Mammatus Clouds Forming At Sunset Ahead Of Severe Thunderstorm. Royalty-Free Stock Photography

Mammatus or mammatocumulus, meaning "mammary cloud" is a cellular pattern of pouches hanging underneath the base of a cloud, typically a cumulonimbus raincloud, although they may be attached to other classes of parent clouds. Mamma is a cloud supplementary feature rather than a genus, species or variety of cloud. The distinct "lumpy" undersides are formed by cold air sinking down to form the pockets contrary to the puffs of clouds rising through the convection of warm air.