Clouds from which Nimbostratus may form

(Section 2.3.6.5)

Nimbostratus evolves:

• Usually from thickening Altostratus (Ns altostratomutatus)
• Rarely from the thickening of a layer of Stratocumulus (Ns stratocumulomutatus)
• Rarely from the thickening of a layer of Altocumulus (Ns altocumulomutatus)
• Sometimes by the spreading out of Cumulonimbus (Ns cumulonimbogenitus)
• Very rarely, when these clouds produce rain, by the spreading out of Cumulus congestus (Ns cumulogenitus)