Climate & Introduction to Earth System Science Meteorology ...maths.ucd.ie/met/cess/LectureNotes/PDFs/MAPH10050-CH04.pdf · 3 Introduction to Meteorology & Climate HUMIDITY Humidity

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Introduction to Meteorology & Climate

Climate & Earth System Science

Peter LynchMeteorology & Climate Centre

School of Mathematical SciencesUniversity College Dublin


Introduction toMeteorology & Climate

MAPH 10050

Peter Lynch

Meteorology & Climate CentreSchool of Mathematical SciencesUniversity College Dublin


Chapter 04Chapter 04

WATER IN THE ATMOSPHEREWATER IN THE ATMOSPHERE


Lecture 07Lecture 07

MEASURING WATER VAPOUR. MEASURING WATER VAPOUR. PHASE CHANGES IN THE ATMOSPHEREPHASE CHANGES IN THE ATMOSPHERE


VIS


IR

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WV


MOISTUREMOISTURE

• Water vapour constitutes only a small fraction of the atmosphere.

• Varies from 0% to about 4%

• Water is probably the most important gas in the atmosphere for understanding atmospheric processes.

• The source of atmospheric water is evaporation.


Crumpled steel electrical transmission towers –Canada, January, 1998

IceStorm


Satellite image of clouds over North America, 9 January, 1998


Sequence of events leading to saturation of water vapour in air


OBSERVATIONS OF VAPOUR PRESSURE AS A FUNCTION OF TEMPERATURE

Six years of hourly observations

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HUMIDITYHUMIDITY

Humidity describes the amount of water Humidity describes the amount of water vapourvapour in the air.in the air.

Humidity is described quantitatively as Humidity is described quantitatively as vapourvapour pressurepressure, , absolute humidity, absolute humidity, mixing ratio mixing ratio andand relative humidityrelative humidity..

SaturationSaturation is achieved when the number of is achieved when the number of water water vapourvapour molecules leaving a water molecules leaving a water surface is equal to the number returning surface is equal to the number returning from the atmosphere to the water surface.from the atmosphere to the water surface.


HUMIDITYHUMIDITYSaturation Saturation vapourvapour pressurepressure is the pressure is the pressure exerted by the water exerted by the water vapourvapour at saturation.at saturation.

Absolute humidityAbsolute humidity is the mass of water per is the mass of water per unit volume . Units are usually grams per unit volume . Units are usually grams per cubic meter (g/mcubic meter (g/m³)³)..

Mixing ratioMixing ratio is the mass of water is the mass of water vapourvapour in in an unit mass of air. Usually in grams per an unit mass of air. Usually in grams per kilogram (g/kg).kilogram (g/kg).

Relative humidityRelative humidity is the actual amount of is the actual amount of water water vapourvapour in the air over the amount of in the air over the amount of water water vapourvapour required for saturation.required for saturation.

Introduction to Meteorology & Climate Introduction to Meteorology & Climate

Climatology of hourlytemperature and relative humidity


Condensation: ExampleWhen the temperature of the air around this web cooled to the

dew-point temperature, dew formed, making the web more visible


RELATIVE HUMIDITYRELATIVE HUMIDITY

Relative humidity changes as daily Relative humidity changes as daily temperature changes.temperature changes.

It changes from one It changes from one locationlocation to another.to another.It changes when air moves It changes when air moves verticallyvertically in in

the atmosphere. the atmosphere. There is a There is a daily variationdaily variation of temperature of temperature

and relative humidityand relative humidityHowever the water However the water vapourvapour content of the content of the

air can stay the same.air can stay the same.Dew pointDew point is the temperature at which is the temperature at which

water water vapourvapour will condense out of the will condense out of the atmosphere.atmosphere.

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4.44.420.53.53.516.216.2Dew point Dew point depression (°C)depression (°C)

15.615.6-0.5--13.513.5--26.226.2Dew point Temp (°C)Dew point Temp (°C)

23.4723.4723.472.882.882.882.88Sat. Sat. vapourvapour pressure pressure (mb)(mb)

17.6017.605.872.162.160.720.72VapourVapour Pressure Pressure (mb)(mb)

11.1511.153.671.351.350.450.45Mixing Ratio (g/kg)Mixing Ratio (g/kg)

75752575752525Relative HumidityRelative Humidity

202020--1010--1010Temperature (°C)Temperature (°C)


Heat index table


Lecture 08Lecture 08

FORMATION OF FOG

FORMATION OF CLOUDS

CLASSIFICATION OF CLOUDS


FOG FORMATIONFOG FORMATION

Fog is cloudFog is cloud with its base at or near with its base at or near the groundthe ground

Fogs result eitherFogs result eitherwhen when air is cooledair is cooled,,

ororby the by the addition of wateraddition of water vapourvapour to to cause saturationcause saturation



Fogs are composed of Fogs are composed of fine droplets of fine droplets of waterwater suspended in the air near the suspended in the air near the Earth's surface. Earth's surface. The presence of these droplets act to The presence of these droplets act to scatter the lightscatter the light and thus reduce the and thus reduce the visibility near the ground.visibility near the ground.The formation of a fog layer occurs The formation of a fog layer occurs when a when a moist air mass is cooledmoist air mass is cooled to its to its saturation point (dew point). saturation point (dew point).



This cooling can be the result of: This cooling can be the result of: radiativeradiative processesprocesses (radiation fog)(radiation fog)advectionadvection of warm air over cold of warm air over cold surfaces (advection fog)surfaces (advection fog)evaporationevaporation of precipitation of precipitation (precipitation or frontal fog)(precipitation or frontal fog)air being adiabatically cooled while air being adiabatically cooled while being being forced up a mountainforced up a mountain (upslope (upslope fog).fog).

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Another type of fog is the soAnother type of fog is the so--called called valley valley fogfog. This fog forms when air is . This fog forms when air is radiativelyradiativelycooled on the slopes of hills.cooled on the slopes of hills.The air becomes denser than its The air becomes denser than its surroundings and surroundings and sinks downsinks down the slope the slope

(this is called a (this is called a katabatickatabatic wind).wind).The result is a pool of cold air at the valley The result is a pool of cold air at the valley floor. floor. If the air is cold enough to reach its dew If the air is cold enough to reach its dew point, fog occurs.point, fog occurs.


RADIATION FOG


VALLEY FOG


UPSLOPE FOG


PRECIPITATION FOG


ADVECTION FOG

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Diurnal Variation of VisibilityDiurnal Variation of Visibility

A fog layer is reported A fog layer is reported whenever the whenever the horizontal visibility horizontal visibility at the surface is at the surface is less than 1 km.less than 1 km.

A typical evolution of A typical evolution of visibility during a visibility during a radiation fog episode radiation fog episode is shown here:is shown here:


Lifting Mechanisms that form CloudsLifting Mechanisms that form Clouds

Air raised to the Lifting Condensation Level (LCL) becomes saturated.

Orographic liftingFrontal liftingConvectionConvergence


Four mechanisms that cause air to ascend


““The Cloud” The Cloud” –– Percy Percy ByssheBysshe ShelleyShelley

I am the daughter of Earth and Water,I am the daughter of Earth and Water,And the nursling of the skyAnd the nursling of the skyI pass through the pores of the ocean and shores;I pass through the pores of the ocean and shores;I change but I cannot die.I change but I cannot die.For after the rain when with never a stainFor after the rain when with never a stainThe pavilion of Heaven is bare,The pavilion of Heaven is bare,And the winds and sunbeams with their convex gleamsAnd the winds and sunbeams with their convex gleamsBuild up the blue dome of air,Build up the blue dome of air,I silently laugh at my own cenotaph,I silently laugh at my own cenotaph,And out of the caverns of rain,And out of the caverns of rain,Like a child from the womb, like a ghost from the tomb,Like a child from the womb, like a ghost from the tomb,I arise and I arise and unbuildunbuild it again.it again.


CLASSIFICATION OF CLOUDS


Major cloud types arranged by altitude.

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LOW CLOUDS: Below 2 km

MEDIUM CLOUDS: Between 2 and 6 km

HIGH CLOUDS: Above 6 km

CLOUD CLASSIFICATION BASED ON HEIGHT


STRATIFORM CLOUDSOccur as horizontal layers with limited vertical extent

CUMULIFORM CLOUDSOccur as cauliflower-like heapswith large vertical extent

CIRRIFORM CLOUDSOccur as whisps or thin sheets

CLOUD CLASSIFICATION: STRUCTURE




HIGH CLOUDSHIGH CLOUDSAbove 6000 metersThree main types

Cirrus - detached clouds composed of delicate icy filaments, have some vertical extent (mares’ tails)Cirrostratus - transparent cloud veil -produces a halo around the sun or moon.Cirrocumulus - very small cells or ripples -mackerel sky

High clouds can portend stormy weather

“Mackerel scales and mares' tails make tall ships carry low sails”


Cirrus Clouds


Cirrostratus clouds showing halo around the sun

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Cirrocumulus clouds


MEDIUM CLOUDSMEDIUM CLOUDS

2000 to 6000 meters.2000 to 6000 meters.Composed of water droplets Composed of water droplets AltocumulusAltocumulus -- large patches composed of large patches composed of rounded masses or rolls.rounded masses or rolls.AltostratusAltostratus -- formless layer of grayish formless layer of grayish clouds covering all or a large portion of clouds covering all or a large portion of the sky the sky NimbostratusNimbostratus –– dense, dark cloud with dense, dark cloud with large vertical extent.large vertical extent.


Altocumulus clouds


Altostratus clouds


Nimbostratus clouds


LOW CLOUDSLOW CLOUDS

Up to 2000 meters. Up to 2000 meters. Composed of water droplets Composed of water droplets

StratusStratus -- formless layer of grayish clouds formless layer of grayish clouds covering all or a large portion of the skycovering all or a large portion of the skyStratocumulusStratocumulus –– layers or rolls of cloud layers or rolls of cloud with modest vertical extend. Often covers with modest vertical extend. Often covers the sthe skyky completelycompletelyCumulusCumulus –– fluffy heaped clouds.fluffy heaped clouds.CumulonimbusCumulonimbus –– Large convective cloud Large convective cloud with large vertical extent.with large vertical extent.

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StratusStratus


StratocumulusStratocumulus


Cumulus clouds


Cumulonimbus




Effect of clouds on temperature

Do clouds have a warming or cooling effect?

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My Favourite: Altocumulus Lenticularis


Formation of

Precipitation


PRECIPITATION GROWTHPRECIPITATION GROWTH

Cloud droplets are typically Cloud droplets are typically 10 microns10 micronsin size. Small raindrops are typically in size. Small raindrops are typically 1000 microns (more than a million 1000 microns (more than a million cloud droplets to make a raindrop)cloud droplets to make a raindrop)Raindrops grow by two processesRaindrops grow by two processes

(1) (1) CollisionCollision--coalescencecoalescence –– warm cloudswarm clouds

(2) (2) Bergeron processBergeron process –– cold clouds.cold clouds.


From droplets to dropsFrom droplets to drops


CONDENSATION AND DEPOSITION CONDENSATION AND DEPOSITION

SupersaturationSupersaturation –– relative humidity relative humidity can be above 100% without can be above 100% without condensationcondensationNucleationNucleation –– droplets usually form droplets usually form around particles around particles –– condensation condensation nuclei.nuclei.Condensation nuclei can be Condensation nuclei can be hydroscopichydroscopic or or hydrophobichydrophobic..

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Aerosols: particle sizes

1 nm 1 mm1 um|


Dust Plume off Mauritania Dust Plume off Mauritania


AEROSOLS & CCNsParticles suspended in the atmosphereParticles suspended in the atmosphereDiameters of microns Diameters of microns –– one millionth of one millionth of

a meter.a meter.Act as Act as cloud condensation nucleicloud condensation nuclei ((CCNsCCNs) )

for formation of water droplets.for formation of water droplets.

PRIMARY SOURCES:PRIMARY SOURCES:Sea salt spraySea salt sprayWind erosionWind erosionVolcanoesVolcanoesFiresFiresHuman activity Human activity

Introduction to Meteorology & ClimateFig. 4-7, p. 93


DROPLET GROWTH BY DIFFUSIONDROPLET GROWTH BY DIFFUSION

Curvature effectCurvature effect –– even if air is saturated even if air is saturated over a flat surface, it may not be for a over a flat surface, it may not be for a curved surface:curved surface:

Diffusion effectDiffusion effect -- big drops grow at the big drops grow at the expense of small ones.expense of small ones.


CollisionCollision--coalescencecoalescenceCollisionCollision--coalescence process is dominant in coalescence process is dominant in warm (warm (tropicaltropical) clouds) clouds

Large droplets have greater Large droplets have greater fallfall--speedspeed than than small dropletssmall droplets

Relative motionsRelative motions allow collision between allow collision between droplets and coalescence to occurdroplets and coalescence to occur

In convective clouds, In convective clouds, dwelldwell--timetime of droplets of droplets may be very long. may be very long.

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Collision-coalescence process


Collision-coalescence process


BergeronBergeron--WegenerWegener--FindeisenFindeisenProcessProcess

Cold clouds occur in middle and high Cold clouds occur in middle and high latitudeslatitudes

In cold clouds, snow and ice crystals In cold clouds, snow and ice crystals are present as well as water dropletsare present as well as water droplets

Ice crystals grow at the expense of Ice crystals grow at the expense of water droplets.water droplets.


Saturation Vapour Pressure


Saturation vapour pressure over ice and water


Attraction of water vapourto ice versuswater

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Bergeron

Process


Process of Aggregation


ExtraExtra--tropical rainfalltropical rainfall

In cold clouds, snow and ice crystals In cold clouds, snow and ice crystals are present as well as water dropletsare present as well as water droplets

Rain in middle and high latitudes is Rain in middle and high latitudes is the result of the melting of the snowthe result of the melting of the snow--flakes/ice crystals as they descend to flakes/ice crystals as they descend to temperatures above zero.temperatures above zero.


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WARM FRONT


FORMS OF PRECIPITATIONFORMS OF PRECIPITATIONRainRain -- droplets of water greater than 0.5 mm in droplets of water greater than 0.5 mm in diameter. Droplets smaller than 0.5 mm called diameter. Droplets smaller than 0.5 mm called drizzledrizzle..

Much rain starts out aloft as ice crystals.Much rain starts out aloft as ice crystals.

SnowSnow -- ice crystals. If air is cold (low humidity), we get ice crystals. If air is cold (low humidity), we get light and fluffy snow (powder). If air is warm than about light and fluffy snow (powder). If air is warm than about --55ºCºC, then we get wet snow (good for snowballs)., then we get wet snow (good for snowballs).

SleetSleet -- small particles of ice. Raindrops encounter small particles of ice. Raindrops encounter freezing air on descent. If freezing not complete freezing air on descent. If freezing not complete --freezing rain.freezing rain.

HailHail -- layers of ice form as the hailstorm travels up and layers of ice form as the hailstorm travels up and down in a strong convective cloud.down in a strong convective cloud.

RimeRime -- formed by freezing of formed by freezing of supercooledsupercooled fog on objects.fog on objects.


THE EFFECTS OF AIRFLOW OVER A MOUNTAIN


Passage of air over a MountainPassage of air over a Mountain

As As air ascendsair ascends mountain it cools adiabatically, mountain it cools adiabatically, clouds form, and precipitation occurs.clouds form, and precipitation occurs.Above this altitude the relative humidity stays Above this altitude the relative humidity stays at at 100%100%At the peak of the mountain the absolute At the peak of the mountain the absolute humidity is determined by the humidity is determined by the saturationsaturationvapourvapour pressure at pressure at --1212ººCC..As the air As the air descendsdescends its absolute humidity its absolute humidity remains the same as at the peakremains the same as at the peak

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As the air As the air descendsdescends it is compressed, so it it is compressed, so it warmswarmsHence the saturation Hence the saturation vapourvapour pressure will pressure will increase, and the increase, and the relative humidity will relative humidity will decreasedecreaseThe net effect of the air ascending and The net effect of the air ascending and descending the mountain is that the air descending the mountain is that the air becomes becomes drier and warmerdrier and warmer..On the island of On the island of Hawaii,Hawaii, the west side of the west side of the coast (westerly winds) has rain the coast (westerly winds) has rain forests, the eastern side has deserts.forests, the eastern side has deserts.

Passage of air over a MountainPassage of air over a Mountain


THE EFFECTS OF AIRFLOW OVER A MOUNTAIN


Global Annual RainfallGlobal Annual Rainfall


NEXT LECTURE

Cloud Recognition Quiz

Class discussion on clouds


End of Lecture 08End of Lecture 08

Documents

Climate & Introduction to Earth System Science Meteorology ...maths.ucd.ie/met/cess/LectureNotes/PDFs/MAPH10050-CH04.pdf · 3 Introduction to Meteorology & Climate HUMIDITY Humidity