Mammatus Clouds

Mammatus clouds, pouch-like formations that are generally found protruding from the underside of cumulonimbus clouds. However, they can also develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus. In the U.S., mammatus occurring in cumulonimbus clouds are more common and are often indicative of a particularly strong - or even tornadic - storm.

Image credit: ©University Corporation for Atmospheric Research