C_M = 6

(a) TECHNICAL SPECIFICATION

Altocumulus cumulogenitus (or Altocumulus cumulonimbogenitus).

(b) EXPLANATION

(i) Altocumulus cumulogenitus results generally from the spreading out of the summits of Cumulus that reach a stable layer. Occasionally, Cumulus congestus in vertical development meet stable layers that cannot stop their growth completely; in this case, the Cumulus, after a temporary spreading out, resume their growth above the stable layer, at least in places. Thus, the Altocumulus cumulogenitus may appear on the lateral portion of Cumulus congestus.

(ii) Owing to their mode of formation, Altocumulus cumulogenitus occurs in patches. Initially, these patches with large and dark elements are fairly thick and opaque, and their undersurface may show a rippled relief. Later on, the patches thin out and finally break into separate elements. The same sky often shows Altocumulus patches in various stages of evolution.

(iii) When the Altocumulus cumulogenitus patches are seen in profile, they may show, especially at their borders, a cumuliform appearance. Care should be exercised not to confuse such patches with Altocumulus castellanus.

(iv) Altocumulus cumulogenitus should not be confused with the anvil of a Cumulonimbus or with Cirrus spissatus cumulonimbogenitus, both of which may show mamma at their lower surface and may resemble Altocumulus. However, Altocumulus never has the fibrous structure, the silky sheen and the whiteness of the anvils or of the Cirrus spissatus.

(v) Altocumulus accompanying Cumulonimbus (Ac cumulonimbogenitus) is also coded C_M = 6; it often forms while the Cumulonimbus is in its Cumulus stage of development.