Editing Altostratus cloud (section)

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{{Short description|Type of middle-altitude cloud}}
[[File:2020-12-31 16 06 58 Altostratus with a wavy undulating base above the Dulles section of Sterling, Loudoun County, Virginia.jpg|upright=1.6|thumb|right|alt=An altostratus radiatus cloud showing the characteristic parallel lines of cloud.|''Altostratus radiatus'' cloud showing distinctive parallel bands]]
'''Altostratus''' is a middle-altitude [[cloud]] genus made up of water droplets, [[ice crystal]]s, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite [[Opacity (optics)|opaque]].

Altostratus clouds usually predict the arrival of [[warm front]]s. Once altostratus clouds associated with a warm front arrive, continuous rain or snow will usually follow in the next 12 to 24 hours. Although altostratus clouds predict the arrival of warmer, wetter weather, they themselves do not produce significant precipitation. Thunderstorms can be embedded in altostratus clouds, however, bringing showers.

Because altostratus clouds can contain ice crystals, they can produce some optical phenomena like [[iridescence]] and [[Corona (optical phenomenon)|coronas]].