Formation
Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system, and they can be found over widespread areas. Their altitude is from 2,400 to 6,100 metres (7,900 to 20,000 ft). They are primarily composed of water droplets. They can rise at velocities up to 0.05 to 0.1 metres per second (0.16 to 0.33 ft/s). They can also be the result of the thickening of a cirrostratus cloud. Altostratus is rarely seen alone in the sky, as there is usually some accompanying stratocumulus or stratus fractus underneath it, this being due to the increased moisture beneath the altostratus or cumulus clouds dying as the altostratus reduces surface heating by the sun.
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