Clouds consist of an aggregation of minute particles of water droplets or ice crystals suspended in the air and usually not touching the ground. The mass of small water droplets or ice crystals suspended in the atmosphere is visible during daytime, or under moonlight or city light at night. Clouds may also include larger particles of liquid water and ice, as well as particles of fumes, smoke or dust.

The work of the first published classification of clouds was done by the French naturalist Jean Lamarck in 1802. Lamarck defined and named a certain number of interesting cloud forms. In 1803, the English scientist Luke Howard published a more completed cloud classification covering nearly all possible cases. Subsequently, a number of meteorologists enriched the content of cloud classification in further details by adding new species or varieties.

The skies are changing eternally. Clouds are continuously evolving and can appear in numerous or infinite variety of forms including cumuliform, stratiform and cirriform. Clouds can be identified and categorized by their dimension, appearance, shape, structure and texture, luminance and transparency, colour, precipitation, cloud base height, altitude and vertical extent. Different types of clouds are related to weather. The World Meteorological Organization (WMO) has drawn up schemes of cloud classification for distinguishing and grouping clouds according to their appearance, altitudes, and, where possible, to their process of formation.

The characteristic forms of clouds are classified in terms of genera (plural of genus). There are 10 main groups of genera or principal cloud types, which are mutually exclusive (see Table 1).

By convention, the part of the atmosphere in which clouds are usually present is vertically divided into 3 etages which are high, middle and low. So, clouds are further categorized according to their height above the ground (etages) into high, middle and low level clouds. The vertical limits of etages vary with latitude, which increases from polar regions to tropical regions. The approximate cloud base altitude and vertical limits of etages are listed in the following table (see Table 2).

* If the height of some clouds in one etage extends into another etage, the etages could overlap. Examples are the Altostratus, Nimbostratus, Cumulus and Cumulonimbus. In particular, the cloud bases of Cumulus and Cumulonimbus are usually in the low etage but often extend upward into the middle and high etages.