Clouds have been classified by various systems according to form, altitude, and the physical processes generating them. The World Meteorological Organization (International Cloud Atlas, 2017 online) classifies 10 genera in three major groups (cumulus or heap clouds, stratus or sheet clouds, and cirrus or fibrous clouds) by criteria essentially based on cloud form. Some of the genera are subdivided according to variations in internal shape and structure, to give fifteen species. Additional or supplementary features, e.g. transparency, arrangement, and characteristics of growth, are defined by Latin names as variants and accessory types of cloud. The cloud genera, with their abbreviations, are: cirrus (Ci), cirrocumulus (Cc), cirrostratus (Cs), altocumulus (Ac), altostratus (As), nimbostratus (Ns), stratocumulus (Sc), stratus (St), cumulus (Cu), and cumulonimbus (Cb). Clouds can also be referred to, according to their composition, as water or ice clouds; combinations of these are called mixed cloud. Clouds are also classified by the average height of their bases as: high (3–8 km, including Ci, Cc, and Cs); middle (2–4 km, including Ac, As, and Ns); and low (0–2 km, including St, Sc, Cu, and Cb). Cloud bases of high and medium clouds are highest at the equator and decrease with increasing latitude.
The 2017 additions to the International Cloud Atlas are:
asperitas A supplementary cloud feature comprising large, wave-like features resembling exaggerated undulations in the cloud base, as seen from below. It occurs in association with altocumulus and stratocumulus.
cataracta See cataractagenitus.
cataractagenitus (cataracta) Stratus cloud that forms above a waterfall, where water is broken into spray.
cauda (tail cloud) A supplementary cloud feature comprising a tail-shaped extension from the main precipitation region of a murus, seen in association with a cumulonimbus.
cavum An accessory cloud comprising a thin veil with small cloud elements, seen in association with cirrocumulus, altocumulus, and stratocumulus.
flamma See FLAMMAGENITUS.
flammagenitus (flamma) Cumulus cloud that develops in air rising by convection above a forest fire, wildfire, or volcanic eruption.
fluctus A short-lived supplementary cloud feature comprising a formation resembling curls or breaking waves on the upper surface of clouds. It occurs in association with cirrus, altocumulus, stratocumulus, stratus, and cumulus.
flumen (inflow band) An accessory cloud feature associated with a supercell cumulonimbus marking the region where air is flowing into the cloud.
homogenitus A variety of cirrus cloud formed from a condensation trail that has persisted for at least 10 minutes.
homomutatus A variety of cirrus cloud that develops when strong winds modify the form of long-lasting cirrus homogenitus.
inflow band See flumen.
murus (wall cloud) A localized, persistent, supplementary cloud feature that often develops abruptly from the base of a supercell or multicell cumulonimbus. It comprises a lowering of the base in the rain-free portion of the cloud. If the murus exhibits significant rotation it may produce funnel clouds and tornadoes.
silva See SILVAGENITUS.
silvagenitus (silva) Stratus cloud that forms above a forest canopy due to high humidity caused by evaporation and transpiration.
tail cloud See CAUDA.
wall cloud See MURUS.
wedge tornado A tornado in which the funnel is at least as wide as the distance from the ground to the cloud base.