Clouds form when rising air cools to a point where it can no longer hold its water vapour. The vapour condenses in the form of tiny droplets. Air is lifted because of:
Widespread ascent i.e. up a warm/cold front
Encountering orographic barriers - mountains and hills
Convection - when air is heated at low levels by contact with sun warmed ground - creating rising bubbles of air i.e. thermals.
How do we Describe Clouds?
Clouds can be cirrus (wispy), cumulus (heaped) and stratus (in layers). They are classified into 10 main cloud types according to height and shape. Not all clouds bring rain, some are signs of fine weather. The word ‘nimbus' added to the beginning or end of a cloud type, e.g. cumulonimbus or nimbostratus, means that the cloud is a rain cloud and will usually appear to be dark grey. Generally cumulonimbus are the most dangerous clouds associated with hail, lightning, tornadoes, downdraughts, downbursts and flashflooding.
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The Weather 
