Altostratus is gray or bluish (never white) sheet or layer of striated, fibrous, or uniform appearance. This cloud type has varying depth, thin enough to reveal the position of the Sun as though viewing it through frosted glass even though it often completely covers the sky. Altostratus may cover an area of several thousand square miles to a thickness of several hundred to several thousand feet. It often contains ice crystals and/or snowflakes mixed with super-cooled (below freezing) water droplets near the top of the cloud and water droplets mixed with super-cooled water droplets nearer the bottom. Altostratus may be identified by its mid-level altitude and the lack of sharply outlined shadows on the ground.
Altostratus is a precipitating cloud often accompanied by virga or mamma. Its base may be indistinct due to the presence of falling precipitation aloft that evaporates due to drier air below the cloud base. If precipitation continues this Principal Type may evolve into Nimbostratus. Altostratus may form from thickening cirrostratus or by the thinning of Nimbostratus. Both Altostratus and Cirrostratus types may be confused with Nimbostratus but it should be remembered that cirrostratus allows shadows on the ground and Nimbostratus is darker, hides the Sun, and is uniform in thickness.