Clouds are grouped in three main classes, based on where they are located in the atmosphere - low, middle or high. This triple division of clouds is based on the range or altitude (etage, level) on which a cloud genus normally occurs. In addition the World Meteorological Organization (International Cloud Atlas, 1956) classifies 10 cloud genera (types) in three major groups (cumulus or heap clouds, stratus or sheet clouds, and cirrus or fibrous clouds) by criteria essentially based on cloud form. However, as the cloud form is influenced by the level or etage where the cloud formed both classifications lead to the same results.
The ten fundamental cloud types are: cirrus (Ci), cirrocumulus (Cc), cirrostratus (Cs) - also known as the high clouds; altocumulus (Ac), altostratus (As) and nimbostratus (Ns) - the middle level clouds; cumulus (Cu), cumulonimbus (Cb), stratocumulus (Sc), and stratus (St), which are classified as low clouds.
Note that certain cloud types often extend into other levels: altostratus into high, nimbostratus into high or low; cumulus and cumulonimbus into both middle and high.
The cloud genera (types) are further subdivided into cloud species.