Clouds from which Altostratus may form

(Section 2.3.5.5)

Altostratus may form:

• By the thickening of a veil of Cirrostratus (As cirrostratomutatus)
• Sometimes by the thinning of a layer of Nimbostratus (As nimbostratomutatus)
• From an Altocumulus layer when widespread ice crystal virga falls from the latter (As altocumulogenitus)
• Sometimes, particularly in the tropics, by the spreading out of the middle or upper part of Cumulonimbus (As cumulonimbogenitus)