C_M5 Ac progressively invading the sky

The altocumulus clouds of C_M5 gradually spread from one part of the horizon, often passing overhead, and may eventually reach the opposite horizon. These clouds generally thicken and usually appear thickest towards the direction from which they first appeared (top photograph). The advancing edge may consist of small cloudlets, often in the process of dissipation, which may cover a large expanse of the sky. The clouds often lie in parallel bands and may be in one or more layers. The coding C_M=5 is no longer applicable once the cloud stretch from horizon to horizon, or when the forward edge no longer progresses.

If during its progress across the sky, parts of the altocumulus change to altostratus or nimbostratus, the coding becomes C_M=7 instead of C_M=5. The bottom photographs were taken a short time apart and together they illustrate the spread of altocumulus with time.