Altocumulus

Definition

A genus of middle-level cloud, usually a layer of patches, rolls or rounded masses, sometimes with a rippled appearance. Elements are thumbnail-sized at arm's length and often show slight shading on their undersides.

Field note

The thumbnail test separates it from stratocumulus (hand-sized) and cirrocumulus (fingertip-sized). Watch for castellanus turrets in mid-morning: they signal a thunderstorm risk later in the day.

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Altocumulus: the mid-level checkerboard and what it warns about

Altocumulus is the most useful cloud in the sky for short-range forecasting, and it is also the one people most often mistake for something else. It sits between roughly 2000 and 6000 metres, is made of water droplets rather than ice at its lower levels, and often arranges itself into rows, patches or a sheep-like mottling that gives away what the atmosphere above is doing.

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Related terms

• Altostratus · Altostratus
• Stratocumulus · Stratocumulus
• Lenticularis · lenticularis