Cloud, visible collection of water droplets suspended in the atmosphere. Clouds whose lower surfaces touch the ground are usually called fog. The water droplets are very small, indeed of colloidal size; they must coagulate or grow before falling as rain or snow. There are 3 main cloud types. Cumulus (heap) clouds, formed by convection, and often mountain- or cauliflower-shaped, are found from about 2,000 ft (610 m) up as far as the tropopause, even temporarily into the stratosphere. Cirrus (hair) clouds are composed almost entirely of ice crystals. They appear feathery, and are found at altitudes above about 20,000 ft (6,100 m). Stratus (layer) clouds are lowlying, found between ground level and about 5,000 ft (1,524 m). Other types of cloud include cirrostratus, cirrocumulus, altocumulus, altostratus, cumulonimbus, stratocumulus, and nimbostratus.
See also: Weather.