Mammatus Clouds

Mammatus cloud formation at the passing of a storm,

Characterized as rounded, sagging, pouch-like structures, mammatus clouds are a rare formation created by sinking air on the underside of cumulonimbus clouds. They are not, in fact, a sign of a tornado, but rather an indication that the worse of the weakening storm has passed.

The drooping structures are formed from precipitation particles occurring in high concentration due to momentum loss. The updraft that carries the precipitation enriched air loses that upward momentum causing the air to start spreading horizontally. The heavily saturated air sinks below the lighter surrounding air. Temperature increase as the subsiding air descends evaporating precipitation particles; however, more energy is required for evaporation than subsidence produces resulting in the sinking air being cooler than its surroundings.

Mammatus clouds will dissolve in time, but the lifespan depends on the contents. If the sinking air contains snow crystals or large drops of precipitation, greater energy for evaporation is needed resulting in long lived clouds.

More of my photos of this phenomenon can be found here showing the progression of formation.