A pouch-like cloud feature often seen beneath a thunderstorm’s anvil is called mammatus. It consists of rounded lobes that hang from the underside of a cloud, creating a striking, cell-like pattern.
Mammatus most often develops beneath cumulonimbus anvil clouds, where sinking cold air creates pockets beneath regions of rising, convective air. The lobes may be smooth, ragged, translucent, or opaque, and they can be made of ice, liquid water, or a mixture of both.
The name comes from the Latin word mamma, meaning “udder” or “breast.” Although mammatus can accompany severe thunderstorms, the formations themselves do not cause tornadoes and do not guarantee that severe weather is occurring at ground level. The World Meteorological Organization classifies mamma as a supplementary cloud feature, not a separate cloud genus. The feature was described scientifically by William Clement Ley in 1894.