Mammatus Clouds
sagging pouch-like structures

Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air.

Photograph by: Manikin
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.

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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.