Mammatus Clouds

Mammatus clouds form in sinking air. (Most clouds form in rising air.) Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus, and stratocumulus. For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance. The baglike sacs that hang beneath the cloud resemble cow's udders.

Here are some examples of Mammatus clouds:

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