C_M = 7

(a) TECHNICAL SPECIFICATION

Altocumulus duplicatus, or Altocumulus opacus in a single layer, not progressively invading the sky, or Altocumulus with Altostratus or Nimbostratus.

(b) EXPLANATION

The specification C_M = 7 includes the following skies:

(i) Patches, sheets or layers of Altocumulus at different levels (duplicatus); these patches, sheets or layers may be Altocumulus translucidus usually opaque in places, or Altocumulus opacus. The elements of this Altocumulus are not changing continually; the clouds are not progressively invading the sky.

(ii) Patches, sheets or a layer of Altocumulus opacus at a single level. The elements are not changing continually; the clouds are not progressively invading the sky. The sky may contain several Altocumulus patches or sheets at the same level and of different optical thicknesses. The definition of the variety opacus permits individual patches or sheets to be identified as Altocumulus opacus when their greater part is sufficiently opaque to hide the position of the Sun or Moon. However, when the code refers to Altocumulus opacus, it relates to the total amount of Altocumulus at the same level. The case of C_M = 7 can only be used when opaque Altocumulus predominates at that level.

(iii) Altocumulus together with Altostratus or Nimbostratus may be observed in the following arrangements:

• A single or a multiple layer showing partly the characteristics of Altocumulus, partly those of Altostratus or Nimbostratus. This sky often results from the transformation when Altocumulus locally changes and acquires the appearance of Altostratus or Nimbostratus, or Altostratus or Nimbostratus break up into Altocumulus.
• Altostratus translucidus or opacus above patches of Altocumulus at one or several levels.
• A rather low veil, often hardly discernible, together with higher Altocumulus.