C_M = 4

(a) TECHNICAL SPECIFICATION

Patches (often lenticular) of Altocumulus translucidus, continually changing and occurring at one or more levels.

(b) EXPLANATION

(i) The irregularly arranged elements of Altocumulus patches of the specification C_M = 4 are continually changing in shape; they often appear to be dissolving in some places and forming in others. The cloud patches are of limited horizontal extent, and their elements are continually changing; as a consequence, they usually belong to the variety translucidus and only rarely to the variety opacus. The patches as a whole may have the form of large lenses and may occur at one or more levels. The clouds are not progressively invading the sky.

(ii) The coding C_M = 4 is applicable not only to the above-described patches that consist of numerous, relatively small, continually changing elements, but also to those relatively stable clouds that consist of one single smooth lenticular element or of a pile of such elements.

(iii) These clouds may occur in the form of accessory clouds (pileus or velum) either near or fairly distant from the upper part of Cumulus or Cumulonimbus.

(iv) Lenticular clouds are frequently observed in hilly or mountainous regions.