Clouds from which Altocumulus may form

(Section 2.3.4.5)

Altocumulus may evolve from:

• An increase in size or a thickening of at least some elements of a patch, sheet or layer of Cirrocumulus (Ac cirrocumulomutatus )
• Subdivision of a layer of Stratocumulus (Ac stratocumulomutatus)
• Transformation of Altostratus (Ac altostratomutatus)
• Transformation of Nimbostratus (Ac nimbostratomutatus)
• Spreading of the tops of Cumulus when they reach a middle level stable layer (Ac cumulogenitus). The vertical development of the Cumulus may:
  • Cease at the stable layer, resulting in patches of Altocumulus spreading out from the top of the Cumulus
  • Temporarily stop at the stable layer and then resume their growth in places or throughout, resulting in Altocumulus on the sides of the Cumulus

Altocumulus may also be observed on or near the sides of Cumulonimbus. This Altocumulus often forms while the mother-cloud is still in the Cumulus stage. Nevertheless, the Altocumulus is classified as cumulonimbogenitus, not cumulogenitus.