​​​​What is Mammatus Clouds?

Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air. Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form; a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed.

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.