Mammatus clouds, meaning “mammary cloud”, is a cellular pattern of pouches hanging underneath the base of a cloud. They are most often associated with cumulonimbus clouds and severe thunderstorms, although they may be attached to other classes of parent clouds.
The name mammatus is derived from the Latin mamma, which means “udder” or “breast”.
They are formed by cold air sinking down to form the pockets contrary to the puffs of clouds caused by rising air through the convection of warm air.
Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or translucent. Because mammatus occur as a grouping of lobes, the way they clump together can vary from an isolated cluster to a field of mammae that spread over hundreds of miles. A lobe can last an average of 10 minutes, but a whole cluster of mamma can range from 15 minutes to a few hours.
While they may appear foreboding they are merely the messengers – appearing around, before or even after severe weather.