From Glossary of Meteorology


(Abbreviated Ac.) A principal cloud type (cloud genus), white and/or gray in color, that occurs as a layer or patch with a waved aspect, the elements of which appear as laminae, rounded masses, rolls, etc.

These elements usually are sharply outlined, but they may become partly fibrous or diffuse; they may or may not be merged; they generally have shadowed parts; and, by convention, when observed at an angle of more than 30° above the horizon, an altocumulus element subtends an angle between 1° and 5°. Small liquid water droplets invariably compose the major part of the composition of altocumulus. This results in sharpness of outline, small internal visibility (both common cumuliform characteristics), and in the occurrence of coronae and irisation (colored diffraction phenomena). With sufficiently low temperatures, ice crystals may appear in all forms of altocumulus, but mainly in the species castellanus or floccus, each unit of which may produce an individual snow shower. The crystals that fall from altocumulus sometimes produce parhelia, or a moon or sun pillar, any of which indicates the presence of tabular crystals. Should the composition become entirely ice crystals, the cloud would lose its characteristic sharpness of outline. Altocumulus often forms directly in clear air. It may be produced by an increase in size or thickening of the elements of an entire layer or patch of cirrocumulus (Ac cirrocumulomutatus); by subdivision of a layer of stratocumulus (Ac stratocumulomutatus); by transformation of altostratus (Ac altostratomutatus) or nimbostratus (Ac nimbostratomutatus); or by the spreading of cumulus or cumulonimbus (Ac cumulogenitus or Ac cumulonimbogenitus). Altocumulus frequently occurs in a given sky at different levels; also, it often is associated with clouds of other genera. Virga may appear with most of the species of altocumulus. This supplementary feature, however, should not be confused with the very white trails of ice crystals that frequently are formed with the dissipation of altocumulus floccus. When detached, the ice crystal trails are cirrus. Sometimes mamma occur with altocumulus. Cirrocumulus and stratocumulus are the clouds most easily confused with altocumulus. The elements of cirrocumulus never have shadows of their own, and nearly always are smaller. Stratocumulus elements are larger than those of altocumulus. The rolls or cells that are associated with altocumulus are thought to be a result of the absorption of terrestrial radiation and/or the presence of wind shear, which drives Rayleigh–Bénard convection or Kelvin–Helmholtz shear instability.
See cloud classification.