Mammatus clouds

Mammatus are bulbous or pouch-shaped protrusions on the underside of a cloud, especially the flattened anvil spreading out from a cumulonimbus thunderstorm. The name comes from the Latin mamma, meaning breast. They are not a separate cloud genus: the term describes a supplementary cloud feature, which can appear on different cloud types. Individual pouches may form patterns across part of a cloud base, making the feature conspicuous in photographs and from the ground.

Mammatus formation involves small-scale interactions between clouds and the air beneath them. A leading explanation is that ice crystals or water droplets descend from the anvil into drier air below. Melting and evaporation—or sublimation of ice—remove heat from the surrounding air, making it denser and encouraging downward motion. This can create an instability that shapes the pouch-like lobes. Observations and models indicate that the process depends on atmospheric moisture, cloud particles and their settling; the precise mechanisms and conditions remain an active research subject rather than a fully settled single theory.

Mammatus are commonly seen beneath thunderstorm anvils, so they may occur in the vicinity of storms capable of heavy rain, hail or lightning. But the pouches themselves are not a type of severe weather and do not generate severe conditions. Their presence alone does not establish how strong a storm is or predict a tornado. For safety, people should rely on official warnings and forecasts rather than interpreting a cloud's appearance as a hazard forecast.

Because mammatus form at the boundary between cloud and surrounding air, they offer a visible example of atmospheric fluid dynamics, phase changes and buoyancy acting together. Researchers have used field observations and numerical models to test possible formation mechanisms. Studies emphasize that a process capable of producing mammatus under some conditions need not explain every case; different cloud and environmental conditions may contribute to the variety of observed forms.