C_M = 3

(a) TECHNICAL SPECIFICATION

Altocumulus translucidus at a single level.

(b) EXPLANATION

(i) The coding C_M = 3 applies to Altocumulus in patches or sheets at the same level, or to Altocumulus in a layer; the various elements of these clouds are neither very large nor very dark. If the cloud elements change at all, they do so in a hardly perceptible manner and they do not progressively invade the sky.

(ii) The sky may contain several Altocumulus patches or sheets at the same level and of different optical thicknesses. The definition of the variety translucidus permits individual patches or sheets to be identified as Altocumulus translucidus when their greater part is sufficiently translucent to reveal the position of the Sun or Moon. However, when the code refers to Altocumulus translucidus, it relates to the total amount of Altocumulus at the same level. The coding C_M = 3 can only be used when semi-transparent Altocumulus predominates at that level. A similar convention applies when code specifications refer to Altocumulus opacus. 

(iii) When Altocumulus translucidus (not invading the sky) is present at two or more levels, refer to C_M = 7.