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Cloud Atlas

The recognition of a cloud by having a name and being in a certain class is a useful indication of the nature of a cloud and the atmospheric movements and processes which are present. Classification is based on heights, appearance, whether the cloud consists of liquid water or ice, on the processes which lead to the formation of the clouds. Clouds are classified as low, middle or high level according to their base heights. There are three cloud forms, namely stratiform, cumuliform and cirriform cloud. Stratiform cloud are layers of cloud formed by widespread ascent and cumuliform cloud form as air rises by convection, while fibrous cloud composed of ice grystals are called cirriform cloud. There are then basic cloud types:

Cirrus (Ci)

	Formation High-level ascent Shape due to wind shear Characteristics Detached elements White, without shading Delicate filaments Patches of narrow bands Fibrous or silky appearance Hook-shaped feathery filaments Composed of ice crystals Precipitation Ni
Cirrocumulus (Cc)

	Formation Convection Develops from cirrus of cirrostratus Characteristics Thin white patch, sheet or layer Very small elements - grains, ripples Width of less than one finger held at arm's length Merged or separate More or less regularly arranged Precipitation Nil

Cirrostratus (Cs)

	Formation Widespread upper-level ascent Characteristics Transparent whitish veil Fibrous or smooth May cover whole sky Commonly produces halo Precipitation Nil

Altocumulus (Ac)

	Formation Convection Wave flow near mountains Characteristics Variable in form - continuous or patchy Usually waved or in lumps or layers May be lens-shaped near mountains or islands White or grey or both white and grey Smallest elements have apparent width of two fingers when held at arm's length. Precipitation Virga Light showers occasionally

Altostratus (As)

	Formation Widespread ascent Characteristics Grey, featureless sheet or layer cloud Can be fibrous or uniform Covers whole or part of sky Sun shines weakly (as if through ground glass) Great horizontal extent Precipitation Rain

Nimbostratus (Ns)

	Formation Widespread ascent Characteristics Dark grey cloud layer Generally covering the sky Dense and thick enough to hide sun or moon Base indistinct as a result of continuous rain or snow Base often lower than 2 500 m Precipitation Rain or snow - almost continuous

Cumulus (Cu)

	Formation Convection Surface heating Instability Characteristics Detached cloud Develops vertically upwards In the form of turrets, towers Tops domed or cauliflower-shaped Shaped outlines Sunlit parts are bright white Base nearly horizontal, relatively dark Size depends on stage of development Precipitation Showers of rain Snow from large cumulus

Cumulonimbus (Cb)

	Formation Convection Surface heating Instability Characteristics Bulging, dense cloud masses Huge cumulus cloud Fibrous top, often anvil-shaped or plume shaped Base dark and stormy looking Thunder and lightning common Low tattered clouds below base Associated with gusts and squalls Precipitation Showers of rain, snow - may be heavy Hail

Stratocumulus (Sc)

	Formation Spreading out of cumulus (usually evenings) Turbulent mixing under stable layer Characteristics Grey or white layer with darker areas Often regular undulations Elements have apparent width of three or more fing Precipitation Occasional light rain Drizzle

Stratus (St)

	Formation Low-level ascent or cooling Due to precipitation that has saturated the air Lifting fog or mist Characteristics Grey uniform layer May be continuous or patchy Resembling fog, but not on the ground May appear as shreds of fragments below nimbostratus May cover tops of mountains or hills Precipitation Drizzle