Ominous-looking mammatus clouds at Żebbuġ Malta

Ominous-looking mammatus clouds at Żebbuġ Malta. A big thanks to Philip Ellul for the pic.

Mammatus clouds often form on the underside of cumulonimbus clouds, but are sometimes seen underneath other clouds as well. They can appear threatening, but the sinking air required to make these clouds actually indicates weakening of the storm associated with them.

While mammatus clouds usually form beneath weakening thunderstorms, the kinds of thunderstorms that have them at one time contained intense updrafts, which indicates conditions in the area were, and might still be, favourable for severe weather.