Chapter 7 Clouds, Precipitation, and Weather Radar Weather Studies Introduction to Atmospheric Science American Meteorological Society Credit: This presentation

Chapter 7Chapter 7

Clouds, Precipitation, and Clouds, Precipitation, and Weather RadarWeather Radar

Weather StudiesWeather Studies Introduction to Atmospheric ScienceIntroduction to Atmospheric Science

American Meteorological SocietyAmerican Meteorological Society

Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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Case-in-PointCase-in-Point Contrails may be affecting weather and Contrails may be affecting weather and

climateclimate– They are formed when hot, humid air from the jet’s They are formed when hot, humid air from the jet’s

exhaust mixes with cold, drier air at high altitudesexhaust mixes with cold, drier air at high altitudes Exhaust turbulence causes mixingExhaust turbulence causes mixing

– May dissipate within minutes or hours, or they May dissipate within minutes or hours, or they may spread laterally, formally cirrus clouds that may spread laterally, formally cirrus clouds that last for a daylast for a day

– Effect on local radiation budgetEffect on local radiation budget During day, reflect solar radiation and cool Earth’s During day, reflect solar radiation and cool Earth’s

surfacesurface At night, absorb/emit infrared radiation from below At night, absorb/emit infrared radiation from below

and enhance greenhouse effect by warming Earth’s and enhance greenhouse effect by warming Earth’s surfacesurface

– May stimulate local precipitation by providing ice May stimulate local precipitation by providing ice crystal nuclei for lower cloudscrystal nuclei for lower clouds

– Effect on climate may increase in the future with Effect on climate may increase in the future with the increase in global air trafficthe increase in global air traffic

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Driving QuestionDriving Question How do clouds and precipitation form?How do clouds and precipitation form?

– Clouds are products of condensation, or in the Clouds are products of condensation, or in the case of ice clouds, deposition, within the case of ice clouds, deposition, within the atmosphereatmosphere

– Clouds are essential to the global water cycleClouds are essential to the global water cycle Without clouds, there would be no precipitationWithout clouds, there would be no precipitation Yet most clouds do not yield precipitationYet most clouds do not yield precipitation

– This chapter covers:This chapter covers: Development and classification of clouds and fogDevelopment and classification of clouds and fog Formation and types of precipitationFormation and types of precipitation Weather radarWeather radar

– A valuable tool for monitoring air motion within weather A valuable tool for monitoring air motion within weather systemssystems

Methods for measuring precipitationMethods for measuring precipitation

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Cloud FormationCloud Formation

Water vapor is invisibleWater vapor is invisible Clouds are visible products of condensation Clouds are visible products of condensation

or deposition of water vapor within the or deposition of water vapor within the atmosphereatmosphere

Clouds are increasingly likely to form as air Clouds are increasingly likely to form as air nears saturationnears saturation

Clouds do not form in laboratory clean air Clouds do not form in laboratory clean air unless it is supersaturated due to the unless it is supersaturated due to the curvature effectcurvature effect

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Cloud FormationCloud Formation The curvature effectThe curvature effect

– The curvature of a water surface affects the ability of a The curvature of a water surface affects the ability of a water molecule to evaporate from the surfacewater molecule to evaporate from the surface

– The smaller the droplet, the greater the water vapor The smaller the droplet, the greater the water vapor concentration needed for droplet growthconcentration needed for droplet growth Smaller droplets have greater curvature than large dropletsSmaller droplets have greater curvature than large droplets They also have the fewest surrounding molecules, so they are They also have the fewest surrounding molecules, so they are

more weakly bondedmore weakly bonded Water vapor more readily escapes a curved surface than a flat Water vapor more readily escapes a curved surface than a flat

surfacesurface– There is more surface area on a curved surfaceThere is more surface area on a curved surface– Saturation vapor pressure is greater surrounding a small Saturation vapor pressure is greater surrounding a small

droplet vs. a large onedroplet vs. a large one– For droplets with a radius of 0.001 micrometer, saturation For droplets with a radius of 0.001 micrometer, saturation

vapor pressure is up to 3.4 times greater than for larger vapor pressure is up to 3.4 times greater than for larger surface surface → t→ this equates to 340% relative humidityhis equates to 340% relative humidity

– For droplets with a radius For droplets with a radius > 1.0 micrometer, RH only slightly > 1.0 micrometer, RH only slightly higher than 100% is requiredhigher than 100% is required

– What then makes cloud formation possible?What then makes cloud formation possible?

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Cloud FormationCloud Formation Role of NucleiRole of Nuclei

– Outside the laboratory, clouds form readily as the Outside the laboratory, clouds form readily as the relative humidity nears 100%relative humidity nears 100%

– Suspended in the air are solid or liquid particles, known Suspended in the air are solid or liquid particles, known as nuclei, which provide a relatively large surface area as nuclei, which provide a relatively large surface area for condensation or deposition to occurfor condensation or deposition to occur Nuclei have radii Nuclei have radii > 1 micrometer> 1 micrometer When condensation starts on the surface of the nuclei, they When condensation starts on the surface of the nuclei, they

become comparably sized cloud droplets (or ice crystals) and become comparably sized cloud droplets (or ice crystals) and additional growth is more likelyadditional growth is more likely

– Nuclei are essential for cloud formationNuclei are essential for cloud formation They are abundant within the atmosphereThey are abundant within the atmosphere Sources include volcanic eruptions, wind Sources include volcanic eruptions, wind

erosion of soil, forest fires, and ocean sprayerosion of soil, forest fires, and ocean spray

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Cloud FormationCloud Formation Role of Nuclei, continuedRole of Nuclei, continued

– Types of nuclei:Types of nuclei: Cloud condensation nuclei (CCN) promote Cloud condensation nuclei (CCN) promote

condensation at temperatures both above and below condensation at temperatures both above and below the freezing point of waterthe freezing point of water

– Cloud droplets can remain liquid even at temperatures well Cloud droplets can remain liquid even at temperatures well below 0 below 0 °°C (32 C (32 °°F) - F) - supercooledsupercooled droplets droplets

Ice-forming nuclei (IN) promote ice crystal formation at Ice-forming nuclei (IN) promote ice crystal formation at temperatures well below freezingtemperatures well below freezing

– Freezing nuclei – surfaces on which water first condenses, Freezing nuclei – surfaces on which water first condenses, then freezesthen freezes

Active at temperatures below about -9 Active at temperatures below about -9 °°C (16 C (16 °°F) F) – Deposition nuclei – surfaces on which ice deposits directly Deposition nuclei – surfaces on which ice deposits directly

from vaporfrom vapor Not fully active until temperature is below -20 Not fully active until temperature is below -20 °°C (-4 C (-4 °°F) F)

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Cloud FormationCloud Formation Role of Nuclei, continuedRole of Nuclei, continued

– Types of nuclei, continued:Types of nuclei, continued: CCN are much more common than INCCN are much more common than IN Hygroscopic nuclei – special category of CCNHygroscopic nuclei – special category of CCN

– They have a chemical attraction for water moleculesThey have a chemical attraction for water molecules– Clouds form more readily in their presence, for example, Clouds form more readily in their presence, for example,

magnesium chloride in ocean spray can promote magnesium chloride in ocean spray can promote condensation at relative humidity as low as 70%condensation at relative humidity as low as 70%

– Many sources exist in urban-industrial areasMany sources exist in urban-industrial areas– Cities also spur clouds and precipitation development by Cities also spur clouds and precipitation development by

contributing water vapor (raising the relative humidity) and contributing water vapor (raising the relative humidity) and adding heat (adding to the buoyancy of air)adding heat (adding to the buoyancy of air)

Rainfall has been shown to be greater downwind from an Rainfall has been shown to be greater downwind from an urban-industrial areaurban-industrial area

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Supercooled waterSupercooled water– Supercooling = fresh water cooled below its Supercooling = fresh water cooled below its

freezing point remains liquidfreezing point remains liquid– Within very small droplets, kinetic energy is Within very small droplets, kinetic energy is

sufficient to prevent growth of ice embryos to sufficient to prevent growth of ice embryos to the critical size needed for continued growth the critical size needed for continued growth and freezingand freezing Hence, water droplets can remain liquid even at Hence, water droplets can remain liquid even at

temperatures well below freezingtemperatures well below freezing As droplet temperature falls, the probability of an ice As droplet temperature falls, the probability of an ice

embryo growing to critical size increases because embryo growing to critical size increases because average kinetic molecular activity decreasesaverage kinetic molecular activity decreases

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Supercooled water, continuedSupercooled water, continued– Homogeneous nucleationHomogeneous nucleation

The formation of ice embryos of critical size due to the chance The formation of ice embryos of critical size due to the chance aggregation of water moleculesaggregation of water molecules

If no foreign particles acting as IN are present, a cloud droplet If no foreign particles acting as IN are present, a cloud droplet can cool to as low as -39 can cool to as low as -39 °°C (-38.2 C (-38.2 °° F) without freezing F) without freezing (Schaefer point)(Schaefer point)

Additional cooling leads to homogeneous nucleationAdditional cooling leads to homogeneous nucleation

– Heterogeneous nucleationHeterogeneous nucleation If a supercooled droplet contains (or comes in contact with) If a supercooled droplet contains (or comes in contact with)

foreign particles that are IN, the cloud droplet will freeze at a foreign particles that are IN, the cloud droplet will freeze at a temperature below freezing, but well above the Schaefer pointtemperature below freezing, but well above the Schaefer point

Water molecules collect on the IN and form an embryo that is Water molecules collect on the IN and form an embryo that is close enough to critical size that additional growth causes the close enough to critical size that additional growth causes the droplet to freezedroplet to freeze

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Cloud ClassificationCloud Classification

Clouds are classified based on:Clouds are classified based on:– General appearanceGeneral appearance

Cirriform – wispy, fibrousCirriform – wispy, fibrous Stratiform – layeredStratiform – layered Cumuliform – heaped, or puffyCumuliform – heaped, or puffy

– Altitude of cloud baseAltitude of cloud base High, middle, or lowHigh, middle, or low

– TemperatureTemperature Warm cloud – 0 Warm cloud – 0 °C (32 ° F)°C (32 ° F) Cold cloud – at or below 0 Cold cloud – at or below 0 °C (32 ° F)°C (32 ° F)

– Composition Composition Corresponds to temperatureCorresponds to temperature See Table 7.1 (next slide)See Table 7.1 (next slide)

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High CloudsHigh Clouds Bases above 5000 m (16,000 ft) where average Bases above 5000 m (16,000 ft) where average

temperatures are typically below -25 temperatures are typically below -25 °°C (-13 C (-13 °°F)F) Clouds are composed almost exclusively of ice Clouds are composed almost exclusively of ice

crystals and have fibrous appearancecrystals and have fibrous appearance Names include the prefix Names include the prefix cirrocirro

cirruscirrus cirrostratuscirrostratus cirrocumuluscirrocumulus

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Middle CloudsMiddle Clouds

Bases between 2000 and 5000 m (6600 and 16,000 ft) Bases between 2000 and 5000 m (6600 and 16,000 ft) where average temperatures are typically between 0 where average temperatures are typically between 0 °°C C and -25 and -25 °°C (32 C (32 °°F and -13 F and -13 °°F)F)

Clouds are composed of supercooled water droplets or a Clouds are composed of supercooled water droplets or a mixture of droplets and ice crystalsmixture of droplets and ice crystals

Names include the prefix Names include the prefix altoalto

altostratusaltostratus altocumulusaltocumulus

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Low CloudsLow Clouds Bases from Earth’s surface (fog) to 2000 m (6600 ft) where Bases from Earth’s surface (fog) to 2000 m (6600 ft) where

average temperatures are typically above -5 average temperatures are typically above -5 °°C (23 C (23 °°F)F) Clouds are composed of mostly water dropletsClouds are composed of mostly water droplets Include stratocumulus, stratus, and nimbostratusInclude stratocumulus, stratus, and nimbostratus Usually only drizzle falls from stratus, but significant Usually only drizzle falls from stratus, but significant

amounts of rain or snow may fall from thicker nimbostratusamounts of rain or snow may fall from thicker nimbostratus

stratocumulusstratocumulus stratusstratus

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Clouds Having Significant Vertical Clouds Having Significant Vertical DevelopmentDevelopment

cumuluscumulus cumulus congestuscumulus congestus

cumulus clouds on acumulus clouds on avisible satellite imagevisible satellite imagecumulonimbuscumulonimbus

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Sky WatchingSky Watching

Cumuliform and stratiform clouds are often Cumuliform and stratiform clouds are often observed in the sky togetherobserved in the sky together

Clouds at different levels may move in different Clouds at different levels may move in different directions at different speedsdirections at different speeds– Caused by wind shear – change in wind direction or Caused by wind shear – change in wind direction or

speed with distancespeed with distance– Strong vertical shear in horizontal wind speed or Strong vertical shear in horizontal wind speed or

direction may cause cloud streets to formdirection may cause cloud streets to form

Cloud holes may be formed by aircraftCloud holes may be formed by aircraft– Turbulence may cause expansional cooling of Turbulence may cause expansional cooling of

supercooled dropletssupercooled droplets Droplets freeze to ice crystals that fall out of the cloudDroplets freeze to ice crystals that fall out of the cloud

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Unusual CloudsUnusual Clouds Stationary disk-shaped Stationary disk-shaped

clouds (altocumlus clouds (altocumlus lenticularis) are generated lenticularis) are generated when a prominent mountain when a prominent mountain range disturbs large scale range disturbs large scale windswinds

Mountain-wave clouds – Mountain-wave clouds – lenticular clouds situated lenticular clouds situated over the mountainover the mountain

Lee-wave clouds - lenticular Lee-wave clouds - lenticular clouds formed in the wave clouds formed in the wave crests downwind of the crests downwind of the mountainmountain

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Unusual CloudsUnusual Clouds

In a stable atmosphere, waves and wave-type clouds can In a stable atmosphere, waves and wave-type clouds can develop along the interface between two layers of air develop along the interface between two layers of air moving horizontally at different speedsmoving horizontally at different speeds

In the figure, a layer of relatively warm air moving In the figure, a layer of relatively warm air moving horizontal more rapidly overlies a colder, denser air layer horizontal more rapidly overlies a colder, denser air layer

Vertical shear in the horizontal wind creates Kelvin-Vertical shear in the horizontal wind creates Kelvin-Helmholtz waves along the boundaryHelmholtz waves along the boundary

Billow clouds are formed in the wave crestsBillow clouds are formed in the wave crests

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Unusual CloudsUnusual Clouds

Noctilucent clouds form in the upper mesosphere above an Noctilucent clouds form in the upper mesosphere above an altitude of about 80 km (50 mi)altitude of about 80 km (50 mi)– Seldom observed, and then only at high latitudesSeldom observed, and then only at high latitudes– Key to formation is exceptionally low temperatures; ice clouds can Key to formation is exceptionally low temperatures; ice clouds can

form at extremely low vapor pressuresform at extremely low vapor pressures– Water vapor can be from volcanic eruptions or chemical reactions Water vapor can be from volcanic eruptions or chemical reactions

involving methaneinvolving methane

Nacreous clouds occur in the stratosphere at altitudes of Nacreous clouds occur in the stratosphere at altitudes of 20 to 30 km (12 to 19 mi) and form on sulfuric acid nuclei20 to 30 km (12 to 19 mi) and form on sulfuric acid nuclei

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FogFog Fog is simply a cloud Fog is simply a cloud

in contact with the in contact with the groundground– Restricts visibility to Restricts visibility to

1000 m (3250 ft) or less1000 m (3250 ft) or less Otherwise, the Otherwise, the

suspension is called suspension is called mist (light drizzle)mist (light drizzle)

Types:Types:– Radiation – radiational Radiation – radiational

cooling causes humid cooling causes humid air near the ground to air near the ground to saturatesaturate

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FogFog Types:Types:

– Advection fogAdvection fog The advecting air The advecting air

passes over a passes over a relatively cool relatively cool surface causing surface causing condensation to condensation to occur as the air occur as the air coolscools

This happens in This happens in the summer over the summer over the surface of the the surface of the Great LakesGreat Lakes

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FogFog

Types of fog:Types of fog:– Steam fogSteam fog

Also called Arctic sea smokeAlso called Arctic sea smoke Develops in late fall or winter when extrememly cold Develops in late fall or winter when extrememly cold

and dry air flows over a large unfrozen body of waterand dry air flows over a large unfrozen body of water Evaporation and sensible heating cause the lower Evaporation and sensible heating cause the lower

portion of the air mass to become more humid and portion of the air mass to become more humid and warmer than the air abovewarmer than the air above

The air is destabilized, and the consequent mixing of The air is destabilized, and the consequent mixing of mild, humid air with cold, dry air causes saturation mild, humid air with cold, dry air causes saturation and fog formationand fog formation

It looks like steam or smoke coming off the waterIt looks like steam or smoke coming off the water

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FogFog Types of fog:Types of fog:

– Upslope fog – Upslope fog – fog develops on fog develops on mountainsides mountainsides or hillsides as or hillsides as the ascending the ascending humid air humid air reaches reaches saturationsaturation

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Precipitation ProcessesPrecipitation Processes Precipitation is water in solid or liquid form that Precipitation is water in solid or liquid form that

falls from clouds to Earth’s surface under the falls from clouds to Earth’s surface under the influence of gravityinfluence of gravity

Terminal velocityTerminal velocity– Terminal velocity is defined as the constant downward-Terminal velocity is defined as the constant downward-

directed speed of a droplet (or other particle within a directed speed of a droplet (or other particle within a fluid) due to a balance between gravity acting downward fluid) due to a balance between gravity acting downward and air (or fluid) resistance acting upwardand air (or fluid) resistance acting upward

– Helps explain why tiny water droplets and ice crystals Helps explain why tiny water droplets and ice crystals composing clouds will not fall as precipitation unless composing clouds will not fall as precipitation unless they are sufficiently largethey are sufficiently large The two basic methods for cloud particle growth are collision-The two basic methods for cloud particle growth are collision-

coalescence (warm clouds) and the Bergeron-Findeisen coalescence (warm clouds) and the Bergeron-Findeisen process (cold clouds)process (cold clouds)

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The terminal velocity of a particle falling through The terminal velocity of a particle falling through the air increases with the size of the particlethe air increases with the size of the particle

Precipitation ProcessesPrecipitation Processes

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Precipitation ProcessesPrecipitation Processes Warm-Cloud Precipitation (collision-coalescence Warm-Cloud Precipitation (collision-coalescence

process)process)– Warm cloud = cloud at temperatures > 0 Warm cloud = cloud at temperatures > 0 °°CC– Droplets may grow by colliding and coalescing with one Droplets may grow by colliding and coalescing with one

anotheranother– Takes place in a cloud with a mixture of droplet sizes, Takes place in a cloud with a mixture of droplet sizes,

ideally with some having diameters of at least 20 ideally with some having diameters of at least 20 micrometersmicrometers Laboratory simulations demonstrate that colliding droplets will not Laboratory simulations demonstrate that colliding droplets will not

coalesce unless they are of significantly different sizescoalesce unless they are of significantly different sizes– Eventually, the droplets become large enough that their Eventually, the droplets become large enough that their

terminal velocity overcomes updraftsterminal velocity overcomes updrafts Then, precipitation will occurThen, precipitation will occur A key factor is collision efficiencyA key factor is collision efficiency

– This is the fraction of all droplets in the path of the falling This is the fraction of all droplets in the path of the falling larger droplet that come into contact with the larger dropletlarger droplet that come into contact with the larger droplet

– Collision efficiency increases with the size of the larger dropletCollision efficiency increases with the size of the larger droplet

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The relatively large droplet The relatively large droplet falls through a cloud of falls through a cloud of much smaller dropletsmuch smaller droplets

The larger drop falls faster The larger drop falls faster and collides with the and collides with the smaller droplets in its pathsmaller droplets in its path

The larger drop grows via The larger drop grows via coalescence coalescence

This is the collision-This is the collision-coalescence processcoalescence process

Warm-Cloud PrecipitationWarm-Cloud Precipitation

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Precipitation ProcessesPrecipitation Processes Cold-Cloud Precipitation (Bergeron-Findeisen Cold-Cloud Precipitation (Bergeron-Findeisen

process)process)– Cold cloud = clouds or portions of clouds at temperatures Cold cloud = clouds or portions of clouds at temperatures

< 0 < 0 °°CC– Most middle and high latitude clouds form precipitation in Most middle and high latitude clouds form precipitation in

an environment of supercooled water and ice crystalsan environment of supercooled water and ice crystals– At sub-freezing temperatures, water molecules more At sub-freezing temperatures, water molecules more

readily vaporize from a liquid water surface than an ice readily vaporize from a liquid water surface than an ice surface because water molecules have stronger bonds in surface because water molecules have stronger bonds in the solid phase the solid phase

– At the same sub-freezing temperature, the saturation At the same sub-freezing temperature, the saturation vapor pressure is greater over water than over ice (see vapor pressure is greater over water than over ice (see next slide showing Table 6.3)next slide showing Table 6.3)

– A vapor pressure that is saturated for water droplets is A vapor pressure that is saturated for water droplets is actually supersaturated for ice crystals, hence the water actually supersaturated for ice crystals, hence the water vapor deposits on the ice and the crystals grow at the vapor deposits on the ice and the crystals grow at the expense of surrounding water dropletsexpense of surrounding water droplets

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Precipitation ProcessesPrecipitation Processes Bergeron-Findeisen Bergeron-Findeisen

process, cont.process, cont.– Terminal velocity increases Terminal velocity increases

with increased ice crystal with increased ice crystal sizesize

– Larger crystals overtake, Larger crystals overtake, collide, and agglomerate collide, and agglomerate with smaller crystals and with smaller crystals and water dropletswater droplets

– May fall out of cloud base May fall out of cloud base when large enough, and when large enough, and may reach Earth’s surface may reach Earth’s surface as snow or rain depending as snow or rain depending on ambient air temperatures on ambient air temperatures below the cloudbelow the cloud

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Precipitation ProcessesPrecipitation Processes

Once a falling raindrop or snowflake leaves the base of a Once a falling raindrop or snowflake leaves the base of a cloud, it enters unsaturated air and begins to vaporizecloud, it enters unsaturated air and begins to vaporize

Amount that vaporizes increases with cloud base height Amount that vaporizes increases with cloud base height and decreased relative humidity of ambient airand decreased relative humidity of ambient air

Virga is streaks of water and ice crystals falling from a Virga is streaks of water and ice crystals falling from a cloud that mostly vaporize before reaching Earth’s surfacecloud that mostly vaporize before reaching Earth’s surface

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Forms of PrecipitationForms of Precipitation Precipitation – water in solid or liquid form that Precipitation – water in solid or liquid form that

falls from clouds to the Earth’s surfacefalls from clouds to the Earth’s surface Occurs in many formsOccurs in many forms

– Liquid (rain, drizzle)Liquid (rain, drizzle)– Freezing (freezing rain and freezing drizzle)Freezing (freezing rain and freezing drizzle)– Frozen (snow, snow pellets, snow grains, ice pellets, Frozen (snow, snow pellets, snow grains, ice pellets,

hail)hail) Liquid precipitationLiquid precipitation

– Rain (diameters of 0.5 – 6 mm or 0.02 to 0.2 in.)Rain (diameters of 0.5 – 6 mm or 0.02 to 0.2 in.) Flattened spheres (not teardrop shaped)Flattened spheres (not teardrop shaped) Unstable at larger diameters and break apartUnstable at larger diameters and break apart Fall mostly form nimbostratus and cumulonimbus cloudsFall mostly form nimbostratus and cumulonimbus clouds

– Drizzle (diameters of 0.2 – 0.5 mm or 0.01 to 0.1 in.)Drizzle (diameters of 0.2 – 0.5 mm or 0.01 to 0.1 in.) Originates mostly in stratus cloud and has limited growth by Originates mostly in stratus cloud and has limited growth by

collision-coalescencecollision-coalescence

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Forms of PrecipitationForms of Precipitation

Frozen precipitationFrozen precipitation– Snow (aggregate diameters Snow (aggregate diameters

can reach 5 – 10 cm or 2 – can reach 5 – 10 cm or 2 – 4 in.)4 in.) An agglomeration of ice An agglomeration of ice

crystals in the form of flakescrystals in the form of flakes Vary in size depending on Vary in size depending on

water vapor concentration water vapor concentration and the temperature in the and the temperature in the portion of the cloud where portion of the cloud where they growthey grow

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Forms of PrecipitationForms of Precipitation Frozen precipitation, continuedFrozen precipitation, continued

– Snow pellets (diameters of 2 – 5 mm or 0.08 – 0.2 in.)Snow pellets (diameters of 2 – 5 mm or 0.08 – 0.2 in.) Soft conical or spherical white ice particlesSoft conical or spherical white ice particles Form when supercooled cloud droplets collide with an ice Form when supercooled cloud droplets collide with an ice

crystal and freezecrystal and freeze

– Snow grains (diameters < 1 mm or 0.04 in.)Snow grains (diameters < 1 mm or 0.04 in.) Flat or elongated opaque white ice particlesFlat or elongated opaque white ice particles Form in similar way to drizzle, but freeze prior to reaching the Form in similar way to drizzle, but freeze prior to reaching the

groundground

– Ice pellets (sleet) (diameters < 5 mm or 0.02 in.)Ice pellets (sleet) (diameters < 5 mm or 0.02 in.) Spherical or irregularly shaped transparent or translucent ice Spherical or irregularly shaped transparent or translucent ice

particlesparticles Form when snowflakes partially or completely melt below cloud Form when snowflakes partially or completely melt below cloud

base and then refreeze into ice particles before striking the base and then refreeze into ice particles before striking the groundground

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Forms of PrecipitationForms of Precipitation Frozen precipitation, Frozen precipitation,

continuedcontinued– Freezing rain (freezing Freezing rain (freezing

drizzle)drizzle) Supercooled liquid Supercooled liquid

precipitation that freezes precipitation that freezes (totally or partially) on (totally or partially) on contact with subfreezing contact with subfreezing objectsobjects

Forms a coating of ice Forms a coating of ice (glaze) on exposed (glaze) on exposed surfacessurfaces

Sounding favorable to freezing rain formationSounding favorable to freezing rain formation

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Forms of PrecipitationForms of Precipitation Frozen precipitation, Frozen precipitation,

continuedcontinued– HailHail

Characterized by concentric, Characterized by concentric, onion-like layers of iceonion-like layers of ice

Develops within severe Develops within severe thunderstorms characterized by thunderstorms characterized by vigorous updrafts, abundant vigorous updrafts, abundant supercooled water droplet supercooled water droplet supply, and great vertical cloud supply, and great vertical cloud developmentdevelopment

Updrafts lift ice pellets into Updrafts lift ice pellets into higher portion of cloud, they higher portion of cloud, they grow by collecting supercooled grow by collecting supercooled droplets and eventually may exit droplets and eventually may exit the cloud basethe cloud base

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Forms of PrecipitationForms of Precipitation

Schematic cross-Schematic cross-section of the lower section of the lower atmosphere showing atmosphere showing temperature conditions temperature conditions required for the required for the formation of frozen, formation of frozen, freezing, and liquid freezing, and liquid forms of precipitationforms of precipitation

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Acid DepositionAcid Deposition Rain and snow are naturally slightly acidicRain and snow are naturally slightly acidic

– They dissolve some atmospheric COThey dissolve some atmospheric CO22, producing weak , producing weak carbonic acid Hcarbonic acid H22COCO33

When air is polluted with oxides of sulfur and When air is polluted with oxides of sulfur and oxides of nitrogen, acid rain may formoxides of nitrogen, acid rain may form– These gases interact with moisture in the atmosphere to These gases interact with moisture in the atmosphere to

form tiny droplets of sulfuric acid Hform tiny droplets of sulfuric acid H22SOSO44 and nitric acid and nitric acid HNOHNO33

– These acids dissolve in precipitation and may increase These acids dissolve in precipitation and may increase its acidity by as much as 200 timesits acidity by as much as 200 times

– Without precipitation, sulfuric acid droplets convert to Without precipitation, sulfuric acid droplets convert to acidic aerosols that reduce visibility and may cause acidic aerosols that reduce visibility and may cause human health problems – acidic aerosols may settle to human health problems – acidic aerosols may settle to ground as dry depositionground as dry deposition

– Acid deposition is the combination of acid precipitation Acid deposition is the combination of acid precipitation and dry depositionand dry deposition

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Acid DepositionAcid Deposition

An acid is a hydrogen-An acid is a hydrogen-containing compound that containing compound that releases hydrogen ions releases hydrogen ions (H(H++) when it dissolves in ) when it dissolves in waterwater

An alkaline substance An alkaline substance releases hydroxyl ions releases hydroxyl ions (OH(OH--) when dissolved in ) when dissolved in waterwater

Acidity and alkalinity are expressed in terms of pH, a measure of the hydrogen ion concentration

Rain with a pH < 5.6 is considered acid rain

pH Scale (note that scale is logarithmic)

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Weather Radar:Weather Radar:Locating PrecipitationLocating Precipitation

Radar is an acronym (Radar is an acronym (raradio dio ddetection etection aand nd rranging)anging) Emits microwave signals and receives reflected signals from targets as Emits microwave signals and receives reflected signals from targets as

it continually scans a large volume of the lower atmosphereit continually scans a large volume of the lower atmosphere Can detect potentially dangerous weather, such as a tornado’s Can detect potentially dangerous weather, such as a tornado’s

circulation within a parent thunderstormcirculation within a parent thunderstorm Monitors rainfall rates and cumulative rainfall totalsMonitors rainfall rates and cumulative rainfall totals Widespread use began after WWIIWidespread use began after WWII Mid-1950s, Congress allocated funds to purchase long-range radar Mid-1950s, Congress allocated funds to purchase long-range radar

units for meteorological purposes following several tornado and units for meteorological purposes following several tornado and hurricane disastershurricane disasters

1959, coastal hurricane radar stations established (WSR-57)1959, coastal hurricane radar stations established (WSR-57) Late 1960s – common to television weather reportingLate 1960s – common to television weather reporting 1974 – major upgrade (WSR-74)1974 – major upgrade (WSR-74) 1990s – implementation of Doppler radar network (WSR 88-D)1990s – implementation of Doppler radar network (WSR 88-D)

– Operates in a reflectivity or velocity modeOperates in a reflectivity or velocity mode Reflectivity – detects location, movement, and intensity of areas of precipitationReflectivity – detects location, movement, and intensity of areas of precipitation Velocity – detect motion directly toward or away from the radar unitVelocity – detect motion directly toward or away from the radar unit

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Reflectivity modeReflectivity mode– WSR 88-D emits short pulses of microwave energy with wavelengths of 10 to WSR 88-D emits short pulses of microwave energy with wavelengths of 10 to

11.1 cm11.1 cm– At these wavelengths, radar pulses are scattered readily by rain, snow, or hail, At these wavelengths, radar pulses are scattered readily by rain, snow, or hail,

but not significantly by cloud dropletsbut not significantly by cloud droplets– Falling precipitation reflects some of radar signal back to a receiving unit, where it Falling precipitation reflects some of radar signal back to a receiving unit, where it

is processed and displayed on a computer screen as a radar echois processed and displayed on a computer screen as a radar echo– Some radar echoes may be caused by fixed objects on ground (ground clutter) – Some radar echoes may be caused by fixed objects on ground (ground clutter) –

typically this is electronically subtracted from displaytypically this is electronically subtracted from display

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The radome for a WSR-88D unit houses aThe radome for a WSR-88D unit houses arotating radar dish antennarotating radar dish antenna

A radar reflectivity product in which echoA radar reflectivity product in which echointensity is graduated by colorintensity is graduated by color

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Doppler EffectDoppler Effect– The shift in frequency of sound or electromagnetic waves that The shift in frequency of sound or electromagnetic waves that

accompanies the motion of the wave source(s) or wave receiveraccompanies the motion of the wave source(s) or wave receiver– (A) = The sound wave source is stationary and wave frequency is (A) = The sound wave source is stationary and wave frequency is

uniform everywhereuniform everywhere– (B) = The wave source is in motion so that wave frequency is greater (B) = The wave source is in motion so that wave frequency is greater

ahead of the source than behind the sourceahead of the source than behind the source

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Velocity (Doppler) modeVelocity (Doppler) mode– Determines horizontal air motions within a weather systemDetermines horizontal air motions within a weather system– There is a shift in frequency of sound and There is a shift in frequency of sound and electromagnetic waves electromagnetic waves

coming from a moving sourcecoming from a moving source Doppler radar can detect this frequency shift, and displays it as motionDoppler radar can detect this frequency shift, and displays it as motion

– Doppler radars monitor movement of precipitation particles directly Doppler radars monitor movement of precipitation particles directly toward or away from the radar unittoward or away from the radar unit

– Doppler radar allow advance notification of severe approaching weatherDoppler radar allow advance notification of severe approaching weather

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Doppler image – greens and blues indicate motion directly towards the radar, while reds and yellows indicate motion directly away from the radar

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Measuring PrecipitationMeasuring Precipitation Rainfall and snowfall are routinely measured in terms of Rainfall and snowfall are routinely measured in terms of

accumulation depth over a specified time interval, usually accumulation depth over a specified time interval, usually hourly and every 6 hrs and 24 hrshourly and every 6 hrs and 24 hrs

Measurements are made with gauges or remotely by Measurements are made with gauges or remotely by weather radar or satellite sensorsweather radar or satellite sensors

Standard non-recording rain gaugeStandard non-recording rain gauge– Cylinder with a cone-shaped funnel openingCylinder with a cone-shaped funnel opening– Can resolve rainfall into increments as small as 0.01 in. (0.25 mm)Can resolve rainfall into increments as small as 0.01 in. (0.25 mm)– A graduated stick measures depth of water in cylinderA graduated stick measures depth of water in cylinder– Rainfall is measured manually at a fixed time and then the gauge is Rainfall is measured manually at a fixed time and then the gauge is

emptiedemptied Weighing-bucket rain gaugeWeighing-bucket rain gauge

– calibrates the weight of rainwater in terms of water depthcalibrates the weight of rainwater in terms of water depth– Marks a chart on a clock-driven drum that sends an electronic Marks a chart on a clock-driven drum that sends an electronic

signal to a computersignal to a computer– May melt frozen precipitation with antifreeze or a heaterMay melt frozen precipitation with antifreeze or a heater

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Measuring PrecipitationMeasuring Precipitation

Tipping-bucket rain gaugeTipping-bucket rain gauge– Consists of a free-swinging Consists of a free-swinging

container partitioned into 2 container partitioned into 2 compartments, each of which can compartments, each of which can collect 0.01 in. of rainfallcollect 0.01 in. of rainfall

– Each compartment alternately Each compartment alternately fills with water, tips, and spills its fills with water, tips, and spills its contents, tripping an electronic contents, tripping an electronic switch that marks a chart or switch that marks a chart or sends a signal to a computersends a signal to a computer

NWS will be replacing heated NWS will be replacing heated tipping-bucket gauges with tipping-bucket gauges with weighing bucket gaugesweighing bucket gauges

A standard NWS non-recording rain gaugeA standard NWS non-recording rain gauge

A new weighing-bucket rain gaugeA new weighing-bucket rain gauge

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Measuring PrecipitationMeasuring Precipitation Snowfall MeasurementSnowfall Measurement

– Meteorologists are interested in the depth of snow that Meteorologists are interested in the depth of snow that falls over a certain time period, the melt water falls over a certain time period, the melt water equivalent of the snowfall, and the depth of snow on the equivalent of the snowfall, and the depth of snow on the ground at observation timeground at observation time

– New snowfall accumulates on a board placed on top of New snowfall accumulates on a board placed on top of the old snow cover and is measured with a rulerthe old snow cover and is measured with a ruler

– The melt water equivalent can be determined by The melt water equivalent can be determined by weighing-bucket gauge measurements or melting snow weighing-bucket gauge measurements or melting snow in a non-recording gaugein a non-recording gauge As a very general rule, 10 cm of fresh snow melts to 1 cm of As a very general rule, 10 cm of fresh snow melts to 1 cm of

waterwater Actual ratio depends on crystalline form of snowflakes and the Actual ratio depends on crystalline form of snowflakes and the

temperature of the air through which the snow fallstemperature of the air through which the snow falls– The depth of snow on the ground is determined by an The depth of snow on the ground is determined by an

average of ruler measurements at several locationsaverage of ruler measurements at several locations

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Measuring PrecipitationMeasuring Precipitation

Remote Sensing – Weather Remote Sensing – Weather RadarRadar– Reflectivity is proportional to Reflectivity is proportional to

raindrop surface area and raindrop surface area and rainfall rate is proportional to rainfall rate is proportional to the volume of the raindropsthe volume of the raindrops

– Reflectivity data is converted to Reflectivity data is converted to rainfall rates by estimating rainfall rates by estimating raindrop size distributionraindrop size distribution

Remote Sensing – SatelliteRemote Sensing – Satellite– Tropical Rainfall Measuring Tropical Rainfall Measuring

Mission (TRMM) satellite uses Mission (TRMM) satellite uses radar, a microwave imager, radar, a microwave imager, and a visible/IR scanner to and a visible/IR scanner to estimate rainfallestimate rainfall Cumulative rainfall as determined by a Cumulative rainfall as determined by a

computer analysis of radar echoescomputer analysis of radar echoes

Documents

Chapter 7 Clouds, Precipitation, and Weather Radar Weather Studies Introduction to Atmospheric Science American Meteorological Society Credit: This presentation