(a) TECHNICAL SPECIFICATION
(i) The irregularly arranged elements of Altocumulus patches of the specification CM = 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 CM = 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.
(iv) Lenticular clouds are frequently observed in hilly or mountainous regions.