09 Cloud Types

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Clouds

ENVI1400: Lecture 9


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ENVI 1400 : Meteorology and Forecasting 2

Cloud Classification

Four latin terms form the basis for the naming of

clouds:

Cirrus: fibrous or hair-like

Cumulus: a heap or pile Stratus: a horizontal sheet or layer

Nimbus: rain-bearing

The prefix Altois used to indicate medium altitude

clouds.

Terms, and basic classifications first proposed by Luke Howard in 1803.

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml - good online guide


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High Level:

Cloud base above 6000m

Are all forms of cirrus (ice clouds)

Medium Level: Cloud base

2000-6000m

Low level: Cloud base below 2000m (within boundary

layer)


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High-Level Clouds

Cirrus (Ci):White, delicate, fibrousin appearance. Forms in patches ornarrow bands. May for comma-shaped streaks or mares tails(cirrus uncinus)

Cirrus clouds are formed entirelyof ice crystals. These grow andevaporate slowly, leading to softedges to clouds.


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Cirrostratus (Cs): Thin,transparent sheet or veil; sunclearly visible & casting shadowsat surface. A halo may be seenaround the sun (or moon).Sheets of cirrostratus may cover

entire sky, and be up to several1000m deep.


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Cirrocumulus (Cc): Thin whitepatch or sheet of cloud; appears

dappled or rippled. Dapplingresults from convectiveoverturning within the cloud,ripples from gravity waves.

Aircraft contrails: condensationfrom aircraft exhaust. May

dissipate quickly, or be very long-lived depending on conditions.


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Medium-Level Clouds

Altostratus (As): A greyishsheet of cloud, may be fibrousor uniform in appearance.Thin enough in parts to makeout the sun, but no halo.



Altocumulus (Ac): white or greypatches arranged in sheets.

Shape and texture are variable.

There are several distinct sub-classes of altocumulus



Altocumulus lenticularis (Ac len):white or grey lenticular (lens

shaped) clouds formed by the liftingof air over a topographic barrier.



Altocumulus castellanus(Ac cas): white or grey, brokencumulus-like clouds; upper part

appearing castle-like.

Sometimes arranged in lines.



Altocumulus undulatus (Ac und):white or grey patches or sheets ofcloud with an undulating or rippled

appearance.



Low-Level Clouds

Cumulus (Cu): Brilliant white to grey,dense detached clouds. Forms clumpedor heaped (cauliflower-like) shapes,usually with sharp outlines and flat base.Field of Cu often have bases all at same(lifting condensation) level.


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Cumulus humilis (Cu hum):small cumulus, of limited

vertical extent, may have aflattened appearance. Also

called fair-weather cumulus


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Cumulus mediocris : cumulus,of moderate vertical extent.


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Cumulus congestus:crowded (congested) field of

cumulus or greater verticalextent. May produce rain.


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Pileus : cap clouds that form above large cumulus as theupward motion of the convective cloud distorts the layer of airabove (pileus is latin for skull-cap)


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Cumulonimbus (Cb) : huge towering cloud, dark base and whitesides. Associated with heavy rain, thunderstorms, and hail. Frequentlyhas an anvil shaped top.


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mammatus : smooth, rounded shapes sometimes formed onthe underside of cumulonimbus; they result from downdraftswithin the cloud.


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Stratocumulus (Sc) :white or grey sheet ofcloud, usually formed inmounds or rolls.


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Stratocumulus with virgahair-like strands of falling rain,which evaporate below cloud before reaching the surface.


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Stratus (St) : grey featureless layer of cloud with a uniform base.Often associated with drizzle or snow.


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Nimbostratus (Ns) : Dark grey,featureless, thick layer of cloud.

Associated with prolongedprecipitation. Commonly forms

in frontal systems


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Precipitation

Cloud droplets require a condensation nucleion which

to form; growth then occurs by deposition of water

molecules from vapour.

Growth is limited by local supersaturation of the vapour

Growth rate decreases as droplet size increases

Cloud droplets are typically 10 to 30 m in diameter.

Growth/evaporation can occur within a few 10s of

seconds.

Rain drops are typically 0.5 to 5 mm in diameter, growthfrom the vapour would take several hourslonger than

the lifetime of typical convective clouds.


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In order to grow into rain drops, cloud droplets must

grow by coalescence

Larger droplets fall faster than small ones, and can collide with

them

The process by which a sufficient number of large enough

droplets is generated remains a topic of active research

Ice crystals provide a more efficient process

Saturation vapour pressure over ice is less than that over waterice crystals grow at expense of water droplets

If ice crystal touches a droplet, the droplet freezes

Once large enough, ice crystalsor clumps of crystalsfall past

droplets and collect them. Rapid growth of soft hail pellet

(graupel) by riming.

Graupel falls from cloud, melting before reaching the surface as

rain


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Cloud Radiative Effects

Clouds play an important controlling role in theglobal radiation budget.

Reflection of incoming solar (short-wave) radiation

Absorption of both solar and thermal infra red (long-

wave) radiation (incoming & outgoing)

Emission of infra red radiation (up and down)

The altitude, type, and thickness of cloud, along

with that of clouds above & below determineswhether the local net effect is to warm or cool

the air & surface below.


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Vertical Transport

Deep convective clouds play a major role

in mixing boundary layer airalong with

moisture, aerosol particles, and gases

(both natural and man-made pollutants)up into the free-troposphere.


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Chemistry

Clouds provide an environment within whichaqueous phase chemical reactions can takeplace within the atmosphere

Aerosol particles can be substantially modifiedwithin clouds Aqueous phase reactions with gases dissolved in

droplets

Coalescence of droplets brings multiple aerosol

particles together Chemically different aerosol may react

On evaporation of droplet, a single aerosol particle is formed,containing material from all contributing droplets

09 Cloud Types

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