic field to produce aurorae and geomag-netic storms.

solarimeter /soh-lă-rim-ĕ-ter/ See pyra-nometer.

solar radiation The radiation emittedfrom the Sun, nearly all of which is in theform of ELECTROMAGNETIC RADIATION. Avery small fraction is emitted in the form ofparticles, known as particulate or corpus-cular radiation.

solar wind The electrically chargedelectrons and protons that stream out intospace at about 400 km s–1, produced by theionization of particles in the high solar at-mosphere. Though this corpuscular radia-tion (plasma) comprises less than amillionth of the total solar energy emittedby the Sun it interacts with the geomag-netic field of the Earth and can producestorms in the Earth’s magnetosphere.

SOLAS See Surface Ocean–Lower At-mosphere Study.

solstice 1. Either of the two points atwhich the Sun reaches its greatest or leastdeclination (i.e. is at its greatest angular

snow line

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distance above or below the celestial equa-tor) during a year.2. The date on which the Sun reaches eitherof these points. In the northern hemispherethe summer solstice is on about June 21and the winter solstice on December 21. Inthe southern hemisphere the dates are re-versed.

sonde /sond/ See radiosonde; rocket-sonde.

sounding A vertical profile of atmos-pheric properties, such as DRY-BULB TEM-PERATURE, RELATIVE HUMIDITY, and WIND

DIRECTION and WIND SPEED produced by in-struments attached to a RADIOSONDE bal-loon or ROCKETSONDE.

sounding balloon See balloon sound-ing.

source region An extensive area associ-ated with an ANTICYCLONE (high) over anocean or continent from which air spiralsout. It gives an AIR MASS its distinctiveproperties; for example, warm and moistfrom the SUBTROPICAL ANTICYCLONES andcold and dry from the winter CONTINENTAL

ANTICYCLONES. The areas over which theanticyclones lie are of generally even sur-face and light winds. Once air flows outfrom the source regions it becomes modi-fied, especially in the lower levels, by ex-changes of heat and moisture with theunderlying surface. For example, air fol-lowing a maritime track passing overocean surfaces will have its moisture con-tent increased in the lower layers.

southeast trade winds (southeast trades)The tropical TRADE WINDS, best markedover the oceans between about 5° and 15°latitude, that blow with significantstrength and notable constancy from thesoutheast. They occur over the southern-hemisphere tropics, flowing toward the In-tertropical Convergence Zone (ITCZ),where they converge with their counterpartNORTHEAST TRADE WINDS. They cross theEquator over much of the year to flow intothe ITCZ, and on doing so are deflectedinto southwesterlies.

South Equatorial Current See Equato-rial Current.

southerly buster An intense coldsqually southerly wind that advects polarair across southern and southeastern Aus-tralia, especially the coast of New SouthWales, in spring and summer. Thirtybusters affect this region in an averageyear, the majority of which occur betweenOctober and March. The southerly bustertypically follows the BRICKFIELDER, the180° switch in wind direction producing asudden fall in temperature of 10–15°Cwithin a few minutes. It usually occurs onthe westward limb of a low-pressuretrough, which sweeps a COLD FRONT north-ward from Antarctica; the mountains ofthe Great Dividing Range along the southcoastline tend to block the cold air andchannel it round to the east coast. Thesoutherly buster is frequently accompaniedby thunderstorms and strong gale-forcewinds with gusts up to 60 knots (70 mph).In South American countries, such as Ar-gentina and Uruguay, a similar phenome-non is known as the PAMPERO.

Southern Annular Mode See AntarcticOscillation.

southern lights See aurora.

Southern Ocean (Antarctic Ocean) Thebody of water encircling Antarctica. Thisocean comprises the southern parts of thePACIFIC, ATLANTIC, and INDIAN OCEANS. Un-like the northern-hemisphere oceans, theSouthern Ocean is not divided into geo-graphic sectors by the continental land-masses. The narrowest part of the ocean,only 970 km (600 miles) wide, is the DrakePassage between South America and theAntarctic Peninsula. Currents in the South-ern Ocean are dominated by the sinking ofwater that has been cooled by coastal land-masses. This cold water flows north alongthe ocean floor (see Antarctic bottomwater) and is replaced by water flowingsouth at the surface from the Indian, Pa-cific, and Atlantic Oceans.

Southern Oscillation A large-scale

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Southern OscillationFREEDOM PALESTINE


fluctuation in monthly/seasonal mean-sea-level pressure across part of the low-lati-tude Pacific Ocean. The oscillation is anoccasional ‘see-saw’ effect exhibited bychanges in the average pressure observed inthe South Pacific SUBTROPICAL ANTICY-CLONE (high) and the low-pressure areaacross Indonesia and northern Australasia.The Southern Oscillation Index (SOI) ismapped by monitoring the difference ofmonthly average mean-sea-level pressurebetween Tahiti (in the high) and Darwin(in the low). Tahiti is always higher, butwhen it is unusually high, the value at Dar-win is anomalously low. In addition, whenTahiti’s high is weaker than average (alower-pressure value), Darwin’s is higherthan average (a higher-than-average pres-sure value). A time series of Tahiti minusDarwin monthly mean pressure exhibitsthe oscillation that is closely linked to theEL NIÑO. Most often the difference in pres-sure is strong enough to drive the SOUTH-EAST TRADE WINDS across the South Pacific,but when it is weak, the trade windsweaken or even reverse into westerlies nearthe Equator. This is linked to the develop-ment of an El Niño event. See also El NiñoSouthern Oscillation.

Southern Oscillation Index (SOI) SeeSouthern Oscillation.

South Pacific Convergence Zone(SPCZ) A major year-round feature ofthe tropospheric circulation that is charac-terized by marked CONVERGENCE and anassociated cloud band that stretches fromthe low-latitude Pacific Ocean, southeast-ward into the middle-latitude central SouthPacific.

SPARC See Stratospheric Processes andtheir Role in Climate.

SPCZ See South Pacific ConvergenceZone.

specific heat capacity See heat capac-ity.

specific humidity A measurement of at-mospheric moisture content (WATER

VAPOR), usually expressed as the dimen-sionless ratio of the mass of water vapor tounit mass (kilogram) of moist surroundingair (air including water vapor); it is almostidentical to, but slightly smaller than thatof MIXING RATIO. The specific humidity ofan air parcel remains constant unless watervapor is added to or taken from the parcel.

speed shear The rate of change of windvelocity with height. This is a function ofatmospheric STABILITY and the roughnessof the surface. In the neutral stability state,the variation of wind speed (v) with height(Z) can be expressed as: v = kZr, where k isa constant and r depends on the roughnessof the surface. In this state, speed shear istypically greater over a city than a woodedarea, which in turn is greater than that ex-perienced over undulating grassland. Re-gardless of the surface, speed shear isgreatest in UNSTABLE conditions and leastpronounced or sometimes non-existent inSTABLE conditions. It is rare for the speedshear to exceed 10 knots (11 mph) between10–40 m (30–130 ft) above the Earth’s sur-face; typically it is of the order of 3 knots(3.5 mph) per 30 m (100 ft).

spissatus /spi-say-tŭs/ A cloud species;CIRRUS that is thick enough to appear graytoward the Sun, it may also obscure theSun completely. See also cloud classifica-tion.

splitting storm A THUNDERSTORM thatsplits into two separate systems. The newsystems tend to follow diverging paths, oneto the north and the other to the south. Ingeneral (in the northern hemisphere) thesouthern storm is most likely to developinto a SUPERCELL, while the north-movingstorm tends to dissipate and weaken.

SPOT (Système Probatoire d’Observationde la Terre) A series of French commer-cial satellites with high spatial resolutionthat images the Earth’s surface. The firstsatellite was launched in 1986; the most re-cent, SPOT 5, was launched in 2002. Itcovers the whole Earth once every 26 days,with an orbit similar to those of LANDSAT 4and 5.

Southern Oscillation Index

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spring In the middle latitudes, the SEA-SON of the year in which the Sun ap-proaches the summer solstice (June 21/22in the northern hemisphere), usually refer-ring to the months of March, April, andMay in the northern hemisphere and Sep-tember, October, and November in thesouthern hemisphere. It is marked by a de-crease in the strength of the mid-latitudewesterly winds and their associated storms.Air frosts become less frequent and dailytotals of incoming solar radiation show amarked rise. In parts of Western EuropeDROUGHT may occur as temperatures in-crease without a commensurate increase inmoisture.

sprite A weakly visible reddish tendril-shaped electrical discharge of considerablevolume, occurring above a THUNDER-STORM.

squall A sudden and violent gust ofwind that can disappear as suddenly as itappeared. It is similar to a gust but has alonger duration. The World Meteorologi-cal Organization defines a squall as a sud-den increase of wind speed by at least 16knots (18 mph), rising to a wind speed of22 knots (25 mph) or more, and lasting forat least one minute. It is commonly accom-panied by rain, sleet, and snow.

squall line A line or band of activeTHUNDERSTORMS that produce strongwinds and rain orientated along the line.The band of thunderstorms may only be acouple of kilometers wide resulting insharp increases in wind speed followed byheavy rainfall.

SSP See subsatellite point.

SST See sea-surface temperature.

stability The atmospheric state in whichan air parcel returns to its original positionafter an initial perturbation. Consequently,vertical air motion cannot be sustained in astable atmosphere. It is sometimes referredto as a state of stable equilibrium, leadingto stable air. See absolute stability.

stable air See absolute stability; stabil-ity.

stable boundary layer The stable layerof air that develops close to the Earth’s sur-face and spreads upward where there is notendency for the air to rise. It occurs typi-cally at nighttime and the stability is pro-duced by near-surface cooling underconditions dominated by outgoing radia-tion, known as radiative cooling, on calmclear nights, leading to the creation of tem-perature INVERSIONS. The boundary layermay also become stable for long periodsover snow and ice surfaces due to the ab-sence of turbulence, or be created by activesubsiding air under anticyclones (high-pressure systems).

stade (stadial) During an ICE AGE (glacialperiod), a cooling of conditions and sec-ondary advance of glaciers constituting asubdivision (substage) of a glacial stage.Compare interstade.

stadial /stay-dee-ăl/ See stade.

standard atmosphere A hypotheticalatmosphere chosen to represent the aver-age distribution of pressure, temperature,and density in the real atmosphere. A num-ber of standards are used. That defined bythe International Civil Aeronautical Orga-nization (ICAO) assumes a mean-sea-leveltemperature of 15°C, a standard sea-levelpressure of 1013.25 hPa, and a tempera-ture lapse rate of 0.65°C per 100 m up to11 km (7 miles) in the atmosphere. The USstandard atmosphere is identical to theICAO standard atmosphere up to 32 km(20 miles) and then extends to 1000 km(620 miles).

static lapse rate See environmentallapse rate.

stationary front (quasi-stationary front)A narrow elongated frontal region thatshows little or no horizontal progressionover periods of several hours or occasion-ally more than a day since its last synopticposition. It is usually associated with ex-tensive cloud and precipitation that can

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lead to serious flooding or severe snowstorms. See front.

station model The internationallyagreed method of plotting surface observa-tions around the station ‘circle’. The prin-cipal information represented, either as anumber or a symbol, is DRY-BULB TEMPERA-TURE, DEW-POINT temperature, MEAN-SEA-LEVEL PRESSURE, PRESSURE TENDENCY,VISIBILITY, PRESENT WEATHER, PAST

WEATHER, CLOUD TYPE, CLOUD AMOUNT,and CLOUD BASE height, and WIND DIREC-TION and WIND SPEED (see diagram on pp206–207).

station plot The complete set of surfaceobservations that are plotted in the form ofa STATION MODEL.

statistical–dynamical model (SDM) Acomputer-based MODEL based on statisticalaverages of the climatic system. One suchmodel is the statistical-dynamical down-scaling model in which statistical relation-ships are developed between large-scaleatmospheric parameters (such as sea-levelatmospheric pressure) and localized para-meters (such as precipitation or tempera-ture).

steam fog A low thin fog that formswhen cold air moves across warm water ora warm wet surface. Water evaporates intothe rising air, increasing the dew point, andthe air above becomes saturated; the watervapor condenses in the cold air to form vis-ible water droplets, which resembles steamrising from the water. ARCTIC SEA SMOKE isa type of steam fog that occurs over openwater in the Arctic.

steering The process by which the direc-tion and speed of motion of large-scalepressure systems, such as FRONTAL CY-CLONES, ANTICYCLONES, TROPICAL CYCLONES,and THUNDERSTORMS, is controlled. Seesteering current.

steering current (steering flow, steeringwind) A circulation feature that is of im-portance in determining the track of arange of different synoptic and smaller-

scale pressure disturbances. TROPICAL CY-CLONES, for example, are often embeddedin the broad steering current of the TRADE

WINDS. However, the wind at one particu-lar level, or in one particular layer, is rarelythe complete answer to how such systemsare steered. See also steering.

Stefan–Boltzmann law /stef-ăn bolts-măn/ The law, which applies only toblack bodies, states total radiation in all di-rections from a black-body radiator is pro-portional to the fourth power of itsthermodynamic (absolute) temperature.Originally formulated in 1879 by the Aus-trian physicist, Josef Stefan (1835–93), itwas derived thermodynamically by an-other Austrian physicist, Ludwig Boltz-mann (1844–1906), in 1889.

Stefan’s law See Stefan–Boltzmann law.

stepped leader In a LIGHTNING stroke, ashort discharge path that initiates the finallightning discharge; it occurs as an advanc-ing region of ionization propagating in lu-minous steps or sections from the base ofthe cloud and is often faintly visible. Thesequence of steps establishes the channelfor the RETURN STROKE, during which themajority of the lightning current flows.

Stevenson screen See instrument shel-ter.

STJ See Subtropical Jet Stream.

storm 1. In general, a severe atmos-pheric phenomenon, for example a gale,thunderstorm, or rainstorm, probablyleading to disruption of human life.2. In SYNOPTIC METEOROLOGY, regions oflow pressure carrying increases of windspeed, cloud, and precipitation.3. Force 10 on the BEAUFORT SCALE.

storm-force wind A damaging wind,corresponding to either force 10 (storm) orforce 11 (violent storm) on the BEAUFORT

SCALE, with a wind speed of at least 43knots (49 mph).

Storm Prediction Center An organiza-

station model

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tion in the US that provides forecasts andwatches for severe THUNDERSTORMS andTORNADOES across the contiguous US. Inaddition it monitors heavy rain, heavysnow, and fire weather across the States.Located in Norman, Oklahoma, it is a con-stituent body of both NOAA’s NATIONAL

WEATHER SERVICE (NWS) and the NATIONAL

CENTERS FOR ENVIRONMENTAL PREDICTION

(NCEP).

storm surge A deviation from the nor-mal predicted sea level caused by wind ve-locity and/or low air pressure over the sea.Storm surges occur with a TROPICAL CY-CLONE (HURRICANE or TYPHOON), and arealso associated with high winds in advanceof a severe storm or low-pressure system.The height of the storm surge is the differ-ence between the observed level of the seasurface and the level that would have oc-curred in the absence of the storm. Alonggently shelving coastlines a storm surgemay be many meters high and can cause se-vere damage and danger to human life,with flooding far inland. Storm surges alsocause concern for low-lying cities. Thestorm surge that accompanied HurricaneKatrina in August 2005, reached heights ofsome 6–9 m (20–30 ft); in combinationwith the wave action and extremely highwinds, severe flooding occurred alongparts of the Gulf Coast of Louisiana andMississippi and levees and floodwallsfailed leaving many parts of New Orleansunder flood water.

storm tracks (depression tracks) A setof lines that express the route followed bytraveling disturbances or cyclonic circula-tions (low-pressure systems or depres-sions). They are produced by, for example,plotting a mid-latitude cyclone’s central(minimum) pressure every 6, 12, or 24hours. Mid-latitude cyclones in the NorthAtlantic most commonly track from south-west to northeast, from off the northeastUS and Maritime Provinces of Canada toN West Europe. They tend, on average, toreach their deepest central pressure nearIceland. In the North Pacific, a broadlysimilar track orientation occurs, runningfrom off NE Asia/Japan toward Alaska,

British Columbia, and the Pacific North-West. Cyclones tend, on average, to reachtheir deepest central pressure near theAleutian Islands.

Strahler climate classification /shtrah-ler/ A genetic CLIMATE CLASSIFICATION

system proposed by the American geologistand geographer Arthur N. Strahler(1918–2002) in 1951, with subsequent re-visions. The classification is based on themain air masses and primary groups areidentified: (1) LOW-LATITUDE CLIMATES,produced by tropical air masses; (2) MID-LATITUDE CLIMATES, produced by tropicaland polar air masses; and (3) HIGH-LATI-TUDE CLIMATES, produced by polar andArctic air masses. These main groupingsare subdivided into 14 climatic types,based on temperature and precipitationvariations, together with the undifferenti-ated highland climates. The classificationhas been widely used. See also Flohnclimate classification; Köppen climate clas-sification; Thornthwaite climate classifica-tion.

straight-line wind (straight wind) 1.Any wind that does not show rotation.2. A THUNDERSTORM wind, associated witha gust front or a squall line, in which astraight-line wind originates from down-drafts in the MESOSCALE storm system andrapidly spreads out on reaching theground. It can cause considerable damageto property, for example as experienced inthe Mississippi and Ohio Valleys on April15, 1994. Damage from such winds occursin a straight line, unlike tornadoes in whichthe affected area is more circular in shape.

stratiform /strat-ă-form/ Of or relatingto layered cloud at any level that exhibitslittle or no vertical development. It in-cludes STRATUS, STRATOCUMULUS, NIMBO-STRATUS, ALTOSTRATUS, and CIRROSTRATUS.Compare cumuliform. See also cloud clas-sification.

stratiformis /strat-ă-for-mis/ A cloudspecies in which the cloud is spread out asan extensive and uniform horizontal sheetor layer. It applies to cirrocumulus, alto-

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206

TT

CHCM

PPP

PPa

dd

ff

CL

TdTd

VV ww

cloud cover(overcast)

temperature

present weather(rain)

dew-point

winddirection

wind speed(17 knots)

pressuretendency(falling)

pressure(1023.5 hPa)

15

11

2353 \N

W1W2

Station model (enlarged) Example of a station plot (enlarged)

The station model. Observations from a weather station are plotted automatically orby hand onto the surface weather chart.

TT Temperature (top left of station)Degrees Celsius (degreesFahrenheit in the US)

TdTd Dew-point temperature (bottomleft of station)Expressed in same units as dry-bulb temperature

N Cloud cover (within circle)Assessed to the nearest eighth(okta) or nearest tenth (in theUS)

VV Visibility (left of station)A 2-digit code, expressed inunits of kilometers or miles

ww Present weather conditions (leftof station, inside of VV)The weather when the observa-tion is made, e.g. type of precipi-tation, shown as a symbol

ff Wind speedeach long barb = 10 knotseach short barb = 5 knots = 50 knots

dd Wind directionLine represents direction wind isblowing from

PPP Mean-sea-level pressure (topand right of station)Units are coded in hPa or mb,given to nearest tenth but with-out decimal point or leading 9 or10, e.g. a pressure of 1034.5 hPais represented by 345

PPa Pressure tendency (right of sta-tion)Change in mean-sea-level pres-sure in previous 3 hours, repre-sented by a value and a lineindicating how changing

CH Cloud Type (CH and CM aboveCM station, CL below station)CL Represented by 9 symbols for

each of 3 levels (27 in total)

W1W2 Past weather conditions (bottomright of station)

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207

Cloud amount symbols

light drizzle rain shower

moderate drizzle snow shower

heavy drizzle thunderstorm

light rain freezing drizzle

moderate rain freezing rain

heavy rain light fog

light snow thick fog

moderate snow

heavy snow

Common weather symbols

clear sky 5/8 covered

covered 1/8 or less, 6/8 covered but not zero

2/8 covered 7/8 covered

3/8 covered sky completelycovered

4/8 covered sky obscured, e.g by fog

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cumulus, and stratocumulus. See alsocloud classification.

strato- A prefix meaning ‘layered’.

stratocumulus /stray-toh-kyoo-myŭ-lŭs/ One of the cloud genera (see cloudclassification); a type of widespread orpatchy LOW CLOUD, often with blue sky vis-ible between the cloud elements, in theform of a sheet that contrasts with STRATUS

because its base has a lumpy surface (thecloud elements have a typical angularwidth of about 5°). It is gray or whiteish incolor, formed of water droplets, and oftenassociated with dull weather. It can be as-sociated with light rain although in verycold conditions it can produce light snowor snow pellets.

stratopause /strat-ŏ-pawz/ The bound-ary that caps the STRATOSPHERE at approx-imately 50 km (30 miles) above the Earth’ssurface and divides it from the MESOSPHERE

above. The stratopause is an isothermallayer.

stratosphere The atmospheric layerthat lies above the TROPOSPHERE (fromwhich it is separated by the TROPOPAUSE) atan altitude of about 10 km (6 miles); it canextend to around 50 km (30 miles) abovethe Earth’s surface to the MESOSPHERE. Theair temperature remains constant up toabout 25 km (15 miles) but above this alti-tude the stratosphere is characterized byincreasing temperature with height toabout 200 kelvins at the STRATOPAUSE cre-ating a temperature inversion above thetroposphere. Temperatures in the stratos-phere tend to be high due to the high con-centrations of OZONE that absorbsincoming ultraviolet radiation convertingsolar energy to kinetic energy: the temper-ature increases with ozone concentration.

The increase in temperature with alti-tude in the stratosphere has a stabilizing ef-fect on atmospheric conditions, in markedcontrast to the troposphere below in whichcolder air overlies warmer air.

Stratospheric Processes and their Rolein Climate (SPARC) A core project of

the WORLD CLIMATE RESEARCH PROGRAM

(WCRP) that focuses on the interaction ofdynamical, chemical, and radiativeprocesses within the stratosphere. The fourmain areas of study and research are the in-fluence on climate of the stratosphere;processes involved with ozone levels;stratospheric variability; and changes in ul-traviolet radiation.

stratus /stray-tŭs/ One of the cloud gen-era (see cloud classification); a type of ex-tensive LOW CLOUD in the form of a sheetwith a flat base that is usually of a uniformgray tone. Stratus is usually composed ofsmall water droplets and can often be lowenough to obscure the upper parts of hills(see hill fog) and tall buildings. The liftingof a thick layer of fog can also result instratus cloud. DRIZZLE and light RAIN occa-sionally fall from such cloud or ICE PRISMS

and SNOW GRAINS in very cold weather.

streak lightning A CLOUD-TO-GROUND

DISCHARGE in which the forked appearanceis absent and there is a single, often appar-ently straight, lightning channel. Compareforked lightning.

streamline A smoothly curved line thatis everywhere parallel to the local directionof the wind vector. A streamline analysis isthus a ‘snapshot’ of the atmospheric flow’sdirection field and can highlight zones ofCONFLUENCE and DIFLUENCE, for example.

striations /strÿ-ay-shŏnz/ Fine groovesor channels in a cloud formation.

Stüve diagram /shtyoo-vĕ/ (pseudoadia-batic chart) A type of THERMODYNAMIC

DIAGRAM that uses straight lines to repre-sent PRESSURE, TEMPERATURE, and the DRY

ADIABATS. It was developed in about 1927by G. Stüve and is commonly used in theUS. See also skew-T log-P diagram; tephi-gram.

subarctic Denoting those latitudes thatlie directly south of the ARCTIC Circle.

Sub-Arctic Current See Aleutian Cur-rent.

strato-

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subcloud layer The layer betweenCLOUD BASE and the surface, normally re-ferring only to the layer below LOW CLOUD.

subhumid In climatology, the zones be-tween the humid zones, in which there isplentiful precipitation, and the arid zones.

sublimation The process in which asubstance changes phase from a solid to agas with no intermediate liquid phase. Inmeteorology, it refers generally to watervapor evaporated directly from an ice sur-face, or the reverse process in which watervapor is deposited directly onto an ice sur-face, for example in the formation of HOAR

FROST.

subsatellite point (SSP) The point onthe Earth’s surface that is vertically belowa satellite. It is where an imaginary straightline that joins the satellite to the center ofthe Earth intersects the surface.

subsidence The process of sinking un-dergone by a mass of air, usually throughsome considerable depth of, for example,the TROPOSPHERE. Air can sink from theupper troposphere down to lower tropos-pheric levels in ANTICYCLONES (highs).Huge volumes of air move slowly down ata rate of perhaps 1–2 centimeters per sec-ond. As the air subsides it is compressedand warms adiabatically so that it is oftenextremely dry and cloud-free. The warm-ing can culminate in a SUBSIDENCE INVER-SION at about 1 km (0.6 mile) above thesurface. In the lower troposphere it is asso-ciated with a layer of DIVERGENCE and in-creased stability, while in the uppertroposphere it is linked to CONVERGENCE.

subsidence inversion A troposphericlayer some few hundred meters deep thathas a marked increase of DRY-BULB TEMPER-ATURE but a marked decrease of DEW-POINT

temperature (i.e. it is a dry INVERSION) andis produced by the deep SUBSIDENCE andadiabatic warming that occurs in manyANTICYCLONES (highs). Its base is typicallyabout 1 km (0.6 mile) above the Earth’ssurface; the layer between it and the sur-

face often has CUMULUS or STRATOCUMULUS

clouds.

subsun (undersun) An optical phenome-non, the most commonly occurring type ofsubhorizon HALO, which appears as a whitespot of light, vertically elongated. belowthe horizon. It is produced by the direct re-flection of the Sun by clouds containing icecrystals, typically plate crystals in whichthe faces are horizontal.

subsynoptic low /sub-să-nop-tik/ Seemesolow.

subtropical Denoting the latitudinalzones that lie between the tropics and thetemperate zones. Although this usage is im-precise the word is often taken to refer tothose zones poleward of the Tropics ofCancer (northern hemisphere) and Capri-corn (southern hemisphere) extending toabout 35–40°N and S. Subtropical is usedin a number of climate classifications todesignate regions in which no month has amean temperature below 6°C (43°F).

subtropical anticyclone (subtropicalhigh) An extensive semi-permanent ANTI-CYCLONE that is found over the subtropicaloceans (i.e. between 35° and 40° latitude).The subtropical anticyclones are presentthroughout the year, shifting poleward inthe summer and equatorward in the win-ter. The AZORES HIGH and BERMUDA HIGH

over the Atlantic Ocean, and the NORTH PA-CIFIC HIGH are examples in the northernhermisphere, with the South Pacific andSouth Indian Ocean highs in the southernhemisphere. The subtropical cyclones aretypified by deep sinking motion, dry condi-tions, and either clear skies or widespreadstratiform cloud.

Subtropical Jet Stream (STJ) A west-erly upper tropospheric JET STREAM locatedon the poleward flanks of the HADLEY CELLS

in each hemisphere, directly above the SUB-TROPICAL ANTICYCLONES (highs) at an eleva-tion of some 13 km (8 miles). It is relativelyfixed in latitude from day-to-day but mi-grates seasonally in association with thenorth–south shift of the Hadley cells.

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suction vortex A small intense vortexfound in addition to the central ‘parent’vortex of a TORNADO. This type of vortex isembedded in the tornadic circulation androtates around the parent vortex.

sukhovey (sukhovei) A hot drySCIROCCO-type wind that affects southernSiberia and Central Asia, especially thesteppes of Ukraine and Kazakhstan. Oftencarrying vast quantities of dust, thesukhovey wind has a desiccating effect,often eliminating cloud cover and allowingintense solar radiation to reach the Earth’ssurface. It can reduce relative humidity atthe ground to very low values both by dayand night.

sulfate aerosol Highly reflective parti-cles in the atmosphere generated over con-tinental areas, mainly from the combustionof sulfurous fossil fuels and also from vol-canic eruptions. They come ultimatelyfrom the release of SULFUR DIOXIDE and sul-fur trioxide, which react with sulfur com-pounds to transform into the minuteaerosol particles. Over oceans, sulfateaerosols originate from the DIMETHYLSUL-FIDE (DMS) produced by phytoplankton.Sulfate aerosol particles have RESIDENCE

TIMES of a few days and are therefore notwidely well mixed in the atmosphere. Theyact as CLOUD CONDENSATION NUCLEI.

sulfur dioxide (SO2) A compound pro-duced primarily by the combustion of sul-furous fossil fuels, but also associated withthe natural processes of volcanic eruptionsand ocean spray. In the atmosphere it cancombine with other sulfur compounds incomplex chemical reactions that produceSULFATE AEROSOLS. In moist air it can alsocombine with water vapor to produce sul-furic acid, and consequently ACID RAIN.

sulfur hexafluoride /heks-ă-floo-ŏ-rÿd/(SF6) A colorless gas, a compound of sul-fur and fluorine, used as an insulator forcircuit breakers and other electrical equip-ment. It has an extremely low atmosphericconcentration enabling it to be used as anatmospheric tracer gas. For example, it hasbeen added to the waste gases leaving

power plants so that it can be monitored byinstrumented aircraft to assess how far at-mospheric pollution from power plantscan reach. Sulfur hexafluoride is a power-ful GREENHOUSE GAS and, although notemitted in large quantities, it has a lifetimeof 3200 years.

sumatra /sû-mah-tră/ The local namefor a squall in the Malacca Strait (betweenpeninsular Malaysia and the island ofSumatra) when a sudden change of winddirection from southerly is accompaniedby a rise in wind speed and a high bank ofcumulus or cumulonimbus cloud. A suma-tra usually occurs at night, commonly be-tween April and November.

summer The SEASON of the year usuallydesignated as June to August in the middle-latitude northern hemisphere and Decem-ber to February in the southernhemisphere. Temperatures reach a maxi-mum with August often the warmestmonth. Precipitation is predominantly con-vectional, with frequent thunderstorms,which may be accompanied by hail, andSUPERCELL storms may develop in continen-tal interiors. Receipt of solar radiation inJune can exceed that of the tropics due toincreased day length.

Sun dog See parhelion.

Sun pillar An optical phenomenon, atype of HALO, in which a luminous streakor pillar appears above and/or below theSun, especially just before sunset or aftersunrise. The pillar can extend to an angu-lar distance of about 20° from the Sun. Itresults from the reflection of the Sun’s lightby horizontal ice plates within clouds or icefog, the flat faces of which are alignednearly parallel to the ground surface.

sunshine DIRECT RADIATION from theSun received at the surface of the Earth (asopposed to DIFFUSE RADIATION). The stan-dard measure is of ‘bright’ sunshine that isstrong enough to burn a trace in the dailysunshine card of a SUNSHINE RECORDER.The Sun is too low in the sky to burn the

suction vortex

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card within about 30 minutes of sunup andsundown.

sunshine recorder An instrument usedfor measuring the duration of bright sun-shine. See Campbell–Stokes sunshinerecorder.

sunspot A relatively dark cool region ofthe disk of the Sun with temperaturesaround 1500 K lower than the surroundingPHOTOSPHERE. This ‘umbra’ is surroundedby an outer hotter ‘penumbra’. The indexof ‘relative sunspot number’, which varyfrom less than ten in quiet sun periods toover 150 in active years, was introduced in1848 by Rudolf Wolf (1816–93) of theZurich Observatory.

Sun-synchronous satellite A satellitethat circles the Earth, in near-polar orbit,in such a way that it has an overhead ornear-overhead pass above any point on theEarth’s surface at or about the same localtime each day, throughout the year. Inorder to achieve this, the plane of its orbitmust precess by about 1° every day, to keeppace with the Earth’s rotation round theSun over the year. As a result, the satellite’sorbital plane appears to be fixed with re-spect to the Sun. See polar-orbiting satel-lite. Compare geostationary satellite.

Supan’s temperature zones /soo-panz/An early climate classification in which theclimatic regions of the world were based ondifferences in mean annual temperatures. Itwas proposed in 1896 by the Austrian cli-matologist Alexander Supan (1847–1920).

superadiabatic lapse rate /soo-per-ad-ee-ă-bat-ik/ A LAPSE RATE that is greaterthan the DRY ADIABATIC LAPSE RATE of about10°C km–1. This lapse rate does not occurwithin the free atmosphere but can occurdirectly above the surface when it isstrongly heated by solar radiation, for ex-ample over arid areas in summer. The airhas ABSOLUTE INSTABILITY.

supercell /soo-per-sell/ A violent rotat-ing thunderstorm with a large rotating up-draft known as a MESOCYCLONE. Supercells

produce severe weather including hail,heavy rainfall leading to flash floods, se-vere winds, and frequent lightning; theyalso produce the most destructive and dan-gerous tornadoes. The thunderstorm canlast up to 6 hours. Supercell formation oc-curs in environments that have large verti-cal wind shear and instability.

supercooled cloud /soo-per-koold/Cloud composed of water droplets inwhich the temperature is below the freez-ing point of water, 0°C (32°F). Suchdroplets freeze instantly if they come intocontact with ice crystals – this process iscritical in the formation of precipitation inmost clouds. If freezing nuclei are absent(i.e. in pure air) supercooled water dropletscan remain liquid at temperatures down to–40°C (–40°F).

supercooling /soo-per-kool-ing/ Thecooling of water to below its normal FREEZ-ING POINT of 0°C (32°F) but it remains inthe liquid state. The supercooling of waterdroplets is common in clouds that extendbeyond the 0°C isotherm. The supercooledwater droplets remain unfrozen if noFREEZING NUCLEI are present; this is possi-ble to temperatures as low as –40°C(–40°F). The inclusion of a seed crystal intosupercooled liquid causes it to freeze im-mediately as the release of the heat of fu-sion raises the temperature rapidly tofreezing point.

superior mirage See mirage.

supersaturation /soo-per-sach-ŭ-ray-shŏn/ A condition in which the atmos-phere contains more water vapor than isneeded to produce saturation with respectto a flat surface of pure water or ice, andRELATIVE HUMIDITY is greater than 100%.Such a condition may occur when there isno surface upon which condensation cantake place (i.e. clean air, in which there isan absence of CLOUD CONDENSATION NU-CLEI).

surface-based convection A bubble ofair that forms and grows in response tosurface heating, moving up through the

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lower TROPOSPHERE. The height of the bub-bles depends partly on the intensity of theheating by the land or sea surface. Drybubbles are also known as THERMALS,while those with condensed water dropletsare seen as CUMULIFORM clouds.

surface boundary layer That part ofthe PLANETARY BOUNDARY LAYER that liesdirectly above the ground surface and inwhich friction effects are generally con-stant, as opposed to decreasing withheight. It extends up from the surface toabout 10 m (33 ft) and within this layer in-solation and radiational cooling are attheir strongest.

surface energy budget See energy bud-get.

surface inversion An INVERSION of tem-perature in the atmosphere directly abovethe Earth’s surface so that the coolest tem-peratures are closest to the ground. Thiscan result, for example, from the influx ofrelatively cold sea air over the ground sur-face via an onshore breeze. It is also oftencaused by the overnight radiative coolingof surface air to form a RADIATION INVER-SION.

Surface Ocean–Lower AtmosphereStudy (SOLAS) A core project of theWORLD CLIMATE RESEARCH PROGRAM

(WCRP), established in 2000, that aims fora quantitative understanding of the mainbiogeochemical–physical interactions andfeedbacks between the ocean and the at-mosphere in relation to climate and envi-ronmental change.

surface weather chart A synoptic chartthat portrays one or more aspects of theweather at the Earth’s surface for a givengeographic area. The most common formis the MEAN-SEA-LEVEL PRESSURE analysisthat consists of ISOBARS and also usuallyFRONTS.

surface weather observations All thevisual and instrumental observations thatare plotted as the STATION MODEL plus oth-

ers that are not, such as daily SUNSHINE

total.

surface wind The velocity and direc-tional characteristics of the wind flow closeto the ground, usually measured at thestandard height of 10 m (33 ft). As the air-flow experiences frictional retardation inthe planetary boundary layer, the surfacewind velocity is usually lower than theGEOSTROPHIC WIND or GRADIENT WIND ve-locities recorded in the free atmosphere.

surge See storm surge.

SWEAT index See severe weatherthreat index.

sweeping In cloud physics, a mechanismthat is thought to contribute to the growthof raindrops in the COLLISION–COALES-CENCE PROCESS in which small droplets areswept into the wake of larger falling waterdroplets and are absorbed by the latter.

synoptic /să-nop-tik/ Denoting the mete-orological conditions at a particular pointin time and over an extensive area from ob-servations made simultaneously (the wordmeans literally viewed simultaneously).

synoptic chart A weather map for aparticular point in time that portrays fea-tures having a SYNOPTIC SCALE. Synopticcharts represent features both at MEAN SEA

LEVEL and in the upper air.

synoptic climatology The study of CLI-MATE over a large related geographic area.In this form of study, synoptic observa-tions are plotted on charts for the purposeof weather analysis and forecasting.

synoptic code (international synopticcode) The internationally agreed methodof representing the range of SYNOPTIC ob-servations made at the surface, fromSOUNDINGS, etc. All the data that surroundthe STATION PLOT are transmitted as groupsof five digits in the synoptic code.

synoptic meteorology The branch ofMETEOROLOGY that is specifically con-

surface boundary layer

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cerned with atmospheric phenomena andsystems associated with weather through-out the world. The name originates fromthe synoptic method, in which observa-tions and atmospheric conditions are plot-ted on a SYNOPTIC CHART. Data is gatheredfrom the observations from a network ofsynoptic stations, ground-based radars,balloon-borne radiosondes, aircraft, ships,and meteorological satellites. From thisdata forecasts of the weather are producedusing such methods as numerical weatherforecasting, which employs complex com-puter models.

synoptic scale A particular scale ofweather phenomenon; synoptic-scale fea-tures are those that are portrayed on alarge-scale MEAN-SEA-LEVEL PRESSURE map,such as ANTICYCLONES (HIGHS), LOWS,RIDGES, and TROUGHS. They are defined byanalyzing surface weather observationsthat have a spacing of some few tens ofkilometers or more, and so are typicallymany hundreds or a thousand kilometersacross, and last a few days.

synoptic symbols The internationallyagreed symbols that are used to define thewhole range of PRESENT and PAST WEATHER

on a SYNOPTIC CHART.

synoptic weather map See synopticchart.

synthetic aperture radar (SAR) Aradar system used in aircraft and on boardpolar- or lower-angle orbiting satellites,that takes advantage of its rapid speedthrough space to create a large syntheticaperture (to improve spatial resolution ofthe image). The size of the synthetic aper-ture is that created during the period oftime that the satellite’s radar is emitting asignal down to the surface. Synthetic aper-ture radar is used to map the roughness ofthe surface in all weathers so that useful en-vironmental parameters can be deducedfor operational purposes.

system In meteorology, some type ofweather-producing feature, such as an AN-TICYCLONE (high) or LOW.

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TAF (Terminal Aerodrome Forecast) Aforecast transmitted to aviation centersand aircraft to include details of the VISIBIL-ITY, CLOUD HEIGHT, and WEATHER pertinentto the landing conditions at the destinationairport.

tail cloud A small feature with a tail-likeappearance associated with a TORNADO. Ithas a horizontal base and consists of frag-ments of cloud that move from an area ofPRECIPITATION toward the WALL CLOUD,which is most often a non-precipitatingfeature. Where the tail cloud joins the wallcloud there is rapid updraft.

talwind See up-valley wind.

TAO/TRITON array An array ofnearly 70 moored buoys in the tropical Pa-cific Ocean that was developed by theTROPICAL OCEAN–GLOBAL ATMOSPHERE PRO-GRAM (TOGA) and completed in 1994, theyear the program came to an end. Thearray was formerly known as the TropicalAtmosphere Ocean (TAO) array but wasrenamed in 2000: sites to the west of 165°Eare supported by the Japanese TRITON(Triangle Trans Ocean Buoy Network).The array gathers data on winds, humidity,sea-surface temperatures, sub-surface tem-peratures (at 10 depths down to 500 m);ocean velocity is also recorded at 5 moor-ings along the Equator. The data is trans-mitted to ground stations by satellite relayand the information gathered contributesto understanding and aiding prediction ofthe El Niño Southern Oscillation (ENSO),to oceanographic studies, and to atmos-phere–ocean exchanges.

teleconnection /tel-ĕ-kŏ-nek-shŏn/ Aconnection or linkage between anomalies

or variables in the atmosphere and/orocean over widely spaced geographicalareas, frequently on a planetary scale. Forexample, the teleconnections associatedwith the EL NIÑO SOUTHERN OSCILLATION

(ENSO) and with the NORTH ATLANTIC OS-CILLATION (NAO).

temperate climate A climate of themiddle latitudes, which is influenced byboth tropical and polar air masses. See alsoclimate classification.

temperate zone One of the three his-toric climatic zones, based on temperatureand sunshine characteristics, into whichthe world was divided by the classicalGreeks; the northern temperate zone liesbetween the Tropic of Cancer (23°27′ N)and the Arctic Circle (66°32′ N), while thesouthern temperate zone lies between theTropic of Capricorn (23°27′ S) and theAntarctic Circle (66°32′ S). The temperatezone has distinct summer and winter sea-sons. See also frigid zone; torrid zone.

temperature The degree of hotness of asubstance (solid, liquid, or gas), which isrelated to the average kinetic energy of theatoms or molecules of the substance. It isone of the most important elements in cli-matology and meteorology. In meteorol-ogy, temperature is recorded at stationsfrom thermometers housed within instru-ment shelters; THERMISTORS are used in au-tomatic weather stations and in upper-levelradiosonde measurements. A number oftemperature scales are in use: the CELSIUS

SCALE is generally used in meteorology al-though the FAHRENHEIT SCALE is still exten-sively used in the US; the KELVIN

TEMPERATURE is more widely used in scien-tific studies.

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temperature-efficiency index Seethermal-efficiency index.

temperature gradient See lapse rate.

temperature–humidity index (THI) An index based on the combination of tem-perature and humidity as a measure of thedegree of comfort or discomfort experi-enced by an individual in outdoor condi-tions in warm weather. Developed by theUS National Weather Service, the indexgives a single numerical value in the range70–80. This is an EFFECTIVE TEMPERATURE

based on air temperature and humidity,and the index is calculated using the fol-lowing formula:

THI = 15 + 0.4(Td + Tw),where Td and Tw are the dry- and wet-bulbtemperatures respectively. Most people arecomfortable with an index below 70, andvery uncomfortable when it is above 80.

temperature inversion See inversion.

temperature inversion layer See inver-sion layer.

temperature range The difference be-tween the maximum and minimum tem-peratures, or the highest and lowest meantemperatures measured during a specifiedtime period, for example DIURNAL VARIA-TION or seasonal (annual) variation.

tendency The rate of change of a vectoror scalar quantity of a meteorological ele-ment (e.g. pressure or temperature) over aparticular period of time at a given loca-tion. See also pressure tendency.

tephigram /teff-ă-gram/ (te-phi-gram) Atype of THERMODYNAMIC DIAGRAM that wasdeveloped in 1922 by the British meteorol-ogist Sir William Napier Shaw (1854–1945), who introduced it into the UK Me-teorological Office (UKMO). The name isderived from the rectangular Cartesian co-ordinates used: temperature and entropy.Strictly speaking, the tephigram should bearranged so that temperature lies along thex-axis, and the DRY ADIABATS along the y-axis. In reality, a right-rotated form of the

diagram has always been used since the late1940s. The lines of a tephigram are: ISO-BARS (almost horizontal); ISOTHERMS (slopefrom bottom left to top right); DRY ADIA-BATS (slope from top left to bottom right);SATURATED ADIABATS (curved lines); SATURA-TION MIXING RATIO lines (slope from bot-tom left to top right). The tephigram is theonly type of thermodynamic diagram com-monly used in the UK. See also skew-T log-P diagram; Stüve diagram.

Terminal Aerodrome Forecast SeeTAF.

Terra See Earth Observing System.

terrestrial radiation See longwave ra-diation.

thaw The melting of snow or ice at theEarth’s surface due to a rise in temperatureabove 0°C (32°F).

thermal A localized but highly buoyantarea of rising air in the atmosphere. Ther-mals can be generated either by intenseheating of the ground surface by the Sun,or by a steep ENVIRONMENTAL LAPSE RATE

(ELR), i.e. a superadiabatic temperatureprofile. A thermal may even reach the CON-VECTIVE CONDENSATION LEVEL if its verticalmotion is sufficient, leading to CUMULUS

cloud formation and possibly precipita-tion.

thermal advection The horizontaltransfer of heat in the atmosphere by thewind. This is distinct from vertical advec-tion, which is usually called convection,and heat that is transferred from one placeto another as a result of non-advectiveprocesses, such as radiative transfer.

thermal capacity See heat capacity.

thermal depression See heat low.

thermal-efficiency index (temperature-efficiency index) 1. An index devised bythe American climatologist Charles War-ren Thornthwaite (1889–1963) for delin-eating climatic types in his empiric climate

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classification (see Thornthwaite climateclassification). The index was used to-gether with an expression for precipitationefficiency to produce the classification.2. (TE) In ecology, an index that is a mea-sure of the long-range effectiveness of tem-perature in contributing to plant growth.

thermal equator The zone around theEarth in which the highest temperaturesoccur, either the mean annual temperatureor the temperature at a given time. Themean annual thermal equator occurs atabout 5°N, rather than along the actual ge-ographical EQUATOR; one factor for thisnorthward location is the influence of thedistribution of the continents. During thecourse of the year the position of the ther-mal equator migrates northward to reachabout 20°N in the northern hemispheresummer, and southward to around 5°S inthe southern hemisphere summer.

thermal infrared The region of the in-frared spectrum between about 10.5 and12.5 µm that is sensed routinely by METEO-

ROLOGICAL SATELLITES. The strength of theradiation emitted to space in this wave-band is directly proportional to the tem-perature of the emitting surface; this is truefor cloud, sea, or land surface, for example.This thermal radiation can therefore beconverted from a flux measured in wattsper square meter at the satellite’s sensor, toa temperature of the cloud top or sea sur-face for each PIXEL. The incessant flow ofthis radiation into space allows all-daycoverage to be achieved, which is very use-ful operationally.

thermal low See heat low.

thermal wind The theoretical wind thatblows parallel to the isohypses on a thick-ness chart, the most commonly drawnchart being that of 1000–500 hPa thick-ness. The thickness (meters) of any atmos-pheric layer is proportional to the meantemperature of that layer. The thermalwind on this chart (see diagram) is definedas the VECTOR which quantifies the differ-ence, and hence the degree of WIND SHEAR,

thermal equator

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4

8

528

536

544

552 CO

LDE

R

AI R

Vg1000

Vg500

VT

536

500 hPa wind

544

552

thermal

wind

WA

RM

ER

AIR

1000

hPa

win

d

0

Thermal wind: 1000 hPa and 500 hPa geostrophic wind components Vg and the resultant thermalwind VT blowing parallel to the 1000–500 hPa thickness lines. All heights in 10s of meters.

FREEDOM PALESTINE


between the GEOSTROPHIC WIND at the bot-tom (1000 hPa) and top (500 hPa) of theatmospheric layer. The thermal wind‘blows’ between cooler air (lower thick-nesses) to its left and warmer air (higherthicknesses) to its right in the northernhemisphere, and vice versa in the southernhemisphere.

thermistor /ther-mis-ter/ A resistor inwhich electrical resistance changes withtemperature so that it can be used as thebasis for a THERMOMETER. It is made froma mixture of metallic oxides that have beenfused together.

thermocline /ther-mŏ-klÿn/ A stablelayer of water in an ocean in which verticalmixing is inhibited and the temperaturegradient (cooling with depth) is at a maxi-mum (greater than in the warmer layerabove it and the cooler layer below). A per-manent thermocline can be found in thelower and middle latitudes at a depth ofseveral hundred meters, extending down toover 1000 meters (3300 ft). Seasonal ther-moclines occur much closer to the surfaceduring summer months due to the action ofsolar heating; these are only temporarystructures and are destroyed by reduced in-solation and increased turbulence duringwinter.

thermocouple /ther-moh-kup-ăl/ An in-strument used to measure temperature.This device, a thermoelectric sensor, con-sists of two wires composed of dissimilarmetals (e.g. copper and iron), joined ateach end (junction) to form a circuit. Onejunction is kept at a constant temperaturewhile the other measures the unknowntemperature creating an electromotiveforce. This emf is related to the tempera-ture difference and is therefore a measureof the temperature difference between thetwo junctions. See also thermometer.

thermodynamic diagram (aerologicaldiagram, adiabatic chart) A diagramused in operational meteorology showingthe vertical pressure, temperature, and hu-midity structure of the atmosphere. Fivesets of lines appear on a thermodynamic di-

agram: ISOBARS; ISOTHERMS; DRY ADIABATS;SATURATED ADIABATS; and SATURATION MIX-ING RATIO lines. Variations in the DRY-BULB

TEMPERATURE (i.e. the environmental lapserate) and DEW-POINT temperature withheight are plotted on to a thermodynamicdiagram. A number of types of diagramsare used, all based on the same principles,but the three in most widespread use arethe TEPHIGRAM and, especially in the US,the SKEW-T LOG-P and STÜVE DIAGRAMS. Athermodynamic diagram can be viewed asa ‘meteorological ready-reckoner’ fromwhich derived variables, such as RELATIVE

HUMIDITY and air STABILITY, can be quicklydetermined without having to result tocomplex and often time-consuming numer-ical calculations. See also absolute instabil-ity; absolute stability.

thermodynamic equation An equa-tion representing the law of conservationof energy. The equation states that an air mass will not change its tempera-ture unless heat is added or removed orthrough changes in surrounding pressureassociated with changes in its vertical posi-tion. The equation can take the followingform:

∆T = –(g/Cp)∆z + ∆QH/Cp.Mair,where ∆T is the change in temperature ofan air parcel with mass Mair; ∆QH/Mairrepresents heat input, which can be causedby latent heat through condensation, ra-diative heating, and heat from chemical re-actions; ∆z is the change in height; g is theacceleration of free fall; and Cp is the spe-cific heat capacity for moist air.

thermodynamics /ther-moh-dÿ-nam-iks/The branch of physics concerned with thestudy of heat and its transformation intoother types of energy, and vice-versa.

thermodynamic temperature Formerlycalled absolute temperature, which wasmeasured on an absolute scale of tempera-ture, e.g. the Kelvin scale of temperature.Now, however, thermodynamic tempera-ture is regarded as a basic physical prop-erty and is usually measured in units calledKELVINS. The concept of a temperaturescale is now restricted to the International

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Practical Temperature Scale, which is de-fined by 16 fixed points, so that this scaleconforms as closely as possible to the ther-modynamic temperature.

thermogram /ther-mŏ-gram/ See ther-mograph.

thermograph /ther-mŏ-graf, -grahf/ Aself-recording THERMOMETER. It consists ofa bimetallic coil, which distorts withchanges in ambient temperature; an armattached to the coil holds a pen thatrecords the distortions on a paper chart,the thermogram.

thermohaline circulation /ther-noh-hay-lÿn/ A circulation or flow within theocean that is dependent on differences inwater density, governed by temperatureand salinity. It has a large vertical compo-nent of flow and is responsible for the mix-ing of deep water in the oceans. Forexample, deep water is carried from theNorth Atlantic to the Pacific, where it thenupwells and is transported back to thedownwelling regions in the North Atlantic(see Atlantic conveyor). The sinking ofwater in the north acts to drag warmerwater from the south.

thermohygrograph See hygrothermo-graph.

thermometer An instrument used formeasuring temperature. The most com-monly used type in meteorology is the liq-uid-in-glass thermometer; the liquid used isoften mercury but as the freezing point forthis element is –38.9°C (–38°F) alcohol,which has a freezing point of –114.4°C(–174°F), is also used, especially for low-temperature measurement and in the MINI-MUM THERMOMETER. The liquid, whichexpands with a rise in temperature, is en-closed in a sealed glass tube; the height ofthe column measures the temperature,which is read on a graduated scale to thenearest 0.1°C or 0.1°F. Electrical instru-ments or sensors are becoming morewidely used and are of necessity used in AU-TOMATIC WEATHER STATIONS, RADIOSONDES,and other automated locations. The sen-

sors include THERMISTORS, THERMOCOU-PLES, and resistance temperature detectors(RTD), which work on the basis of the re-sistance changes of certain metals, such asplatinum and copper, to provide accuratemeasurements. See also black-bulb ther-mometer; dry-bulb thermometer; grassminimum thermometer; maximum andminimum thermometer; maximum ther-mometer; mercury-in-glass thermometer;wet-bulb thermometer.

thermopile /ther-moh-pÿl/ An instru-ment used for measuring radiation. It con-sists of a number of THERMOCOUPLES joinedin series.

thermosphere /ther0mŏ-sfeer/ The layerof the atmosphere lying above the MESOS-PHERE (from which it is separated by theMESOPAUSE) and extending from about 85km (50 miles) above the Earth’s surface. Inthis layer, temperatures increase with alti-tude due to the absorption of solar radia-tion by molecular oxygen.

theta-e /thay-tă ee/ See equivalent poten-tial temperature.

theta-e ridge An axis along which rela-tively high values of EQUIVALENT POTENTIAL

TEMPERATURE (theta-e) occur. Intense rain-fall frequently occurs near to or just aheadof a theta-e ridge.

THI See temperature humidity index.

thickness The geopotential height dif-ference between two specified ISOBARIC

SURFACES, for example the 1000–500 hPathickness. The density of air decreases astemperature increases and as a result thethickness between two pressure levels al-ters with temperature. As the air becomeswarmer it expands and therefore the dis-tance between two pressure levels must in-crease to accommodate the same mass.

Thiessen polygon method /tee-sĕn/ Amethod used in meteorology to assign arealsignificance to point rainfall values. Apolygon is drawn around a precise pointwithin a scatter of points. Polygons are

thermogram

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generated from a set of points mathemati-cally defined by the perpendicular bisectorsof the lines between all points. For exam-ple, Thiessen polygons can be drawnaround a network of rain gauges. Each raingauge will be surrounded by a polygon,drawn so that the boundary is closest to thegauge contained within it than to any othergauge. The area within the polygon is con-sidered to display uniform characteristics,i.e. the area within the polygon, regardlessof its size, is assigned the rainfall amountcollected by the gauge within it.

Thornthwaite climate classification/thorn-thwayt/ An empiric climate classi-fication proposed originally in 1931 andsubsequently revised in 1948 by the Amer-ican climatologist, Charles Warren Thorn-thwaite (1899–1963). The original (1931)classification was based on the PRECIPITA-TION-EFFICIENCY INDEX and the THERMAL-EFFICIENCY INDEX as a method to specify theclimatic effects on vegetation. Five climaticregions with associated vegetation wereidentified according to the index: >128 wet(rain forest); 64–127, humid (forest);32–63, subhumid (grassland); 16–31,semi-arid (steppe); and <16, arid (desert).Thornthwaite’s second classification(1948) is based on an index of POTENTIAL

EVAPOTRANSPIRATION (PE) and the moisturebudget. In this system the thermal effi-ciency is derived from the PE value. Thesystem has been widely applied but is com-plex with a large number of climatic re-gions and has not achieved the popularityof the KÖPPEN CLIMATE CLASSIFICATION. Seealso Flohn climate classification; Strahlerclimate classification.

Thornthwaite’s index of potentialevapotranspiration An index to definePOTENTIAL EVAPOTRANSPIRATION that wasdevised by the American climatologistCharles Warren Thornthwaite in 1948. Itwas calculated from measurements of airtemperature and day length and used inThornthwaite’s second (1948) climate clas-sification. See Thornthwaite climate classi-fication.

three-cell model A basic conceptualiza-

tion of the average large-scale circulationfeatures that occur in the north–south ver-tical plane in each hemisphere. The threecells are the HADLEY CELL, FERREL CELL, andthe POLAR CELL. See also general circula-tion.

threshold wind speed The criticalwind speed required to entrain material(e.g. dust) into the atmosphere. Particleshaving diameters of about 40 µm are mosteasily entrained into the lower atmosphere;such particles possess the lowest thresholdwind speeds. Larger particles are difficultto lift because of their weight; so too arevery small particles because they have agreater surface area per unit volume andcohesive forces tend to be greater forsmaller particles.

throughfall /throo-fawl/ The PRECIPITA-TION that passes through a cover of vegeta-tion to fall to the Earth’s surface.

thunder The audible acoustic shockwave that follows a LIGHTNING flash. It iscaused by the sudden heating and expan-sion of air along the lightning channel,which propagates outward. Thunder trav-els at the speed of sound (300 m s–1) so it isheard after a lightning flash has been seen;it is seldom heard at distances beyond 16km (10 miles). The rumbling of thunder re-sults from atmospheric refraction of thesound produced; a sharp crack is morecommonly heard directly beneath a thun-dercloud.

thunderstorm A severe storm producedby a CUMULONIMBUS cloud in which precip-itation (especially heavy rain and/or hail)and updrafts are accompanied by THUNDER

and LIGHTNING. A thunderstorm has a typ-ical duration of about one hour, duringwhich electrification processes are suffi-ciently rapid for many lightning dischargesto occur. Thunderstorms are of consider-able vertical scale, typically extending fromthe surface to the tropopause, with anupper ANVIL shape comprising ice crystalssheared in upper level winds. Updrafts anddowndrafts can have sharp wind shear as-sociated with them. Electrification proba-

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bly occurs as a result of collisions betweenrising ice crystals and falling soft hail(GRAUPEL) particles, carrying negativecharge downward in the cloud and positivecharge upward into the anvil region. Elec-trification continues until the electric fieldsare sufficiently intense to break down theinsulating properties of air, when a LEADER

is propagated followed by a lightning dis-charge. See also air-mass thunderstorm;multicell storm; supercell.

tidal wave See tsunami.

tide The periodic rise and fall of sea levelwithin the Earth’s oceans, which resultsfrom the relative gravitational attraction ofthe Moon, and to a lesser extent the Sun,and the Earth. Spring tides occur when thegravitational pull of both the Sun andMoon coincide (around new and fullMoon) and produce the maximum tidalrange between high and low water. Neaptides occur when the Sun and Moon are ap-parently at right angles to each other (firstand last quarters). The pattern of semi-di-urnal tides, in which two high waters andtwo low waters occur during a tidal day of24 hrs and 50 minutes is the most com-mon; diurnal tides are one high and onelow water during a tidal day.

tierra /tye-ră/ The series of climaticzones within South America, CentralAmerica, and Mexico that are defined byaltitude. The tierra helada is the zone ofmountain peaks; the tierra fria is the up-land climatic zone above 1800 m (6000 ft)in which frost occurs (it is also the upperlimit for tree growth); the tierra templada,extends between 900 and 1800 m(3000–6000 ft), and the tierra caliente isthe coastal zone extending from the coastup to 900 m (3000 ft).

tilted storm A type of thunderstorm orcloud tower that is tilted, indicating verti-cal wind shear and conditions suitable forsevere storm intensification.

timberline (treeline) The limit, at highlatitudes or high altitudes, beyond whichtrees do not grow.

timescale See Coordinated UniversalTime; Greenwich Mean Time; UniversalTime; Zulu Time.

tipping-bucket rain gauge A recordingRAIN GAUGE in which the delivery tube con-sists of a bucket divided into two equalcompartments, each designed to collect aspecific amount of rain. The bucket is bal-anced on a seesaw so that as one compart-ment fills to a pre-determined amount,usually 0.3 mm (0.01 in), it tips emptyingthe water, and the other compartment ispositioned in its place to collect the water.Each time a tip occurs an electronic signalis sent to a recorder giving a continuousrecord of rainfall against time. Some ver-sions of the gauge incorporate a heating el-ement to melt solid precipitation, such assnow. See also weighing rain gauge.

TIROS /tÿ-ross/ (Television and InfraredObservation Satellite) The first opera-tional meteorological satellite series devel-oped in the US. The first of the series,TIROS–1, was launched on April 1, 1960to open the era of METEOROLOGICAL SATEL-LITE observations. It carried two TV cam-eras, one with high and one with lowresolution. The term TIROS is still usedoccasionally as synonymous with theNOAA polar-orbiting operational satel-lites (POES).

TIROS operational vertical sounder(TOVS) An instrument consisting of asystem of three sensors: the microwavesounding unit (MSU), the High resolutionInfrared Radiation Sounder (HIRS), andthe Stratopheric Sounding Unit (SSU). It iscapable of instantaneously sensing thechanging strength of emitted radiationfrom different levels within the atmos-phere. It does this by measuring these sig-nals in a large number of channels orBANDS.

TOGA See Tropical Ocean–Global At-mosphere Program.

TOMS See total ozone mapping spec-trometer.

tidal wave

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topoclimatology /top-oh-klÿ-mă-tol-ŏ-jee/ The study of the influence of topog-raphy on CLIMATE.

topography /tŏ-pog-ră-fee/ The land-forms and features of the surface of theEarth, including relief, vegetation, andhuman features, such as urban areas andcommunications. See also topoclimatol-ogy.

tornadic vortex signature /tor-nay-dik/(TVS) A significant distribution of radialvelocity components mapped by a DOPPLER

RADAR that indicates the possible develop-ment of a tornado. The pattern is one inwhich a strong component away from theradar is juxtaposed horizontally, over ashort distance, with a component in whichthere is strong flow toward the radar. Thehorizontal WIND SHEAR is extremely largeand CYCLONIC. In the US, tornado warn-ings are issued by the National WeatherService (NWS) based on the strength of thetornadic vortex signature.

tornado /tor-nay-doh/ A violent rotat-ing column of air extending between a con-vective thunderstorm cloud and theground, averaging tens to hundreds of me-ters in diameter, with extreme vertical up-drafts and low pressures in the center.Tornadoes are frequently able to levitateheavy objects and cause widespread devas-tation in the US, which has a greater inci-dence of tornadoes than anywhere else inthe world; around 1000 annually. Torna-does form in conditions similar to thunder-storms, and indeed are usually seenbeneath thunderclouds, reaching near tothe ground. Wind speeds in excess of 500km per hour have been identified in US tor-nadoes using Doppler radar and the centralpressure can be very low. In the UK around30 tornadoes occur annually though theyare seldom reported; they rarely, if ever,reach more than F3 strength. See FujitaTornado Intensity Scale. See also funnelcloud; wall cloud.

Tornado Alley A colloquial name foran area in the Great Plains of the US inwhich TORNADOES occur with the greatest

frequency and intensity. It extends fromwestern Iowa, through Nebraska, Kansas,Oklahoma, Arkansas, and Louisiana,down to Mississippi.

tornado family Conditions for tornadodevelopment cover a wide area with nu-merous thunder cells, it is not unusual forgroups or families of tornadoes to occurwithin a small area or timescale, such asTornado Alley in the US.

torrid zone One of the three historic cli-matic zones, based on temperature andsunshine characteristics, into which theworld was originally divided by the classi-cal Greeks; it is the low-latitude zonebounded by the northern and southernlimit of the Sun’s vertical rays, i.e. theTropics of Cancer (23°27′N) and theTropic of Capricorn (23°27′S). In this zonetemperatures are warm throughout theyear and the noon Sun is always high; dayand night are nearly equal.

total column moisture See precipitablewater.

total ozone mapping spectrometer(TOMS) A satellite-borne instrument formonitoring ozone and sulfur dioxide con-centrations. The instrument was first flownon NASA’s Nimbus-7 satellite, which waslaunched in 1978, and provided data fromthis, including the first maps of the ozonehole, until 1993. Versions of TOMS havebeen carried on three subsequent satellites:the Russian Meteor-3, NASA Earth Probe,and Japanese Advanced Earth Observa-tions Satellite (ADEOS).

TOVS See TIROS operational verticalsounder.

towering cumulus See congestus.

trace gas A constituent gas of the at-mosphere that is present in extremely smallconcentration. It does not imply unimpor-tance, however, as trace components, suchas OZONE and METHANE, for example, areboth very important GREENHOUSE GASES.Other trace gases include CARBON DIOXIDE

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(on which plant life depends), the oxides ofnitrogen, and ammonia.

trade-wind inversion A horizontallyvery widespread temperature INVERSION as-sociated with the base of deeply subsidedair within the Equatorward flank of theSUBTROPICAL ANTICYCLONES (highs). It is anoceanic phenomenon, lying typically some600 m (2000 ft) above the ocean surfacewithin the high, but increasing in depth to-ward the EQUATORIAL TROUGH, where itmay be as high as 2 km (1.2 miles). Its ele-vation also increases westward across thesubtropical oceans as the heating from thesurface increases along its track. Thismeans that the trade-wind CUMULUS cloudsincrease in depth in association with thegradually upward-sloping inversion. Thetrade-wind inversion is also dry, whichmeans that within the inversion layer oftemperature increasing with height, the HU-MIDITY decreases very dramatically up-ward.

trade-wind littoral climate See tropi-cal monsoon and trade-wind littoral cli-mates.

trade winds (trades, tropical easterlies)The winds that blow from the Equator-ward margin of the subtropical anticy-clones (around 30–40°N and S) toconverge in the INTERTROPICAL CONVER-GENCE ZONE: the northeast trades in thenorthern hemisphere; the southeast tradesin the southern hemisphere. Unlike manyother wind systems, which show great vari-ability over time, the direction and velocityof the trade winds are remarkably con-stant, especially over the oceans, being ofthe order of 14 knots (16 mph). The tradewinds are strongest in winter, with theirlatitudinal extent varying by season. Heavyrainfall can result when the trades en-counter a topographic barrier, especiallyon the western side of oceans nearer theEquator where the trade wind temperatureinversion is weaker. Consequently, thenorthern coast of many Caribbean islands(e.g. Jamaica) is considerably wetter thanthe southern coast. Conversely, places lo-cated further away from the Equator and

on the eastern side of ocean basins withinthe trade-wind belt (e.g. the Canary Is-lands) experience very arid conditions be-cause of this temperature inversion.TROPICAL CYCLONES develop in the tropicaleasterlies. See also antitrades.

trajectory The track taken by a parcel ofair as it moves within the atmosphere overa given period. The tracks are nearly al-ways three-dimensional, but are oftenmapped on a two-dimensional weathermap, such as at mean sea level.

tramontana /trah-mon-tah-nah/ A LOCAL

WIND experienced in the Mediterranean,especially in Italy where the name is fre-quently applied to a wind that blows offthe country’s west coast. In Italy, the tra-montana is a cool dry north or northeast-erly wind that blows from the Alpine rangeto the north. A northerly wind that affectsthe Catalan coast of Spain in winter is alsocalled the tramontana.

translucidus /trans-loo-sa—dŭs/ A cloudvariety of extensive sheets, layers, orpatches translucent enough for the Sun orMoon to be visible through it. It is appliedto altocumulus, altostratus, stratocumulus,and stratus. See also cloud classification.

transpiration /tran-spă-ray-shŏn/ Theloss of water vapor through evaporationfrom the surface of a plant. Liquid watercontained in the soil is extracted by plantroots, passed upward through the plant,and discharged as water vapor to the at-mosphere. The rate of transpiration duringthe day is about the same as that from anopen water surface in the same meteoro-logical conditions, but is almost zero dur-ing night hours.

transverse bands Features that havebeen noted on satellite cloud images; theyare linear bands of clouds organized atright angles to the prevailing flow and mayindicate extreme turbulence at upper lev-els. They have been observed in CIRRUS atupper levels in the vicinity of a POLAR

FRONT JETSTREAM.

trade-wind inversion

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treeline See timberline.

tree-ring analysis See dendrochronol-ogy.

trend See secular trend.

triple point of water The temperatureof 0.01°C (273.16 K) at which liquidwater, ice, and water vapor coexist in equi-librium, which is used as a fixed point onthe thermodynamic temperature scale.

tropical air An air mass that has itssource region in the belt of subtropical an-ticyclones of the middle latitudes. Thewarm moist maritime tropical air (mT)originates over the oceans, while the hotand dry continental tropical air (cT) has itssource over the arid and desert areas of theland masses, such as North Africa.

tropical and subtropical desert cli-mate An arid climate type occurringmainly between 15° and 30° latitude inboth north and south hemispheres beneaththe descending Hadley cell circulations; itis dominated by subtropical high pressurefor most or all of the year. Precipitation isvery low and temperatures are high with ahigh diurnal range. The climate is denotedby BW in the KÖPPEN CLIMATE CLASSIFICA-TION.

tropical and subtropical steppe cli-mate A semiarid climate that occursalong the margins of the true arid deserts.It has a short wet season associated withthe Intertropical Convergence Zone(ITCZ) enabling the growth of grasses,which support the grazing of animals. TheSahel region, which borders the Sahara hasa tropical steppe climate. The climate is de-noted by BSh in the KÖPPEN CLIMATE CLAS-SIFICATION.

Tropical Atmosphere–Ocean array(TAO) See TAO/TRITON array.

tropical climate A general term for theclimates of the TROPICS or equatorial zone.Temperatures are high throughout the yearwith no distinct cool season. Precipitation

is also high and in many locations there aredistinct wet and dry seasons. See also trop-ical and subtropical steppe climate; tropi-cal monsoon and trade-wind littoralclimates; tropical wet-dry climate; wetequatorial climate.

tropical cyclone 1. A general term forall cyclonic circulations that originatewithin the tropics. See cyclone; tropical de-pression; tropical disturbance; tropicalstorm.2. The intense low-pressure system withsustained winds (1-minute average) over64 knots (73 mph), i.e. above force 12 onthe BEAUFORT SCALE, that develops overtropical waters and is known in North andCentral America and the Caribbean as aHURRICANE. In the western Pacific region itis known as a TYPHOON, and off Australiaand in the Indian Ocean, as a cyclone ortropical cyclone.

Tropical Cyclone Program An inte-gral part of the World Meteorological Or-ganization’s WORLD WEATHER WATCH

Applications Department. It has the aim ofestablishing nationally and regionally co-ordinated systems to ensure the minimiza-tion of the loss of life and of damage toproperty due to TROPICAL CYCLONES. Oneimportant thrust of the program is there-fore to assist its members in the provisionof high-quality predictions of the tracksand intensity of tropical cyclones.

tropical depression A CYCLONE in trop-ical or subtropical latitudes having one ormore closed isobars and in which the max-imum sustained surface wind speeds (1minute average) are 33 knots (38 mph) orless, or less than force 7 on the BEAUFORT

SCALE. It may form from an easterly waveor a TROPICAL DISTURBANCE. If it continuesto intensify and strengthen if may developinto a TROPICAL STORM, and possibly a hur-ricane (tropical cyclone or typhoon). Trop-ical depressions are assigned a number bywhich they can be recognized and tracked.

tropical disturbance A discrete non-frontal system of clouds and thunder-storms with organized convection that

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occurs in the tropics and subtropics. Itshows slight circulation but no closed iso-bars and is in the range of 150–500 km(100–300 miles) in diameter.

The tropical disturbance may developinto a TROPICAL DEPRESSION, and from thereintensify into a TROPICAL STORM, or possi-bly a hurricane (tropical cyclone or ty-phoon).

tropical easterlies See trade winds.

Tropical Easterly Jet Stream A JET

STREAM that forms a constituent part of thesummer ASIAN MONSOON; it has its EN-TRANCE REGION in the upper TROPOSPHERE

above Indonesia and its EXIT REGION acrosssub-Saharan Africa. It is a major seasonalfeature of the GENERAL CIRCULATION of theatmosphere.

tropical meteorology The study of theatmospheric phenomena of tropical zones,such as convectional clouds, the tradewinds, Hadley cells, Intertropical Conver-gence Zone (ITCZ), monsoons, tropicaldepressions, tropical storms, and tropicalcyclones.

tropical monsoon and trade-wind lit-toral climates Climates of the tropical

zone with high temperatures, a low annualrange of temperatures, abundant precipita-tion, and a short dry season, which occursusually during the low-Sun (winter sea-son). The climate type has two differentorigins. Over southern and southeasternAsia, the largest extent of the type, theASIAN MONSOON brings precipitation insummer; in winter the outflow from theSiberian anticyclone (high) over Asiabrings a drier and cooler season. In theAmericas and Africa the TRADE WINDS bringmoist air and orographic effects produceabundant rainfall along the coastal zones.Short dry seasons occur with the seasonalmigrations in the trade winds. The climatesare denoted by Am in the KÖPPEN CLIMATE

CLASSIFICATION.

Tropical Ocean–Global AtmosphereProgram (TOGA) An international pro-gram, a major component of the WORLD

CLIMATE RESEARCH PROGRAM (WCRP),aimed at the prediction of climate phenom-ena with reference to the tropical oceansand their relationship to the global atmos-phere. It sought to study the feasibility ofmodeling the coupled ocean-atmospheresystem, and to provide a scientific back-ground for the observation and transmis-sion of data. Lasting from 1985 until 1994,

tropical easterlies

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areas with sea-surface temperatureof the warmest month above 27°C

Distribution of tropical cyclones

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one of its major achievements was the es-tablishment of an ocean-observation sys-tem to support seasonal-to-interannualclimate studies. This includes the TAO/TRI-TON ARRAY of nearly 70 moored buoysspanning the equatorial Pacific (which pro-vides measurements of surface winds, sea-surface temperature, upper-ocean tem-perature and currents, air temperature, andhumidity), drifting buoys, observationships, an island wind profiler network, andsatellite data. The data provided by theprogram contributed to understanding theprocesses responsible for the EL NIÑO

SOUTHERN OSCILLATION (ENSO).

Tropical Prediction Center In the US,one of the nine National Centers for Envi-ronmental Prediction (NCEP); it is sited atFlorida International University on theoutskirts of Miami, Florida. It includes theNational Hurricane Center (NHC), therole of which is to provide tropical analysisand forecasts, particularly during the HUR-RICANE season from May 15 to November30 for the Caribbean, the Gulf of Mexico,the North Atlantic, and Eastern Pacific. Italso prepares and distributes hurricanewatches and warnings.

tropical rainy climate One of themajor climate groups in the KÖPPEN CLI-MATE CLASSIFICATION, designated as A.Temperatures are higher than 18°C(64.4°F) for all months of the year andthere is no winter season. The climate hashigh annual rainfall, which exceeds annualevaporation.

tropical storm A CYCLONE developingin tropical latitudes. It is classified as atropical storm when the maximum sus-tained surface wind speed (1 minute aver-age) is between 34 and 63 knots (39–73mph), force 8 to 11 on the BEAUFORT SCALE.Once it reaches this intensity the tropicalstorm is given a name by which it can betracked. The tropical storm maystrengthen further to become a TROPICAL

CYCLONE, HURRICANE, or TYPHOON (de-pending on the part of the world in whichthe tropical cyclone is occurring).

tropical wave See easterly wave.

tropical wet-dry climate (tropical sa-vanna climate) A climate type of tropicalregions in which there are marked wet anddry seasons and temperature are high butshow seasonal variations. Total annualrainfall is around 50–175 cm (20–69 in)and occurs chiefly from convectional rain-fall. The migration of the IntertropicalConvergence Zone (ITCZ) is responsiblefor the seasonal shifts in most of the areas.The climate type occurs in Africa and theAmericas (latitudes 5–20°N and S) and inAsia (10–30°N). The climate is designatedas Aw in the KÖPPEN CLIMATE CLASSIFICA-TION.

Tropic of Cancer The northern limiton the Earth at which the Sun is directlyoverhead (which occurs at the solstice onJune 21), located on the latitude of23°27′ N.

Tropic of Capricorn The southernlimit on the Earth at which the Sun is di-rectly overhead (which occurs at the sol-stice on December 21), located on thelatitude of 23°27′ S.

tropics (equatorial zone) The zone thatlies between the lines of latitude of theTROPIC OF CANCER and TROPIC OF CAPRI-CORN.

tropopause /trop-ŏ-pawz/ The narrowboundary that divides the lowest atmos-pheric layer, the TROPOSPHERE, from theSTRATOSPHERE above. The tropopause actslike a lid to convection that occurs in thetroposphere; for example, this tends to bethe point at which clouds can grow no fur-ther. The tropopause is an isothermal layerin which there is no change in temperaturewith height.

troposphere /trop-ŏ-sfeer/ The atmos-pheric layer that lies directly over theEarth’s surface and below the STRATOS-PHERE. Its height varies, being higher at theEquator at around 20 km (12 miles) abovethe Earth’s surface and decreasing towardaround 10 km (6 miles) at the poles. The

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height also varies with the seasons beinghighest in summer and lowest in winter.The troposphere contains nearly all theEarth’s weather phenomena and clouds aswell as about 75% of the gaseous mass and99% of the water vapor of the atmosphere.It is characterized by a decreasing of tem-perature with height at a rate of approxi-mately 6.5°C/km (the ENVIRONMENTAL

LAPSE RATE) until the TROPOPAUSE isreached. The layer is well mixed by vigor-ous vertical circulations within it related tothe intensity of surface heating.

trough /troff/ A normally elongated fea-ture of low atmospheric pressure on a pres-sure chart, the axis of which is defined by asharp ‘kink’ or angle in the v-shaped iso-bars. The contour pattern on a chart hasthe same appearance as the contours of atrough or valley on a land-surface topogra-phy map. Troughs often coincide withFRONTS, across which wind direction andtherefore isobar alignment change. Theyare frequently, but not always, associatedwith unsettled weather, cloudy conditions,and precipitation. Compare ridge.

trowal In Canada, the trough of warmair that has been lifted and squeezed upalong the upper front of an occluded front.It can be associated with significantweather at the surface, such as may befound with an OCCLUDED FRONT. [From:trough of warm air aloft]

tsunami /tsoo-nah-mee/ A potentiallydestructive wave or series of waves in theocean produced by sudden submarinemovement, such as an underwater earth-quake, volcanic explosion, or a landslide(either a landslide into a body of water ora gravity slide of marine sediments). Theshallow-water wave or waves, which areabout 1 meter high, possess long periods(over 1 hour) and wave lengths (often inexcess of 100 km) and can travel at rapidspeed (e. g. over 700 km/hr across the Pa-cific Ocean). When the tsunami ap-proaches land, and water depth decreases,the wave is slowed and grows considerablyin height, sometimes in excess of 20 m (66ft) with the potential to cause serious dam-

age as it travels ashore. Recent tsunamishave included the devastating waves(about 14 m (46 ft) in height) generated byunderwater earthquakes that hit the Sis-sano Lagoon area of the northwest coast ofPapua New Guinea on July 17 1998, de-stroying villages and causing considerableloss of life. The catastrophic Asian tsunamiof December 26, 2004, which resulted in over 283 000 deaths and considerabledamage, was caused by a massive under-water earthquake of magnitude 9 that hadits epicenter 30 km (20 miles) below theseafloor, 250 km south-southeast of BandaAceh, Indonesia. The consequent tsunamiwaves caused damage along the coastlinesof 13 countries including Indonesia (withwaves up to 10 m (33 ft) high on the coast-line of Sumatra), Sri Lanka, India, Thai-land, Myanmar, Malaysia, The Maldives,and the East African countries of Somaliaand Kenya. The Pacific region has a partic-ularly high incidence of tsunamis associ-ated with the seismically active subductionzone of the Pacific rim that surrounds it.Tsunamis have also been known as tidalwaves but this term is incorrect. [Japanese:‘harbor wave’]

tuba /tew-bă/ A supplementary cloudfeature in the form of a narrow column orinverted cone-like cloud that protrudesfrom the base of a tall convective cloud,typically CUMULONIMBUS but occasionallycumulus CONGESTUS. It is often the precur-sor of the FUNNEL CLOUD with which TOR-NADOES are associated. See also cloudclassification.

tundra /tun-dră/ The treeless plains ofthe northern hemisphere that extendnorthward from the limit of tree growth(the timberline) across North America(Alaska and northern Canada) and Eurasiato the permanently snow- and ice-coveredland of the polar zone. It is generally wetand boggy as a result of the permanentlyfrozen ground – permafrost – which liesbelow the ground surface preventingdrainage, and the vegetation is hardyshrubs, mosses, and lichens.

tundra climate The climate occurring

trough

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along the Arctic coast of North Americaand Eurasia and along the coastal fringesof Greenland between 60° and 75°N. It haslarge annual ranges of temperature andmean annual temperatures are below freez-ing point of 0°C (32°F). Winters are longand cold and summers short and mild. Pre-cipitation is low overall with dry snow inwinter but rises in the brief summer seasonwhen cyclonic storms develop along thesea and sea ice margin. In the KÖPPEN CLI-MATE CLASSIFICATION it is a type E climate(ET); the mean temperature of the warmestmonth is above 0°C (32°F) but below 10°C(50°F).

turbidity The reduction in the trans-parency of the atmosphere to incomingsolar radiation caused primarily throughABSORPTION and SCATTERING by particulatematter and aerosols. It excludes scatteringby air molecules and by clouds.

turbopause /ter-boh-pawz/ The level,approximately 100 km (60 miles) abovethe Earth’s surface, at which the two mainmixing processes in the atmosphere – mol-ecular diffusion and convective motions –are equally important. Below this level tur-bulent fluid motions dominate and above,molecular diffusion prevails.

turbosphere /ter-boh-sfeer/ The part ofthe atmosphere below the TURBOPAUSE

characterized by turbulence in which con-vective motions play the major role in ho-mogenizing the composition of theatmosphere. See also homosphere.

turbulence (turbulent flow) In the at-mosphere, random fluctuations that occurin the wind speed and direction as mechan-ical turbulence when air blows across arough landscape, creating a complex ofEDDY motions. The eddies can produce tur-bulent flow within a layer extending to afew hundred meters deep. Thermal turbu-lence is created within and around CUMULI-FORM clouds, characterized by markedupdrafts and downdrafts. This type of tur-bulence can be troposphere-deep withinCUMULONIMBUS clouds. See also clear-airturbulence.

turbulent boundary layer See mixedlayer.

turbulent diffusion (turbulent transport)The transport of heat, water vapor, mo-mentum, and air pollutants throughout thedepth of the PLANETARY BOUNDARY LAYER

by horizontal and vertical turbulence (theirregular motions embedded in the airflowof the planetary boundary layer). Turbu-lence is much more effective by severalorders of magnitude in transporting quan-tities through the depth of the boundarylayer than the process of molecular diffu-sivity. See also mixed layer.

turbulent flow See turbulence.

turbulent transport See eddy diffusion.

TVS See tornadic vortex signature.

twenty-foot wind The wind speedmeasured or estimated over a 2- to 10-minute period at a standard 20-foot (6-m)level above a vegetative surface or continu-ous tree canopy. The twenty-foot wind isused as one input variable in fire dangermodels in the US.

twilight The time before sunrise (dawn)and after sunset (dusk) when light is in-creasing or waning and is provided by re-flection of sunlight by the upperatmosphere. Specific definitions of twilightare used for the time that twilight begins inthe morning or ends in the evening: civiltwilight begins or ends when the center ofthe Sun is geometrically 6° below the hori-zon; nautical twilight begins or ends whenthis distance is 12°; and astronomical twi-light begins or ends when the distance is18° below the horizon.

twister A colloquial name, especially inthe US, for a TORNADO. It is also used forless severe local events, such as WHIRL-WINDS and DUST DEVILS, in which rotationalmotion occurs.

typhoon The name for an intense TROP-ICAL CYCLONE that occurs in the westernPacific Ocean. Those typhoons with maxi-

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mum sustained winds (1 minute average)of over 130 knots may be defined as super

typhoons. See also hurricane. [Chinese tai-fun: ‘great wind’]

typhoon

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UARS See Upper Atmosphere ResearchSatellite.

UK Met Office See Met Office.

Ulloa’s ring (Ulloa’s circle) SeeBouguer’s halo.

ultraviolet radiation (UV radiation) The part of the electromagnetic spectrumwith wavelengths from 1–400 nm in the re-gion below the VISIBLE RADIATION. Thereare several methods used to subdivide ul-traviolet radiation. The most widelyknown, that based on the biological effectsit produces, has three divisions: UVA(320–400 nm), which is significant in theproduction of photochemical smog, UVB(290–320 nm), and UVC (200–290 nm)which is strongly absorbed by ozone andother gases in the middle stratosphere.Only 1% of solar radiation is within theUVB division, most of which is absorbedby ozone in the atmosphere. The smallproportion that reaches the surface cancause damage at the molecular level, forexample to DNA, which can affect animalsand plants. A number of organizationsnow produce forecasts of UV levels, suchas the UV Index issued by the US NationalWeather Service.

In physics, ultraviolet radiation is sub-divided into regions: near (400–300 nm),middle (300–200 nm), far (200–100 nm),and extreme (less than 100 nm).

uncinus /un-sÿ-nŭs/ A cloud species ofCIRRUS in which the high cloud is shapedlike a comma that terminates in a hook ora tuft; the upper parts do not showrounded protuberances. See also cloudclassification.

undersun /un-der-sun/ See subsun.

undulatus /un-jŭ-lay-tŭs/ A type or vari-ety of cloud that occurs in patches, sheets,or layers and displays an undulating form.Sometimes more than one system of undu-lations is visible. It is a variety of CIRRO-CUMULUS, CIRROSTRATUS, ALTOCUMULUS,ALTOSTRATUS, STRATUS, and STRATOCUMU-LUS. See also cloud classification.

United Nations Conference on Envi-ronment and Development SeeUnited Nations Framework Convention onClimate Change.

United Nations Framework Conven-tion on Climate Change (UNFCCC) An agreement drawn up in 1992 andopened for signature at the United NationsConference on Environment and Develop-ment (UNCED, the Earth Summit) in Riode Janeiro, Brazil, that set out a frameworkfor action to control or cut greenhouse gasemissions to prevent significant anthro-pogenically forced climate change. It en-tered into force in 1994. The first meetingof the Conference to the Parties (COP 1)took place in Berlin, Germany, in 1995,where the Berlin mandate was agreed. Aprotocol to the convention was drawn upin 1997 at the third Conference to the Par-ties (COP 3) in Kyoto, Japan (see KyotoProtocol).

universal gas constant See ideal gaslaws.

Universal Time (UT) The name, intro-duced in 1928, for GREENWICH MEAN TIME

(GMT) when used for precise scientificpurposes. Universal Time is a general des-ignation for timescales based on the rota-

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tion of the Earth and there are several vari-ations. UT1 is related to polar motion andis proportional to the rotation of Earth inspace. See also Coordinated UniversalTime.

unstable air See absolute instability.

unstable equilibrium An atmosphericstate in which a displaced air parcel willcontinue to accelerate vertically after aninitial perturbation, leading to rapid verti-cal air motion and cloud formation. Thisstate occurs when the ENVIRONMENTAL

LAPSE RATE (ELR) is greater than the DRY

ADIABATIC LAPSE RATE (DALR). See also ab-solute instability; lapse rate; stability.

UN World Meteorological Organiza-tion See World Meteorological Organi-zation.

updraft The relatively localized ascend-ing current that is a critical constituent ofCUMULIFORM clouds. Updrafts vary signifi-cantly in magnitude; the updraft that sup-plies or forms a shallow CUMULUS cloudmay have a speed of a few tens of centime-ters per second, whereas that forming theheart of a vigorous CUMULONIMBUS cloudmay be as strong as 50 m s–1 (e.g. in severeweather over the springtime High Plains inthe US). The ascending currents that pro-duce STRATIFORM clouds across a verymuch larger area are sometimes referred toas an updraft.

upper-air station A site at which syn-optic observations of the upper-air levelsare made. RADIOSONDES are released twice aday from upper-air stations, at 00:00 UTCand 12:00 UTC, to provide observations ofATMOSPHERIC PRESSURE, DRY-BULB TEMPERA-TURE, RELATIVE HUMIDITY, and WIND DIREC-TION and WIND SPEED. In addition, at 06:00UTC and 18:00 UTC, upper winds are ob-served by tracking a balloon-borne radartarget. The balloons usually ascend intothe lower STRATOSPHERE before bursting.Upper-air stations are much more widelyscattered than surface-weather stations,and are thus used to define the large-scalesmooth features of the troposphere above

the friction layer and the lower stratos-phere.

upper-air weather chart A map thatdepicts one or more aspects of the atmos-phere at a particular level between thelower TROPOSPHERE and the lower STRATOS-PHERE. A common form is the height analy-sis of, for example, the 500 hPa surface,which takes the form of a field of contoursthat portray the LONG WAVES. Additionally,charts can be for winds at, for example,250 hPa, or temperature at 700 hPa. Oper-ational weather services usually recognizestandard levels for which charts are alwaysproduced: these include 850, 700, 500,400, 300, 250, and 200 hPa.

Upper Atmosphere Research Satellite(UARS) A satellite that was launched in1991 by NASA to measure ozone andchemical compounds within the ozonelayer; winds and temperatures in thestratosphere; and radiation from the Sun.The satellite was designed to be in opera-tion for three years but was finally decom-missioned in 2005. It carried a payload of 10 instruments, including the high-resolution Doppler interferometer (HRDI).

upper-level system Mainly synoptic-scale features in the upper TROPOSPHERE

that may not necessarily be manifested atthe surface. Examples are upper troughsand JET STREAMS.

upslope flow The movement of warmair up the sideslopes of a valley, which isinduced by differential heating rates. It oc-curs during the daytime, the flow beingtypically shallow in its vertical extent, un-like upslope winds, which envelop the en-tire mountain range and are synopticallyforced. See also anabatic wind.

upslope fog A type of FOG formed onthe upwind or upslope side of a hill ormountain when moist STABLE air undergo-ing OROGRAPHIC LIFT cools sufficiently, byadiabatic expansion, to produce condensa-tion of fog droplets at ground level.

up-valley wind (talwind) A weak wind

unstable air

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that blows up a valley, mostly notably on awarm summer’s afternoon. As air is heatedduring the day, it becomes less dense andso rises, the topographic constriction of thevalley sideslopes forcing the air up the val-ley. It generally occurs at the same time asthe upslope ANABATIC WIND.

upwelling A process by which waterfrom deeper levels in the ocean or sea istransported upward to the surface to re-place the surface flow that is taking placeaway from an area. This occurs, for exam-ple, along coasts when warm surface wa-ters are driven away from the shore bywinds generating offshore currents; thesewarm waters are replaced by cold denserwater brought up from moderate depths.The deeper water is often rich in nutrients,which encourage phytoplankton growth,and many of the world’s major fisheries arelocated in such areas of upwelling (e.g. offthe coasts of Peru in South America andOregon, US).

urban canyon (urban street canyon)

The ‘canyon’ formed by a street that isbordered by high-rise buildings on eitherside in an urban area.

urban climate A modification of the re-gional near-surface climate caused by thereplacement of components of the bios-phere, such as vegetation, by urban fabric,such as buildings, roads, and paved zones.This changes the nature of the heat transferfrom the surface by altering its heat capac-ity. In turn, it changes the atmosphericproperties of the SURFACE BOUNDARY LAYER,particularly the temperature, giving rise tothe urban HEAT ISLAND effect. Such urbaneffects on the near-surface climate have sig-nificantly increased since about 1950 asurban centers have expanded and cool airventilation from rural areas has dimin-ished.

UT See Universal Time.

UTC See Coordinated Universal Time.

UV radiation See ultraviolet radiation.

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valley breeze See valley wind.

valley exit jet A strong down-valley aircurrent that blows from a valley above itsintersection with an adjacent plain.

valley wind (valley breeze) The flow ofair predominantly up a valley during theday. Such winds are best developed in calmand clear conditions, and hence on dayswith a high diurnal temperature range. Thedynamics of the valley wind are stronglycontrolled by the topography. See alsoalong-valley wind system; anabatic wind;down-valley wind; funneling; katabaticwind; mountain wind.

Van Allen radiation belts Two bandsor zones of high-energy charged particlestrapped within the Earth’s magnetic field;they are concentrated at 3000 km (2000miles) and 16,000 km (10,000 miles)above the Earth’s surface. The belts arenamed after the American physicist JamesAlfred Van Allen (1914– ), who discov-ered them in 1958 as a result of experi-ments carried by space probes and artificialsatellites.

vane See wind vane.

vane-oriented propeller anemometerSee propeller anemometer.

vapor concentration See absolute hu-midity.

vapor pressure In meteorology, thecomponent of the total atmospheric pres-sure that is exerted by water vapor. It isone way of measuring the humidity of theair. At a given temperature, an increase ofvapor pressure in the air corresponds to an

increase in the humidity of the air. See alsosaturation vapor pressure.

vardar /var-dar/ (vardarac) A cold drynortherly wind that blows from Serbiadown the valleys of Macedonia, such as theVardar, bringing polar air into the AegeanSea region. Like the MISTRAL in its charac-ter, the vardar is particularly felt aroundthe Greek city of Thessaloniki. It occurslargely in winter.

vault See bounded weak echo region.

vector A force possessing both magni-tude and direction. Whereas a scalar quan-tity only possesses magnitude (forexample, the wind speed is a scalar quan-tity), the wind velocity is a vector quantitybecause both direction and speed are spec-ified.

vector wind See thermal wind.

veering (veered) A clockwise change inthe wind direction over time. For example,if a southeasterly (SE) wind (135°) blowingover a town at 10.00 pm has been replacedby a southwesterly (SW) wind (225°) by6.00 am the following morning, then thewind is said to have veered during this 8-hour period. Veering is the oppositechange to BACKING.

velum An ACCESSORY CLOUD; a fine ex-tensive sheet of cloud that lies just above orjoined to one or several CUMULIFORM

clouds that often have grown up throughit. It is most often observed with CUMULUS

and CUMULONIMBUS clouds. See also cloudclassification.

vendavale /ven-dă-vayl/ (vendaval) A

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strong LOCAL WIND associated with a low-pressure system that blows from the south-west and affects the Straits of Gibraltar andthe east coast of Spain, chiefly during thewinter. Given the wind’s Atlantic source,the vendavale wind can bring considerablequantities of rainfall to the region, as wellas very squally conditions.

ventilation index (ventilation factor) An index that represents the ability of anairflow to flush out pollutants from withina known volume of polluted air. It is de-fined as the product of the mean windspeed (in meters per second) through themixed layer and the depth of the mixedlayer itself. In the US the index is forecastby some National Weather Service offices.

Venturi effect The increase in the veloc-ity of a fluid or gas that results from a con-striction of flow. In meteorology, the effectis observed when land-surface elementsconstrict the airflow and hence increase theobserved wind velocities of an area. Exam-ples include URBAN CANYONS (i.e. streetswith high-rise buildings on either side, forexample, downtown Chicago and NewYork) and valleys in mountainous terrain.A low-level temperature inversion can alsoinduce a Venturi effect by constricting andso accelerating the air beneath. This phe-nomenon is sometimes observed in SEA-BREEZE systems.

veranillo /ve-ră-nil-oh/ The secondaryminimum in precipitation in mid- and latesummer in most of Central (Middle) Amer-ica and the Caribbean. This seasonal fea-ture occurs between the two rainfallmaxima in May–June and again in Septem-ber through November. The cause of thislessening in the rains is associated with theslight rise in pressure over the region, pos-sibly linked with an extension southwest-ward of the North Atlantic anticycloneinto the Gulf of Mexico, before low pres-sure and tropical storms dominate the lat-ter part of the year.

verglas /ver-glass/ A thin covering oftransparent ice on exposed rock surfaces

that forms from the freezing of rainfall orthe refreezing of water from melted snow.

vernal See spring.

vertebratus /ver-tĕ-bray-tŭs/ A varietyof cloud, usually CIRRUS, in a pattern inwhich the elements appear like vertebrae,ribs, or a fish skeleton. See also cloud clas-sification.

vertical instability See absolute insta-bility.

vertical motion The component of at-mospheric flow directed either up or down.Its intensity, within a low for example, de-pends directly on the magnitude of the low-level CONVERGENCE and the superimposedupper DIVERGENCE. Gently sloping ascent istied in with CYCLONIC systems, while SUBSI-DENCE occurs with ANTICYCLONES (highs).In meteorology, the vertical motion cannoteasily be observed directly but is calculatedby the way in which divergence changessign and magnitude with height in the TRO-POSPHERE over a region.

vertical stability See absolute stability;stability.

Vienna Convention for the Protectionof the Ozone Layer A convention heldin Vienna, in 1985, in which nationsagreed to take appropriate measure to pro-tect the environment and human healthagainst the adverse affects from human ac-tivities that modify or are likely to modifythe ozone layer. It sought to encourage re-search, cooperation between nations, andthe exchange of information. The urgencyfor action was stressed when evidence ofsevere depletion of the ozone layer aboveAntarctica was published during 1985; in1987 the MONTREAL PROTOCOL was drawnup.

virga /ver-gă/ (fallstreak) A shaft of solidor liquid precipitation that leaves a cloud’sbase but does not reach the surface becausethe air in the subcloud layer is dry enoughto completely evaporate the raindrops orsnowflakes. Virga can be vertical or in-

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clined. In CLOUD CLASSIFICATION virga isclassed as a supplementary feature.

visibility In weather observations, thegreatest (horizontal) distance over whichan object of specified characteristics can beseen clearly by the naked eye. If, when avisibility observation is made, it varies spa-tially, then the smallest distance must bereported. For the SYNOPTIC CODE, visibilityup to and including that of 5 km is re-ported every 100 m. Upward from 6 km, itis reported every 1 km until 30 km, afterwhich it increases in jumps of 5 km.

Visibility Protection Program In theUS, a stated aim of the Clean Air Acts forthe protection of good visibility within des-ignated regions of the US, especially theNational Parks.

visible radiation (photosynthetically ac-tive radiation, PAR) That part of theelectromagnetic spectrum that has wave-lengths between 0.4 and 0.7 µm, to whichthe human eye is sensitive. The range ofcolors across this band is violet, indigo,blue, green, yellow, orange, and red.

visual spectrum See visible radiation.

volatile organic compound (VOC) One of a number of organic compoundsthat readily evaporate into the atmosphereat normal temperatures and pressure.Some contribute to the formation of pho-tochemical smog; others are harmful airpollutants.

volcanic dust Microscopic dry particlesemitted principally during volcanic erup-tions (as is volcanic ash) that can be in-jected into the stratosphere. The mostimportant dust chemically is that with asulfate basis.

vortex flow The air motion linked tolow-pressure areas. It ranges from the cir-culation into a TORNADO on the smallestscale up to that associated with planetary-scale features, such as the CIRCUMPOLAR

VORTEX. Sometimes, however, vortex flowis used to refer to the smaller-scale vorticesonly.

vorticity /vor-tiss-ă-tee/ A measure ofthe spin or circular motion possessed by afluid in motion. It is usually taken to meanthe spin about the local vertical axis, i.e. inthe horizontal plane. Planetary vorticity isthe quantification of the spin possessed bya planet, such as the Earth. Air that is sta-tionary with respect to the Earth’s surface(i.e. in calm conditions) possesses this vor-ticity. It depends on the rotation rate of theplanet (in radians per second) and the sineof the latitude. It is a maximum at the polesand vanishes at the Equator. Relative vor-ticity is the property of spin possessed byfluids (for example, air or water) that moverelative to the Earth’s surface, usuallytaken in meteorology to be that about thelocal vertical axis. It is quantified by con-sidering the horizontal gradients of thewest and south components of the windand is defined as positive for cyclonic cir-culation. It can also be conceived of con-sisting of a contribution from the flow’scurvature and its shear. For example, atightly curved wind circulation withinwhich the wind speed increases to the right,looking along the wind, possesses strongpositive relative vorticity. Anticyclones onthe other hand are characterized by nega-tive relative vorticity. The total rotation orvorticity is the sum of relative vorticity andplanetary vorticity and is known as the ab-solute vorticity (i.e. absolute vorticity =planetary vorticity + relative vorticity). It isa property of the large-scale flow of the at-mosphere that is conserved.

visibility

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Walker cell A tropical circulation fea-ture that exists in the vertical plane alongthe Equator. The ascending branch islinked to deep moist CONVECTION andheavy rain, while the descending current islinked to clear skies and dry weather.These two vertical components are con-nected by westerly or easterly flow in thelower and upper TROPOSPHERE. There are anumber of such Walker cells around theEquator. The cells are named for theBritish mathematician and meteorologist,Sir Gilbert Thomas Walker (1868–1958),who first identified them.

wall cloud An ACCESSORY CLOUD featurerelated specifically to TORNADOES. An im-portant initial sign that a parent cloud is todevelop a tornado is that part of its basewill abruptly lower as a rotating mass ofcloud, 1.5–6 km (1–4 miles) in diameter,which ‘hangs’ beneath – as the wall cloud.It often resembles a vertical wall lying un-derneath an intense updraft of a rapidlygrowing CONVECTIVE CLOUD, the main CU-MULONIMBUS. A funnel cloud or tornadomay develop within a few minutes, or up toan hour, following formation of the wallcloud.

warm advection The horizontal trans-fer of the properties of a warm air massfrom lower latitudes to higher latitudes.For some purposes, vertical components ofmotion may also be important.

warm anticyclone An ANTICYCLONE

(high) that is characteristic of the semi-per-manent subtropical high-pressure areas. Inthe northern hemisphere, the warm anticy-clones include the AZORES HIGH, BERMUDA

HIGH, and NORTH PACIFIC HIGH, while in thesouthern hemisphere they include the sub-

tropical anticyclones located over theSouth Pacific, South Indian, and South At-lantic Oceans. Warm anticyclones aredeep, extending throughout the tropos-phere, and intensifying with height. Theytend to be slow moving and generally bringperiods of stable atmospheric conditionswith settled and warm weather. Comparecold anticyclone.

warm conveyor belt See conveyor belt.

warm front The leading edge of an ex-tensive region of advancing warm and,normally, moist air. The front is a shallowsloping zone (the slope typically 1:100 to1:150) across which temperature and hu-midity increase on its passage over a site.There is a decline in pressure and often a

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1000

10041008

patchydrizzle

brokencloud

mild,moist air

SE wind

extensiverain or snow

thicklayercloud

SW wind

directionof front‘smovement

precipitationregion

0 km 200

Warm front

FREEDOM PALESTINE


shift in the wind direction (VEERING in thenorthern hemisphere). Warm fronts areusually preceded by a belt of widespreadPRECIPITATION that lasts for a few hours be-fore the surface front arrives. In the warmfront warm air overrides colder denser airunderneath, so that it is associated with aclassic sequence of clouds. These rangefrom CIRRIFORM cloud some hundreds ofkilometers ahead of the surface warmfront, gradually lowering to ALTOSTRATUS

as the front moves closer to a site, then fi-nally extensive LOW CLOUD and precipita-tion, typically one to two hundredkilometers ahead of the surface front.

warm occlusion An OCCLUDED FRONT

that has extensive cloud and precipitationgenerally ahead of the surface front. Theair ahead of it is colder than the air follow-ing it; such occlusions are most common inW Europe, for example, in the winter.

warm rain RAIN generated from cloudsin which no ice crystals are present. In suchclouds, the BERGERON–FINDEISEN PROCESS ofraindrop growth does not operate and thegrowth of the droplets occurs only by theCOLLISION–COALESCENCE PROCESS.

warm sector An extensive region ofMARITIME TROPICAL air between the WARM

and COLD FRONTS of a LOW or depression.Warm sectors are characterized by warmmoist air that is often laden with extensiveSTRATIFORM cloud. Precipitation oftentakes the form of widespread drizzle al-though there can occasionally be heavierbursts of rain from deeper cloud.

warm temperate rainy climates Oneof the major climate groups in the KÖPPEN

CLIMATE CLASSIFICATION, designated as C.Temperatures in the coldest month are be-tween –3°C (26.6°F) and 18°C (64.4°F)and in the warmest month greater than10°C (50°F).

warning An announcement issued by aweather service, such as the US NationalWeather Service (NWS), that severeweather has developed, is already occur-ring, or has been detected on radar. The se-

vere weather poses a hazard or imminentdanger, for example a tornado, hurricane,flash flood, or severe thunderstorm.

Wasatch wind /waw-sahch/ A strongeasterly CANYON WIND that blows from themouths of the canyons of the WasatchMountains onto the plains of Utah, US.

washout The removal of solid particlesfrom the atmosphere through collisionwith falling precipitation, a process bywhich some pollution is removed from theatmosphere. The particles are deposited onthe ground surface. See also fallout.

watch An announcement issued by theUS National Weather Service (NWS) indi-cating a need for preparation and increasedawareness that a hazard may be imminent.Watches are produced for tornadoes, thun-derstorms, and flash floods.

water The oxide of hydrogen withchemical formula H20, as well as its liquidform. At atmospheric temperatures andpressures, it can exist in all three phases:solid (ICE), liquid (water), and gaseous(WATER VAPOR).

Water, in its various forms, is of funda-mental importance to weather and climate.Water vapor is one of the most importantgreenhouse gases in the balance of radia-tive energy and in the HYDROLOGIC CYCLE.Through the processes of evaporation,condensation, and advection water vaportransports energy from lower to higher lat-itudes, and through cloud formation re-flects short-wave solar radiation.

water cycle See hydrologic cycle.

water mass A body of water in whichproperties, such as temperature and salin-ity, and therefore density, are generallyuniform and distinctly different to adjacentwater masses.

waterspout A spiraling vertical columnof air or vortex of low pressure that occursover a body of water. Tornadic water-spouts form when a tornado passes fromland to water (these can be particularly de-

warm occlusion

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structive) but the majority form over thewater itself. The fair-weather waterspoutdevelops at the water surface and in associ-ation with warm water temperatures andhigh humidity within the lower levels of theatmosphere. From the surface it developsupward, visible, at its maximum intensity,from the spray vortex above the water, upthrough the funnel cloud to the overheadcloud base, reaching a height of around100 m (300 ft) and moving at speeds of10–15 knots (9–13 mph). It is generallyshort-lived with a typical duration of lessthan 30 minutes. The spout may be subjectto windshear but is otherwise often verti-cal. Waterspouts are most common in thetropics but have also been observed else-where, such as over the Mediterranean Seaand the Great Lakes of North America.

water vapor Water in its gaseous form,sometimes also called moisture or aqueousvapor. The distribution of water vaporvaries depending on temperature andheight, with more than 90% of the watervapor confined to below 500 hPa, usuallywithin the first 5000 m (16,500 ft) abovethe Earth’s surface.

Water vapor plays a fundamental rolein the transfer of moisture and latent heatand consequently weather. It enters the at-mosphere in the vapor phase of the HY-DROLOGIC CYCLE through evaporation andsublimation from water and ice bodies andtranspiration by plants. The energy storedas latent heat is transported through ad-vection and released when water vaporcondenses or freezes in cloud formation.Water vapor is also the most abundantGREENHOUSE GAS and as such is importantin absorbing infrared radiation and warm-ing the Earth’s atmosphere. As the temper-ature of the Earth’s atmosphere increases itis capable of holding more water vapor.The warmer temperatures may increase thefrequency and magnitude of tropicalstorms and hurricanes, flooding, and othersevere weather. Instruments to carry outroutine surface measurements of watervapor include the PSYCHROMETER and DEW-POINT HYGROMETER. The amount of waterin the atmosphere is usually measured via aRADIOSONDE, although satellite infrared

and microwave sensors are used to obtaintotal column moisture and low-resolutionvertical profiles. A number of definitionsare used for the amount of water vapor inair. See absolute humidity; dew-point; mix-ing ratio; relative humidity; specific humid-ity; vapor pressure.

water vapor channel The wavebandswithin which scanners on METEOROLOGI-CAL SATELLITES sense emitted terrestrial ra-diation. Water vapor absorbs andre-radiates electromagnetic radiation atvarious wavelengths, especially in the in-frared of 6.40–7.08 µm. The interceptionof this radiation by satellites provides amethod of remote sensing troposphericwater vapor. The resulting image is of therelative concentration of water vapor inthe middle troposphere.

wave In the atmosphere, a generallysmooth traveling undulation, in the pres-sure field, for example, that exhibits awave-like form. A FRONTAL WAVE is such afeature. A wave can, more widely, describewave-like patterns on a variety of motionscales, ranging from LEE WAVES to LONG

WAVES.

wave cloud A type of cloud that formsin the crest of a LEE WAVE (mountain wave),which is an almost stationary undulationof airstreams at upper levels above or tothe lee of a hill or mountain barrier. Thelenticular clouds (see lenticularis) thatform have smooth lens-shaped outlinesand can appear to be stationary; they maybe individual isolated clouds close to thehill or mountain or form in successions oflines of clouds running parallel to themountain barrier for some considerabledistance downwind.

wave cyclone See frontal wave.

wave depression See frontal wave.

WCRP See World Climate ResearchProgram.

weakening A decrease in the PRESSURE

GRADIENT around a low- or high- pressure

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system over time in conjunction with thedecline in the wind strength across a LOW

(which is said to be FILLING) or around anANTICYCLONE (high). Compare intensifica-tion.

weather The overall condition of the at-mosphere at the surface and in the upperair at one particular time. It is normally de-fined with respect to the data plottedaround the STATION MODEL and representedby a variety of maps including UPPER-AIR

WEATHER CHARTS and SURFACE WEATHER

CHARTS.

weather forecasting The process ofpredicting future weather conditions, overa wide variety of space scales, from theshortest term nowcast to the medium-range or longer. See long-range weatherforecasting; medium-range weather fore-casting; nowcasting; short-range weatherforecasting. See also forecast; seasonal pre-diction.

weather map See surface weather chart;synoptic chart; upper-air weather chart.

weather report A surface, upper-air,aircraft, etc., observation normally trans-mitted to a central collection point by usingthe SYNOPTIC CODE.

weather satellite See meteorologicalsatellite.

weather station An installation de-signed, equipped, and used regularly tomake meteorological observations.

weather symbols See synoptic symbols.

weather system A wide range ofweather-producing features that occur on avariety of space scales. It includes synoptic-scale features, such as CYCLONES and ANTI-CYCLONES and smaller, but important,phenomena, such as THUNDERSTORMS andSEA BREEZES.

wedge See ridge.

weighing rain gauge A type of auto-

matic recording RAIN GAUGE consisting of afunnel that empties into a bucket mountedon a weighing mechanism. The weight isrecorded on a chart mounted on a clock-driven drum, or an electrical signal is sentfor data processing. The weight is con-verted to give an equivalent depth of rain inmillimeters or inches. See also tipping-bucket rain gauge.

West African Monsoon Trough Themost active zone of the southwest moistmonsoon air that invades West Africa fromthe Gulf of Guinea and the neighboringwaters of the tropical Atlantic. The trough,which is closely associated with the In-tertropical Convergence Zone (ITCZ),moves north from the coastlands of south-ern West Africa in January, to a locationaround 18°N in August.

West Australia Current A cold currentin the southern Indian Ocean that flowsnorthward along the west coast of Aus-tralia as part of the South Indian Current.

west coast desert (cold-water desert)Desert areas occurring along the west coastmargins of the hot deserts where upwellingof ocean water is associated with cool cur-rents flowing Equatorward. Such desertsare cooler than expected for their latitude,as a result of air flowing inland off the coolwaters; for example, the Peru desert andAtacama desert of Chile (the driest deserton Earth) in South America, Namib desertof southwest Africa, and Baja California.The deserts are arid but persistant fogs andheavy dew are common. See also tropicaland subtropical desert climate.

westerlies The extensive region of west-erly winds that are commonplace in themiddle latitudes in each hemisphere at thesurface. The term usually refers to the av-erage wind direction over the oceans inthat latitude belt.

western boundary current See bound-ary current.

West Wind Drift See Antarctic Cir-cumpolar Current.

weather

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wet adiabatic lapse rate See saturatedadiabatic lapse rate.

wet-bulb depression The differencebetween the temperatures recorded by theDRY-BULB and WET-BULB THERMOMETERS ofa PSYCHROMETER.

wet-bulb temperature The tempera-ture at which pure water evaporates into asample of air, dry adiabatically, and at con-stant pressure, in order to saturate the air.

wet-bulb thermometer A THERMOME-TER, the bulb of which is kept moist by acovering of muslin wetted with distilledwater. It is used in conjunction with a DRY-BULB THERMOMETER in a PSYCHROMETER tomeasure the relative humidity of the air.The wet-bulb temperature will be lowerthan the dry-bulb due to evaporative cool-ing; the drier the air, the lower the wet-bulb temperature.

wet equatorial climate The climate ofthe region within about 12° latitude of theEquator, and within the influence of the In-tertropical Convergence Zone (ITCZ), inwhich temperatures are consistently high(30°C; 86°F) and abundant annual precip-itation (150–1000 cm) falls. Convergencein weak equatorial lows generates convec-tional rainfall and thunderstorms, often inlate afternoon or early evening. Humidityis high and cloud cover is frequently heavy.Some variations in precipitation occur as-sociated with movements in the location ofthe ITCZ. In the KÖPPEN CLIMATE CLASSIFI-CATION it is designated as Af.

wet microburst See microburst.

wet snow A form of SNOW that occurswhen temperatures are sufficiently highenough to aggregate SNOW CRYSTALS intolarge SNOWFLAKES by the process of regela-tion. During regelation, melted ice crystalsrefreeze under the pressure of the snowpack. Characteristic of more maritimeareas in the mid-latitude belt, wet snow hasa density of about 0.3 g/cm3. Unlike DRY

SNOW, wet snow does not form a very goodmaterial for skiing.

WGNE See Working Group on Numer-ical Experimentation.

whirling psychrometer See sling psy-chrometer.

whirlwind A rapidly revolving columnof air, often with a slightly inclined axis,that can sometimes extend to a consider-able height into the atmosphere and has atits center a mesoscale low-pressure area.Whirlwinds are common in desert areas(e.g. the DUST DEVIL), the phenomenonbeing a product of intense solar heating ofthe ground surface during the day. A whirl-wind occurring over water is known as aWATERSPOUT.

white dew See dew.

white frost See hoar frost.

white-out An optical phenomenon inthe form of a uniform white glow that oc-curs typically in polar regions, and is pro-duced by reflections between a uniformcloud and snow cover. The snow-coveredsurface appears to merge into the sky, andthe horizon and images become blurred orare not visible. This can lead to disorienta-tion and loss of balance on the part of theobserver.

white rainbow See fogbow.

Wien’s law /veenz/ The radiation lawstating that the wavelength of radiationemitted by a body is inversely proportionalto its thermodynamic (absolute) tempera-ture; i.e. hotter objects radiate at shorterwavelengths. It is named for the Germanphysicist Wilhelm Wien (1864–1928).

willy-nilly (willy-willy) In Western Aus-tralia, the local name for a severe TROPICAL

CYCLONE; it is also now frequently used fora DUST DEVIL.

wind The horizontal movement of air atany height, ranging from points close toground level (SURFACE WIND) to levels in theupper atmosphere. A wind is defined interms of its direction and speed. It is always

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named for the direction from which itblows (e.g. a northerly wind blows fromthe north). Wind speed is expressed, by in-ternational convention, in KNOTS, but inthe US miles per hour (mph) are also com-monly quoted; kilometers per hour(km h–1) and meters per second (m s–1) arealso used. Wind speed is often classified bymeans of the BEAUFORT SCALE. Actual windvelocities are measured with an ANEMOME-TER; wind direction is obtained with a WIND

VANE.Fundamentally, the surface wind is a re-

sult of the difference in pressure, the PRES-SURE GRADIENT, between two points on ahorizontal plane, which usually arisesthrough unequal heating of the Earth’s sur-face creating differences of temperatureand thus density. The rotation of the Earthalso has important implications in deflect-ing wind. The global circulation pattern ofwinds is known as the GENERAL CIRCULA-TION of the atmosphere. See also anabaticwind; local wind; katabatic wind; thermalwind.

windbreak (shelterbelt) A physical bar-rier (e.g. row of tall trees) erected to protectcrops or buildings from the force of thewind. Such windbreaks are frequently seenin southern France to provide protectionfrom the MISTRAL. Shelterbelts may also beconstructed to reduce the risk of soil ero-sion, and to provide protection againstdust storms in summer and blizzards inwinter. A windbreak can reduce windspeeds downwind for distances up to20–30 times the height of the obstruction.

wind chill The effect of wind in the lossof heat from the human body in combina-tion with temperature; an increase in windspeed will carry heat away at a faster ratethan on a still day. Several methods areused to calculate wind chill. The wind chillfactor is a measure of the rate of heat lossproduced by the combined effects of windand low temperature, and is expressed inwatts per square meter (W m–2). The wind-chill temperature is the effective tempera-ture experienced by exposed skin whenwind speed is combined with the actualambient air temperature. A wind chill

index, for example, that produced by theUS National Weather Service (NWS), isused to derive the wind chill temperaturefrom the recorded temperature and windspeed.

wind direction The angle (defined asclockwise from true north) from which theair blows. The angles are defined, for ex-ample, as 90° for an east wind (i.e. fromthe east), 180° for a south wind (orsoutherly), 225° for a southwesterly, and360° for a north wind. A wind direction of000 is allotted to calm conditions in theSYNOPTIC CODE. It is sensed by the air flow-ing over an aerodynamically formed flatplate (a wind vane) designed to alwayspoint into the air flow, at a standard heightof 10 m (33 ft) above the surface.

wind drift current (drift current) Anocean CURRENT that is induced by windstresses and in which only the Coriolis andfrictional forces are significant.

wind field The spatial distribution ofwind on a horizontal plane, this eitherbeing at the Earth’s surface (mean sea levelpressure) or at a standard pressure level(e.g. 850 hPa, 700 hPa, 500 hPa). The lo-cations of high and low pressure can be in-ferred from the wind field. Such fields maybe constructed for varying spatial scales,ranging from that of a thunderstorm to thehemispherical or even global scale.

window See atmospheric window.

wind profiler See profiler.

wind rose A diagram that portrays in-formation about the wind at a particularlocation and for a given period of time, forexample showing the relation of wind di-rection to other meteorological variables ata given location, or showing the relativefrequencies of wind direction and speed.

wind shadow A region sheltered by anobstruction from the force of the PREVAIL-ING WIND. See leeward.

wind shear The rate of change of wind-

windbreak

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flow parameters with height in the atmos-phere, these two variables being wind di-rection (DIRECTIONAL SHEAR) and windspeed (SPEED SHEAR). Wind shear is also oc-casionally used in the context of WIND

FIELDS in a horizontal plane.

wind speed The speed of the wind is ex-pressed most usually in knots (nauticalmiles an hour) and, more rarely, in metersper second, in the SYNOPTIC CODE. In sur-face observations it is measured by sensingthe rate at which a three-cup ANEMOMETER

rotates about a vertical axis at a standardheight of 10 m (33 ft) above the surface.The WIND DIRECTION is similarly sensed atthis height.

windthrow /wind-throh/ The uprootingand blowing over of trees by strong winds,especially at gale or hurricane force. Theopening up of stands of trees by windthrowcan result in changes to the microclimate offorested or wooded areas.

wind vane A device used to measureand record the direction from which thewind is blowing. It consists of a horizontalarm mounted at its center of gravity. The

part of the arm with the greatest surfacearea (usually the fin) provides greater resis-tance to the wind causing the forward partof the arm to point into the wind. A windvane is frequently mounted in conjunctionwith an ANEMOMETER and is used to orienta PROPELLER ANEMOMETER into the wind.

windward The upwind side or side di-rectly influenced by the wind, for examplethe side of a mountain range that is ex-posed to the PREVAILING WIND. It is the op-posite to the LEEWARD or lee side.

winter The SEASON of the year usuallydesignated as December to February in themiddle-latitude northern hemisphere andJune to August in the southern hemisphere,characterized by temperatures close to orbelow freezing and dormant growth inperennial plants. Day length is short andthere is often a lack of sunshine. The harsh-ness of winters in the continental interiorsof Europe and North America is indicatedby the many days of temperatures below0°C.

WMO See World Meteorological Orga-nization.

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WMO

variableand calm

9.0%

0–3 knots

4–10 knots

11–21 knots22–23 knots

% of total wind flow

0 5 10 15

N

S

Wind rose

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WOCE See World Ocean CirculationExperiment.

Working Group on Numerical Exper-imentation (WGNE) A group, jointlysponsored by the World Climate ResearchProgramme (WCRP) and the World Mete-orological Organization (WMO), that de-velops atmospheric circulation models forstudies of climate and numerical weatherpredictions.

World Climate Research Program(WCRP) A program established in 1980,jointly sponsored by the World Meteoro-logical Organization (WMO), the Interna-tional Council for Scientific Unions, andthe Intergovernmental OceanographicCommission. The program aims to developa fundamental scientific understanding ofthe climate system and processes, to ex-plore the extent climate can be predicted,and to examine the extent of anthro-pogenic influence. There are five core pro-jects: the CLIMATE AND CRYOSPHERE (CliC)project, CLIMATE VARIABILITY AND PRE-DICTABILITY PROGRAM (CLIVAR), GLOBAL

ENERGY AND WATER CYCLE EXPERIMENT

(GEWEX), STRATOSPHERIC PROCESSES AND

THEIR ROLE IN CLIMATE (SPARC), and SUR-FACE OCEAN–LOWER ATMOSPHERE STUDY

(SOLAS).

World Meteorological Center (WMC)One of the three main centers (Melbourne,Moscow, and Washington) within theWORLD WEATHER WATCH (WWW) systemfor the dissemination of meteorological in-formation on the GLOBAL DATA-PROCESSING

SYSTEM (GDPS) and GLOBAL TELECOMMUNI-CATIONS SYSTEM (GTS).

World Meteorological Organization(WMO) The meteorological agency ofthe UN, based in Geneva, Switzerland. Itcommenced operation in 1951, represent-ing 185 member states as the authoritativescientific center covering the state and be-havior of the Earth’s atmosphere, itsweather, and its climate. It plays a supervi-

sory role for all the member states’ weatherservices in terms of proper observationalpractice, and to ensure the effective opera-tion of the critically important globalweather observation network. One of itsmain aims is the establishment of networksof stations for making meteorological, hy-drological, and other observations; in addi-tion it seeks to promote rapid exchange ofmeteorological information and the stan-dardization of meteorological observa-tions. The WMO operates a number ofprograms, including the HYDROLOGY AND

WATER RESOURCES PROGRAM, the TROPICAL

CYCLONE PROGRAM, the WORLD CLIMATE RE-SEARCH PROGRAM, and WORLD WEATHER

WATCH.

World Ocean Circulation Experiment(WOCE) A former core project of theWORLD CLIMATE RESEARCH PROGRAM

(WCRP), established in 1990, that aimedto predict changes in the worlds oceans,such as circulation, volume, and heat stor-age, that would result from changes in at-mospheric climate and net radiation.WOCE came to an end in 2002.

World Weather Watch (WWW) Oneof the WORLD METEOROLOGICAL ORGANIZA-TION’s (WMO’s) major and fundamentalscientific and technical programs. Its primeaim is to ensure that its member states re-ceive up-to-the-minute weather observa-tions through the efficient organization ofthose member states’ observation systems.Additionally, the WWW ensures that datais made available through the proper main-tenance of telecommunications links fromfour POLAR-ORBITING and five GEOSTATION-ARY SATELLITES, about 7000 ships, 10,000land stations, and 300 moored and driftingbuoys that house automatic weather sta-tions. See Global Data-Processing System;Global Observation System; GlobalTelecommunications system.

WWW See World Weather Watch.

WOCE

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Z See Zulu Time.

Zaïre Air Boundary /zÿ-eer/ See CongoAir Boundary.

zenith 1. That point in the sky verticallyabove a location on the surface of theEarth, especially with respect to an ob-server.2. In general terms, any area of the skymore-or-less directly overhead an observer.

zephyr A name used in the Mediter-ranean region for a gentle BREEZE that usu-ally blows from the west. It occurs mostoften around the time of the summer sol-stice in the northern hemisphere. In Greekmythology, zephyrus was the name givento the west wind.

zodiacal light /zŏ-dÿ-ă-kăl/ A diffusezone of luminous light above the horizonthat is sometimes visible on the ecliptic be-fore sunrise or after sunset. It is producedas a result of sunlight being scattered bydust particles in the solar system.

zonal flow /zoh-năl/ The west-to-east(positive value) component of the wind di-rection and speed or the east-to-west (neg-ative value) component. Zonal essentiallymeans parallel to the lines of latitude.Compare meridional circulation.

zonal index A measure of the strengthof the ZONAL WINDS, usually over a large re-gion for a specific time or short period. Anindex might be the barometric pressure dif-ference between 40° and 60°N over thenortheast Atlantic. A large difference im-plies a high zonal index, with strong west-erlies or easterlies, for example.

zonal mean An average value that isproduced by considering individual stationvalues of a particular property (e.g. Julymean temperature at the Earth’s surface)that are located within a latitude strip, suchas 40° to 50°N. The zonal mean of suchJuly average temperatures is the one valueproduced by averaging all the values forthat strip. Zonal means cannot, therefore,depict any variation of an atmosphericproperty in the east–west direction.

zonal winds The component of the ob-served wind that blows parallel to lines oflatitude, either as a westerly or an easterlycurrent.

zonda /zon-dă/ 1. A hot and dry FÖHN-type wind experienced in western Ar-gentina. This westerly wind occurs mostfrequently in spring and can carry consid-erable quantities of dust. Its properties area result of the warming of the airflow as itdescends the leeward side of the Andeanmountain range.2. A hot and humid southerly wind thatcarries tropical air from Brazil southwardinto Argentina and Uruguay. It results invery uncomfortable conditions and is simi-lar to the Australian BRICKFIELDER in char-acter.

Zulu Time (Z) The time zone centeredon the 0° meridian of longitude at Green-wich, England, which was allocated the let-ter ‘Z’ when used in a military and aviationcontext; it is now equivalent to COORDI-NATED UNIVERSAL TIME (UTC), but was for-merly equivalent to GREENWICH MEAN TIME

(GMT).

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c. 525 BC The Greek philosopher Anaximenes of Miletus (d. c. 528 BC) putsforward the idea that winds, cloud, rain, and hail are formed bythickening of the primary substance, air.

c. 135 BC The Chinese writer Han Ying first refers to the hexagonal shape ofsnowflakes in Moral discourses illustrating the Han text of the“Book of Songs”.

c. 10 BC The Greek geographer Strabo of Amaseia (now in Turkey) dividesthe Earth into frigid, temperate, and torrid zones.

1304 Theodoric of Friebourg (now in Germany) writes De iride (On theRainbow) describing his experiments with bulbs filled with water.

1591 The English mathematician Thomas Harriot (1560–1621) notes thatsnowflakes have six sides.

c. 1592 The Italian scientist Galileo (1564–1642) invents a simple form ofthermometer (the ‘thermoscope’) based on the expansion of air.

1611 The German astronomer Johannes Kepler (1571–1630) publishes ANew Year’s Gift, or On the Six-Cornered Snowflake.

1621 The Dutch physicist Willebrord Snell (1591–1626) discovers his lawof refraction of light (which he did not publish).

1637 The French philosopher René Descartes publishes Discours de laméthode, which includes an appendix Les Météores containing hisideas about the rainbow and the formation of clouds.

The refraction law discovered by Snell is independently published byDescartes in another appendix, La Dioptrique.

1641 Ferdinand II of Tuscany constructs a thermometer containing liquid.

Chronology

Major Events in the Development of Meteorology,

Climatology, and Atmospheric Physics

Appendix I

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246

1643 The Italian physicist Evangelista Torricelli (1608–1647) constructsthe first barometer by inverting a tube, closed at one end and con-taining mercury, in a dish of mercury.

1648 The French mathematician and philosopher Blaise Pascal(1623–1662) investigates Torricelli’s barometer. He arranges for hisbrother-in-law to take an instrument to different heights up a moun-tain, thereby demonstrating that the height of the column decreasedwith altitude.

1686 The English astronomer and physicist Edmond Halley (1656–1742)formulates the law relating height to pressure.

Halley gives a partial explanation of the trade winds, suggesting thathot equatorial air rises and is replaced by cooler air moving in fromthe tropics.

1687 The French physician Guillaume Amontons (1663–1705) invents ahygrometer.

1695 Amontons constructs an improved barometer.

1702 Amontons constructs a constant-volume air thermometer.

1709 The German physicist Gabriel Daniel Fahrenheit (1686–1736) con-structs a thermometer with alcohol as the working fluid.

1714 Fahrenheit uses mercury as the working fluid in thermometers forthe first time and defines a temperature scale based on the freezingpoint of ice and salt (0°) and the temperature of the human body(96°).

1730 The French entomologist and physicist René Réamur (1683–1757)introduces a thermometer using a water/alcohol mixture and devisesa temperature scale based on the freezing point of water (0°) and itsboiling point (set at 80°).

1735 The German explorer Johann Gmelin discovers the permafrost inSiberia.

The English meteorologist George Hadley (1685–1768) explains thedirection of the trade winds (by reference to the Earth’s rotation) inhis paper Concerning the Cause of the General Trade Winds. Thisintroduces the idea of the Hadley cell.

1742 The Swedish astronomer Anders Celsius (1701–1744) devises a tem-perature scale in which he sets the freezing point of water at 100°and the boiling point of water at 0°.

1743 Jean Pierre Christin inverts the fixed points on Celsius’ scale, to pro-duce the scale used today.

1747 The American statesman and inventor Benjamin Franklin

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Appendix I

(1706–1790) begins a series of experiments on electricity. His dis-covery that charge could be drawn by a pointed metal conductor ledto the use of the lightning rod.

1749 Franklin fixes a lightning rod to his house in Philadelphia.

1752 Franklin performs a now famous experiment in which he flies a kitein a thunderstorm to demonstrate that lightning is a form of electric-ity.

1793 The English chemist John Dalton (1766–1844) publishes Meteoro-logical Observations and Essays – an account of his weather obser-vations, which he started in 1787 (and continued for the rest of hislife).

1803 The British pharmacist and amateur meteorologist Luke Howard(1722–1846) proposes the first classification of cloud types (still thebasis of the present classification).

1804 The French physicist Jean Baptiste Biot (1774–1862) and the Frenchchemist Joseph-Louis Gay-Lussac (1778–1850) make a balloon as-cent to a height of three miles to investigate whether the composi-tion of the atmosphere or the Earth’s magnetic field changes at thisheight.

1805 The British admiral Sir Francis Beaufort (1774–1857) proposes ascale of wind speeds (now known as the Beaufort scale).

1820 The English chemist John Frederic Daniell (1790–1845) invents thedew-point hygrometer.

1821 The Swiss geologist Ignatz Venetz (b. 1788) proposes that glaciersformerly occurred throughout Europe.

1823 Daniell publishes Meteorological Essays and Observations, in whichhe puts forward theories of wind movement and the atmosphere.

1827 The French physicist Jean-Baptiste Fourier (1768–1830) suggeststhat human activity may have effects on the Earth’s climate.

1835 The French mathematician Gaspard Coriolis (1792–1843) publishesMémoire sur les équations du mouvement relatif des systèmes decorps (Memoir on the Equations of Relative Motion of Systems ofBodies), in which he describes what is now known as the Coriolis effect.

1837 The Swiss-born American naturalist Jean Louis Agassiz(1807–1873) first begins to use the term Eiszeit (Ice Age).

1840 Agassiz publishes his findings in Etudes des glaciers (Studies onGlaciers).

1854 P. Adie designs the Kew barometer.

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248

1857 The Dutch meteorologist Christopher Buys Ballot (1817–90) pro-poses a rule relating wind direction to the location of the pressurecenter (Buys Ballot’s law).

1869 The Scottish meteorologist Alexander Buchan (1829–1907) attemptsto identify recurring periods of similar weather (Buchan spells).

1853 James Coffin suggests that there are three distinct wind zones in thenorthern hemisphere.

1863 The British explorer and anthropologist Francis Galton(1822–1911) publishes Meteographica (Weather Mapping), whichintroduces the term ‘anticyclone’ and describes modern techniquesof weather mapping.

1871 The American meteorologist Cleveland Abbe (1838–1916) is ap-pointed chief meteorologist with the Weather Service (part of the USArmy).

1874 The American geologist Thomas Chowder Chamberlin (1843–1928)suggests that there were several Ice Ages separated by nonglacialepochs. He published his ideas in a chapter contributed to TheGreat Ice Age (1874–84) by James Geikie.

The International Meteorological Committee adopts the Beaufortscale for use in weather telegraphs

1888 Gustavus Hinrichs, an Iowa weather researcher, first uses the word‘derecho’ for a widespread type of windstorm.

1890s The British astronomer E. Walter Maunder (1851–1928) identifies aperiod of low solar activity (the Maunder minimum).

1890 The German geographer and climatologist Eduard Brückner(1862–1927) investigates the cycle of cold and damp weather alter-nating with warm and dry weather in northwestern Europe (theBrückner cycle).

1891 Luis Carranza, President of the Lima Geographical Society, con-tributes a small article to the Bulletin of the society in which he de-scribes a countercurrent flowing N to S from Paita to Pacasmayooccurring annually after Christmas. He reports that fishermen callthis El Niño.

The US Weather Bureau separates from the Army and becomes anindependent organization under the direction of Cleveland Abbe.

1896 The Swedish chemist Svante Arrhenius (1859–1927) points out thatincreased carbon dioxide in the atmosphere can cause the green-house effect. He suggests that past Ice Ages may have been causedby decreased amounts of carbon dioxide.

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1900 The Russian-born German climatologist Wladimir Peter Köppen(1846–1940) first publishes his classification of climatic types(which he modifies several times before 1936).

1902 The British physicist Oliver Heaviside (1850–1925) and the Ameri-can Arthur Edwin Kennelly (1861–1939) independently suggest theexistence of the ionosphere to explain transatlantic radio transmis-sions.

The French meteorologist Léon Philippe Teisserenc de Bort(1855–1913) identifies and names the troposphere and the strato-sphere as distinct layers of the atmosphere.

The Swedish physicist Vagn Walfrid Ekman (1874–1954) producesa mathematical model for the result of winds blowing over theocean (the Ekman spiral).

1904 The Norwegian meteorologist Jacob Aall Bonnevie Bjerknes(1897–1975) publishes one of the first scientific accounts of meteo-rology in his Weather Forecasting as a Problem in Mechanics.

1910 Jacob Bjerknes publishes Dynamical Meteorology andHydrography.

1920 The Serbian astronomer and mathematician Milutin Milankovitch(1879–1958) proposes a theory of long-term cyclic climate change(Milankovitch cycles).

1922 The United Nations Framework Convention on Climate Change(UNFCCC) is drawn up.

The British meteorologist William Napier Shaw introduces a type ofthermodynamic diagram (the tephigram).

1923 Sir Gilbert Walker, Director of Observations in India, notes “whenpressure is high in the Pacific Ocean it tends to be low in the IndianOcean from Africa to Australia”. He calls the effect the SouthernOscillation.

1925 The British physicist Edward Victor Appleton (1892–1965) discovers a layer in the ionosphere (now called the Appleton layer).

1927 G. Stüve introduces a type of thermodynamic diagram (the Stüve diagram).

1928 Universal Time is adopted as the name for Greenwich Mean Timewhen used for scientific purposes.

1931 The American climatologist Charles Warren Thornthwaite(1889–1963) devises a precipitation-efficiency index.

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250

1932 The Swedish-born American meteorologist Carl-Gustaf ArvidRossby (1898–1957) introduces diagrams for indicating the proper-ties of air masses (now known as Rossby diagrams).

1935 The Swedish meteorologist Tor Harold Percival Bergeron(1891–1977) publishes a paper On the Physics of Clouds and Pre-cipitation, in which he argues that ice crystals in clouds are involvedin the mechanism of rain formation.

1939 Work by Walker Findeisen supports Bergeron’s theory (later knownas the Bergeron–Findeisen process).

1946 The American physicist Vincent Joseph Shaefer (1906–1993) in-duces rainfall by seeding clouds with pellets of solid carbon dioxide.

1947 The American chemist Willard Frank Libby (1908–1980) invents radiocarbon dating, which is used in paleoclimatology.

1948 The American climatologist Charles Warren Thornthwaite(1899–1963) introduces the concept of moisture index.

1950 The German climatologist Hermann Flohn (1912–97) proposes aclimate classification based on global wind belts and precipitation(the Flohn classification).

A team led by the Hungarian-born American mathematician JohnVon Neumann (1903–1957) produces the first computer-generatedweather forecasts using the ENIAC computer.

1951 The World Meteorological Organization (WMO) commences opera-tion.

The American geographer and climatologist Arthur N. Strahler(1918–2002) introduces a climate classification based on air masses(the Strahler classification).

1956 The World Meteorological Organization publishes the InternationalCloud Atlas.

The British government introduces an Act of Parliament to limitsmoke emissions with the aim of reducing smog.

1957 The American oceanographer Roger Revelle (1909–91) instigatesthe start of measurements on the amount of carbon dioxide in theatmosphere.

1958 The American physicist James Van Allen (1914– ) discovers twobands of ionized particles in the atmosphere as a result of satellitedata (the Van Allen belts).

1959 The temperature–humidity index is first introduced by the USWeather Bureau.

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1960 The National Oceanic and Atmospheric Administration (NOAA)launches the weather satellite TIROS-1.

1961 The American meteorologist Edward Norton Lorenz (1917– ) no-tices that computer predictions of weather are highly sensitive tosmall differences in initial conditions. This eventually leads to thedevelopment of chaos theory.

1963 Lorenz publishes his ideas in a paper, Deterministic NonperiodicFlow.

The British climatologist Hubert Horace Lamb (1913–97) intro-duces the dust veil index.

1965 The US government introduces the first Clean Air Act to set nationalair pollution standards.

1966 The American geographer Werner H. Terjung proposes a climateclassification based on comfort.

1969 Jacob Bjerknes in California University, Los Angeles, explains the ElNiño Southern Oscillation (ENSO).

1970s The American engineer Herbert Saffir (1917– ) and the meteorolo-gist Robert Simpson (1912– ) devise a scale for measuring hurri-cane strength (the Saffir–Simpson scale).

1970 The second US Clean Air Act.

1971 The Japanese-born American meteorologist Tetsuya Fujita(1920–98) with Allen Pearson introduces a scale for measuring tor-nado intensities (the Fujita or Fujita–Pearson scale).

1972 Earth Resources Technology Satellite 1 (later called Landsat-1) islaunched.

1974 The Global Atmospheric Research Program (GARP) conducts theGARP Atlantic Tropical experiment (GATE) to further understand-ing of the tropical atmosphere.

The American chemist Sherwood Rowland (1927– ) and MarcoMolina suggest that CFCs (chlorofluorocarbons) could cause long-term depletion of the ozone layer.

1975 The first GOES (Geostationary Operational Environmental Satellite)is launched.

The European Space Agency is formed.

1976 The US National Academy of Sciences publishes a report supportingRowland’s theory concerning CFCs and ozone depletion.

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Appendix I

252

1977 The first Meteosat weather satellite is launched by the EuropeanSpace Agency.

1978 The use of CFCs is banned in the US.

1979 The British scientist James Ephraim Lovelock (1919– ) publishesGaia, expounding the idea that the Earth is a self-regulating ecosys-tem (the so-called Gaia hypothesis).

1980 The World Climate Research Program (WCRP) is established.

1984 The British Antarctic Survey obtains conclusive evidence of a hole inthe ozone layer.

1985 The Vienna Convention for the Protection of the Ozone Layer takesplace.

The Tropical Ocean-Global Atmosphere Program (TOGA) isstarted.

1987 The Montreal Protocol – an international agreement to regulate theuse of CFCs – is drawn up.

The Joint Global Ocean Flux Study (JGOFS) is initiated to study thefluxes of carbon in the oceans.

1988 The Global Energy and Water Cycle Experiment (GEWEX) islaunched to observe the hydrologic cycle.

The German scientist Helmut Heinrich obtains evidence for themarked discharge of large numbers of icebergs from the easternNorth American ice sheet during glacial periods (Heinrich events).

1989 The Global Atmospheric Watch (GAW) is established to collect dataon the atmosphere’s physical characteristics.

Projects start to analyze the Greenland ice sheet. The Greenland IceCore Project (GRIP) is run by the European Science Foundation.The Greenland Ice Sheet Project 2 (GISP 2) is a US project.

1995 Nations meeting in Berlin agree the Berlin mandate to take actionon climate change beyond the year 2000.

1997 The Kyoto Protocol is agreed to achieve reductions in greenhouse-gas emissions.

1999 Landsat-7 is launched.

2001 The US refuses to ratify the Kyoto Protocol.

2004 Russia ratifies the Kyoto Protocol.

2005 The Kyoto Protocol comes into force.

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253

Con

vers

ion

Tab

les

LE

NG

TH

mcm

inft

yd

1 m

eter

110

039

.370

13.

280

841.

093

611

cent

imet

er0.

011

0.39

3 70

10.

032

8084

0.01

0 93

611

inch

0.02

542.

541

0.08

3 33

330.

027

7778

1 fo

ot0.

3048

30.4

812

10.

333

333

1 ya

rd0.

9144

91.4

436

31

km

mile

n

mile

1 ki

lom

eter

10.

621

371

0.53

9 95

71

mile

1.60

9 34

10.

868

976

1 na

utic

al m

ile1.

852

001.

150

781

1 lig

ht y

ear

= 9.

460

70 ×

1015

met

ers

= 5.

878

48 ×

1012

mile

s1

astr

onom

ical

uni

t =

1.49

6 ×

1011

met

ers

1 pa

rsec

= 3

.085

7 ×

1016

met

ers

= 3.

2616

ligh

t ye

ars

App

endi

x II

FREEDOM PALESTINE


254

AR

EA

m2

cm2

in2

ft2

1 sq

uare

met

er1

104

1550

10.7

639

1 sq

uare

cen

tim

eter

10–4

10.

155

1.07

63 ×

10–3

1 sq

uare

inch

6.45

16 ×

10–4

6.45

161

6.94

44 ×

10–3

1 sq

uare

foo

t9.

2903

× 1

0–292

9.03

144

1

m2

km2

yd2

mi2

acre

1 sq

uare

met

er1

10–6

1.19

5 99

3.86

0 19

× 1

0–72.

471

05 ×

10–4

1 sq

uare

kilo

met

er10

61

1.19

5 99

× 1

060.

386

019

247.

105

1 sq

uare

yar

d0.

836

127

8.36

1 27

× 1

0–71

3.22

8 31

× 1

0–72.

066

12 ×

10–4

1 sq

uare

mile

2.58

9 99

× 1

062.

589

993.

0976

× 1

061

640

1 ac

re4.

046

86 ×

103

4.04

6 86

× 1

0–348

401.

5625

× 1

0–31

1 ar

e =

100

squa

re m

eter

s1

hect

are

= 10

000

squ

are

met

ers

= 2.

471

05 a

cres

Appendix IIFREEDOM PALESTINE


255

Appendix II

VO

LU

ME

AN

D C

APA

CIT

Y

m3

cm3

(cc)

in3

gal

1 cu

bic

met

er1

106

6.10

24 ×

104

264.

169

1 cu

bic

cent

imet

er10

–61

0.06

1 02

42.

6416

× 1

0–4

1 cu

bic

inch

1.63

87 ×

10–5

16.3

871

14.

3289

× 1

0–3

1 ga

llon

(US)

3.78

54 ×

10–3

3785

.423

1.01

1

1 ga

llon

(US)

= 0

.832

68

gallo

n (U

K)

1 lit

er =

1 c

ubic

dec

imet

er =

100

0 cu

bic

cent

imet

ers

VE

LO

CIT

Ym

s–1

km h

–1m

ile h

–1ft

s–1

1 m

eter

per

sec

ond

13.

62.

236

943.

280

841

kilo

met

er p

er h

our

0.27

7 77

81

0.62

1 37

10.

911

346

1 m

ile p

er h

our

0.44

7 04

1.60

9 34

41

1.46

6 67

1 fo

ot p

er s

econ

d0.

3048

1.09

7 28

0.68

1 81

71

1 kn

ot =

1 n

auti

cal m

ile p

er h

our

= 0.

514

444

met

er p

er s

econ

d

FREEDOM PALESTINE


Appendix II

256

MA

SS

kg

g lb

lo

ng t

on

1 ki

logr

am1

1000

2.20

4 62

9.84

2 07

× 1

0–4

1 gr

am10

–31

2.20

4 62

× 1

0–39.

842

07 ×

10–7

1 po

und

0.45

3 59

245

3.59

21

4.46

4 29

× 1

0–4

1 lo

ng t

on10

16.0

471.

016

047

× 10

622

401

DE

NSI

TY

kg m

3g

cm3

lb f

t3lb

in3

1 ki

logr

am p

er c

ubic

met

er1

10–3

0.06

2 42

83.

612

73 ×

10–5

1 gr

am p

er c

ubic

cen

tim

eter

1000

162

.428

3.61

2 73

× 1

0–2

1 po

und

per

cubi

c fo

ot16

.018

50.

016

0185

15.

787

04 ×

10–4

1 po

und

per

cubi

c in

ch2.

767

99 ×

104

27.6

799

1728

1

1 lb

/gal

(U

S) =

0.0

83 0

82 k

g/dm

3

FOR

CE

N

kg

dyne

po

unda

l lb

1 ne

wto

n1

0.10

1 97

210

57.

233

000.

224

809

1 ki

logr

am f

orce

9.80

6 65

19.

806

65 ×

105

70.9

316

2.20

4 62

1 dy

ne10

–51.

019

72 ×

10–6

17.

233

00 ×

10–5

2.24

8 09

× 1

0–6

1 po

unda

l0.

138

255

1.40

9 81

× 1

0–21.

382

55 ×

104

10.

031

081

1 po

und

forc

e4.

448

220.

453

592

4.44

8 23

× 1

0532

.174

1

FREEDOM PALESTINE


257

Appendix II

PRE

SSU

RE

Pa(

N/m

2 )hP

a(m

b)in

Hg

atm

os

1 pa

scal

(ne

wto

n pe

r sq

. met

er)

110

03.

3865

× 1

039.

8692

× 1

0–6

1 he

ctop

asca

l10

21

3.38

65 ×

105

9.86

92 ×

10–4

1 in

Hg

2.95

29 ×

10–4

2.95

29 ×

10–2

13.

3422

× 1

0–2

1 at

mos

pher

e1.

013

25 ×

105

1.01

3 25

× 1

0329

.92

1

1 pa

scal

= 1

new

ton

per

sq. m

eter

1 pa

scal

= 1

0 dy

nes

per

sq. c

enti

met

er1

hect

opas

cal =

1 m

illib

ar1

bar

= 10

5pa

scal

s =

0.98

69 a

tmos

pher

e1

torr

= 1

33.3

22 p

asca

ls =

1/7

60 a

tmos

pher

e1

atm

osph

ere

= 76

0 m

mH

g =

33.9

0 ft

wat

er (

at 0

°C)

WO

RK

AN

D E

NE

RG

Y

J ca

l ITkW

hr

btu I

T

1 jo

ule

10.

238

846

2.77

7 78

× 1

0–79.

478

13 ×

10–4

1 ca

lori

e (I

T)

4.18

681

1.16

3 00

× 1

0–63.

968

31 ×

10–3

1 ki

low

att

hour

3.6

× 10

68.

598

45 ×

105

134

12.1

41

Bri

tish

The

rmal

Uni

t (I

T)

1055

.06

251.

997

2.93

0 71

× 1

0–41

1 jo

ule

= 1

new

ton

met

er =

1 w

att

seco

nd =

107

ergs

= 0

.737

561

ft

lb1

elec

tron

volt

= 1

.602

10

× 10

–19

joul

e

FREEDOM PALESTINE


258

°C °F °Rankine °Reamur kelvins

Boiling pt of water 100 212 672 80 373

80 176 636 64 353

60 140 600 40 333

40 104 564 32 313

20 68 528 16 293

Freezing pt of water 0 32 492 0 273

–20 –4 454 –16 253

–40 –40 420 –32 233

Absolute zero –273 –460 0 –218 0

Temperature Conversions

Appendix III

FREEDOM PALESTINE


259

Weather Sites and Services

Accuweather, Inc. www.accuweather.com

BBC Weather Centre, UK www.bbc.co.uk/weather

Meteorological Service of Canada www.weatheroffice.ec.gc.ca

National Centers for Environmental Prediction www.ncep.noaa.gov

National Weather Service (NWS) www.nws.noaa.gov

Unisys Weather www.weather.unisys.com

USA Today Weather www.usatoday.com/weather

The Washington Post Weather www.washingtonpost.com/wp-srv/weather

The Weather Channel (US) www.weather.com

Organizations

Bureau of Meteorology, Australia www.bom.gov.au

Climate Prediction Center www.cpc.ncep.noaa

Environment Canada www.ec.gc.ca/envhome

Hadley Centre for climate prediction and www.met-office.gov.uk/research/research, UK hadleycentre/index

Met Office, UK www.met-office.gov.uk

National Oceanic and Atmospheric www.noaa.govAdministration (NOAA)

US Environmental Protection Agency www.epa.gov

World Meteorological Organization www.wmo.ch

Education, data, and reference sources

britannica.com www.britannica.com

Climatic Research Unit, University of www.cru.uea.ac.ukEast Anglia, Norwich, UK

Web Pages

Appendix IV

FREEDOM PALESTINE


Appendix IV

260

Encyclopedia of the Atmospheric www.doc.mmu.ac.uk/aric/eae/indexEnvironment

Mount Washington Observatory www.mountwashington.org

NASA's Earth Observatory www.earthobservatory.nasa.gov

University Corporation for Atmospheric www.edu/ucar/indexResearch

WW2010 (weather world 2010 project) www.2010.atmos.uiuc.edu/(Gh)/homeUniversity of Illinois

Meteorological Societies

American Meteorological Society, Boston www.ametsoc.org/AMS

Royal Meteorological Society, UK www.rmets.org

Canadian Meteorological and www.cmos.caOceanographic Society

El Niño

El Niño site, NOAA www.pmel.noaa.gov/toga-tao/el-nino-story

Satellites

European Space Agency (ESA) www.esa.int

EUMETSAT www.eumetsat.de

NASA www.nasa.gov

NESDIS www.nesdis.noaa.gov

Severe weather

Storm Prediction Center, NOAA www.spc.noaa.gov

National Severe Storms Laboratory, www.nssl.noaa.govNOAA

Climate change and global warming

Global warming site, US Environmental www.epa.gov/globalwarmingProtection Agency

Climate change site, European www.europa.eu.int/comm/environment/ Commission climate/home_en

US Global Change Research Information www.gcrio.orgOffice

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261

Ahrens, C. Donald. Essentials of Meteorology: an invitation to the atmosphere. 3rd ed.Pacific Grove, Calif.: Brooks/Cole, 2000.

——— Meteorology Today: an introduction to weather, climate and the environment.7th ed. Pacific Grove, Calif.: Brooks/Cole, 2003.

Allaby, Michael. Encyclopedia of Weather and Climate. New York: Facts On File,2007.

Bader et al. Images in Weather Forecasting: a practical guide for interpreting satelliteand radar imagery. Cambridge, U.K.: Cambridge University Press, 1996.

Barry, R. G. & Chorley, R.J. Atmosphere, Weather & Climate. 8th ed. London:Routledge, 2003.

Greer, I. W. (ed). Glossary of Weather and Climate. Boston, Mass.: AmericanMeteorological Society, 1996.

Houghton, J. T. (ed). Climate Change 1995: the science of climate change. Cambridge,U.K.: Cambridge University Press, 1996.

Huschke, R. E. (ed). Glossary of Meteorology. 2nd ed. Boston, Mass.: AmericanMeteorological Society, 2001.

Intergovernmental Panel on Climatic Change. IPCC First Assessment Report, 1990.IPCC Second Assessment Report: Climate Change, 1995. IPCC Third AssessmentReport: Climate Change, 2001.

Lewis, R. P. W. (ed). Meteorological Glossary. 6th ed. London: Met Office, HerMajesty’s Stationery Office, 1991.

Lye, Keith. Equatorial Climates. Austin, Tex.: Raintree Steck-Vaughn Publishers, 1999.

———World’s Climates. Austin, Tex.: Raintree Steck-Vaughn Publishers, 1999.

Reynolds, Ross. Philip’s Guide to Weather. London: George Philip Ltd, 2000.

Schneider, S. H. (ed). Encyclopedia of Climate and Weather. New York: OxfordUniversity Press, 1996.

Silverstein, Alvin, & Silverstein, Virginia, & Silverstein Nunn, Laura. Weather andClimate. Brookfield, Conn.: Twenty-First Century Books Inc., 1998.

Bibliography

FREEDOM PALESTINE


Stevens, William K. The Change in the Weather. New York: Dell Publishing Co. Inc.,1999.

Stull, Roland B. Meteorology Today for Scientists and Engineers. 2nd ed. Pacific Grove,Calif.: Brooks/Cole, 2000.

Wilson, Francis & Mansfield, F. The Weather. EDCP, 1997.

World Meteorological Organization. International Cloud Atlas. Vol. I. 1975. Manualon the observation of clouds and other meteors. Vol. II. 1987 (plates). Geneva:WMO.

Bibliography

262

FREEDOM PALESTINE


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