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 crystals are called cirriform cloud. There are then basic cloud types:

Cirrus (Ci) 

cirrus jrvdm

Formation
High-level ascent
Shape due to wind shear

Characteristics
White, without shading
Delicate filaments
Patches of narrow bands
Fibrous or silky appearance
Hook-shaped feathery filaments
Composed of ice crystals

Precipitation
None

Cirrocumulus (Cc)

cirrocumulus epopich

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
None

Cirrostratus (Cs) 

cirrostratus jrvdm

Formation
Widespread upper-level ascent

Characteristics
Transparent whitish veil
Fibrous or smooth
May cover whole sky
Commonly produces halo

Precipitation
None

Altocumulus (Ac)

altocumulus

Formation
Convection
Wave flow near mountains

Characteristics
Grey, featureless sheet or layer cloud
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)

altostratus epopich

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)

nimbostratus epopich

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 meters

Precipitation
Rain or snow - almost continuous

Cumulus (Cu)

cumulus epopich

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)

cumulonimbus

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)

stratocumulus popich

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 fingers when extended at arms length

Precipitation
Occasional light rain
Drizzle

Stratus (St)

stratus popich

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