Clouds may be classified by form and by height. Luke Howard (1772 - 1864), a British pharmacist was the first to describe cloud forms using Latin terms such as cirrus, cumulus or stratus. The division of clouds into ten basic cloud forms, or cloud genera is based on his publications. Cloud genera are further subdivided into cloud species (desribing shape or structure) and cloud variety. The World Meteorological Organisation (WMO) defined the terminology and standard two- and three-letter abbreviations in its International Cloud Atlas.
Another division of clouds is based upon the range of altitude in which a cloud genus normally occurs.
1. Low-level clouds (0-2 km above mean sea level)
stratus (St) - extensive, shallow cloud sheet, often yielding drizzle or light rain
stratocumulus (Sc) - shallow cloud sheet, broken into roughly recurring cumuliform masses, often yielding drizzle or snow
cumulus (Cu) - separate, hill-shaped clouds, (hence cumuliform) with flat, and often level, bases which are often at the same height
cumulonimbus (Cb) - large, high cumulus, with dark bases, often producing showers. The cloud tops are often formed of ice crystals
2. medium-level clouds (2-4 km above mean sea level)
altocumulus (Ac) - shallow cloud sheet broken into roughly regular, rounded clouds
altostratus (As) - featureless, thin, translucent cloud sheet
nimbus ( Ns) - extensive, very dark cloud sheet, usually yielding precipitation
3. high clouds (tropical regions 6-18 km high, temperate regions 5-14 km high, polar regions 3-8 km high)
cirrus (Ci) - separate, white, feather-like clouds
cirrocumulus (Cc) - shallow, more or less regular patches or ripples of cloud
cirrostratus (Cs) - shallow sheet of largely translucent cloud