A principal cloud type (cloud genus) in the form of a gray or bluish (never white) sheet or layer of striated, fibrous, or uniform appearance.
Altostratus very often totally covers the sky and may, in fact, cover an area of several thousand square miles. The layer has parts thin enough to reveal the position of the sun, and if gaps and rifts appear, they are irregularly shaped and spaced. Within the rather large vertical extent of altostratus (from several hundred to thousands of feet) a very heterogeneous particulate composition may exist. In this most complete case, there may be distinguished 1) an upper part, mostly or entirely ice crystals; 2) a middle part, a mixture of ice crystals and/or snowflakes and supercooled water droplets; and 3) a lower part, mostly or entirely supercooled or ordinary water droplets. A number of partial combinations of these composition types may occur, but never an entire cloud like 3) above. The particles are widely enough dispersed so as not to obscure the sun except by its thickest parts, but rather to impose a "ground-glass" effect upon the sun's image, and to prevent sharply outlined shadows from being cast by terrestrial objects. Halo phenomena do not occur. Altostratus is a precipitating cloud (praecipitatio) and therefore often is accompanied by virga and mamma. Rain, snow, ice pellets, etc., are present in the cloud and under its base, frequently rendering the base quite indistinct, particularly when the precipitation does not reach the ground. When precipitation reaches the ground, it is usually very light and of a relatively continuous nature. Altostratus may be formed by the thickening of cirrostratus (As cirrostratomutatus), or by the thinning of nimbostratus (As nimbostratomutatus). If widespread precipitation develops in altocumulus, altostratus may result (As altocumulogenitus). Sometimes, particularly in the tropics, altostratus may be produced by the spreading of the middle or upper portion of cumulonimbus (As cumulonimbogenitus). Cirrostratus and nimbostratus are the two other forms most easily confused with altostratus. In the first case, it should be remembered that cirrostratus does allow terrestrial shadows and frequently produces halo phenomena. Nimbostratus is darker colored, hides the sun, is more uniform in optical thickness, and always produces precipitation. At night, if precipitation does not reach the ground, it is conventional to call the doubtful layer altostratus. Any stratiform (layered) cloud necessarily forms because further vertical development is inhibited by the presence of a temperature inversion. Rolls and cells in altostratus are thought to be of similar origin to those in altocumulus.