Clouds are formed by the condensation of water vapour into water droplets or ice crystals in the atmosphere. The basic requirements for the formation of a cloud are (a) moist air and (b) a lifting process.
These 4 lifting processes are
- Condensation
- Turbulence
- Mass Ascent and
- Orographic ascent
Clouds are classified according to cloud base:
- (1) High clouds with bases above 20 000 ft
- (2) Medium clouds with bases between 7 000 ft to 20 000 ft and
- (3) Low clouds with bases less than 7 000 ft
These are further catalogued according to form and shape:
- (a) Cumulus - puffy heaped clouds
- (b) Stratus - layer clouds
- (c) Cirrus - feathery clouds
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High Clouds. These are clouds with bases above 20 000 ft and at these levels, the clouds consists mainly of ice particles. There are are 3 different types of High Clouds:
| (a) Cirrus clouds. These have a feathery and fibrous appearance. | 
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| (b) Cirrocumulus clouds. These appear as delicate flakes and cast no shadows. | 
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| (c) Cirrostratus clouds. These have a transparent white veil-like appearance. They do not obscure the sun/moon and the presence of ice crystals in the cloud refract light to create haloes around the sun/moon. | 
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Medium Clouds These clouds have bases between 7 000 ft to 20 000 ft and consists of water or water and ice particles. There are 3 different types of medium clouds:
| (a) Altocumulus clouds. These are layer clouds and appear in patches or rolls and are usually white or greyish. | 
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| (b) Altostratus clouds. These appear as a uniform sheet of clouds. Precipitation in the form of light rain falls from thick altostratus clouds. | 
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| (c) Nimbostratus clouds. These are thick layered clouds. Precipitation in the form of rain or snow falls from these clouds. | 
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Low Clouds These clouds have bases below 7 000 ft and may be heaped or layered. Convective clouds are low clouds with cloud tops extending beyond 40 000 ft. There are 4 different types of low clouds:
| (a) Stratocumulus clouds. These have a rounded mass or roll-like appearance and are aligned in waves. | 
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| (b) Stratus clouds. These are layered clouds with a greyish appearance, usually formed by turbulence or orographic lifting. Precipitation from stratus clouds fall in the form of drizzle, rain or snow. | 
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| (c) Cumulus clouds. These are detached heaped clouds with moderate vertical motion within a rising dome. | 
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| (d) Cumulonimbus clouds. These are thunderstorm clouds whose cloud tops may extend betond 40 000 ft. These clouds cause severe turbulence and have strong vertical motion resulting in severe downdraughts accompanied by hail, snow rain showers and thunderstorms and severe icing. | 
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More about clouds and cloud photos can be found at the following sites:
Plymouth State College Meteorology Program Cloud Boutique
Clouds and Precipitation Online Meteorology Guide - University of Illinois
Clouds - Formation and Classification. Bureau of Meteorology, Australia
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