Cirrocumulus

Definition

A genus of high cloud forming a thin, sometimes rippled sheet of very small white grains, mostly ice with some supercooled water. Elements are fingertip-sized at arm's length and show no shading.

Field note

A widespread rippled sheet is called a mackerel sky. It forecasts rain only when it is thickening and followed by cirrostratus; patchy cirrocumulus in an otherwise clear sky forecasts nothing.

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Cirrocumulus: the mackerel sky and the limits of the folklore

Cirrocumulus is a thin, high sheet of very small white grains at 6000 to 10000 metres, made mostly of ice crystals with some supercooled water. When it covers a large area in ripples it is called a mackerel sky, and the folklore around it is older than any meteorological service. Some of that folklore is good; some of it is not.

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Related terms

• Altocumulus · Altocumulus
• Undulatus · undulatus
• Floccus · floccus