C_M7 Altocumulus with As or Ns

Click the small image for full size version.

Gallery
© R.K. Pilsbury
© S.G. Cornford
© C.J. Richards

When altocumulus occurs together with altostratus or nimbostratus, C_M is coded as 7 (unless C_M=9 applies). Code as C3 if altocumulus predominates, C4 if altostratus predominates or C5 if nimbostratus predominates.

The clouds may occur as a single or a multiple layer, showing partly the characteristics of altocumulus and partly those of altostratus or nimbostratus. This type of sky results from transformation processes by which altocumulus changes locally and acquires the appearance of altostratus/nimbostratus, or altostratus/nimbostratus breaks to altocumulus.

Altocumulus and altostratus at the same level are depicted in the top photograph. However, in the middle photograph the altocumulus and altostratus occur at different levels.

The bottom photograph, taken just before sunset on a winter's day, shows broken altocumulus and altostratus after the clearance of a belt of rain. The ragged appearance of the under surface is due to precipitation trails (virga of rain or snow).

Code figure C_M=7 is also used to report altocumulus in two or more layers, or thick altocumulus in a single layer. The code figures C_M=6, 5 and 4 take precedence over C_M=7. Refer to the pictorial guide for descriptions of these other cloud types of C_M7.

Classification
Description: Opaque layer of Altocumulus not progressively invading the sky.	Designation: CM=7 (C3 if altocumulus predominates, C4 if altostratus predominates, C5 if nimbostratus predominates)
	Latin name: Altocumulus opacus (Ac op) [plus other varieties].

Related pages: Cloud classification | C_L guide | C_M guide | C_H guide | Cloud index