Mammatus (mamma) clouds are distinctive pouch-like protuberances that hang from the underside of a cloud layer, typically the anvil of a cumulonimbus. The name comes from the Latin mamma (breast or udder), describing the rounded, bulging shapes that can look dramatic when lit by low-angle sunlight. Each individual lobe is typically 1–3 km wide and 0.5–1 km deep.
The formation mechanism remains debated among atmospheric scientists, but the leading theory involves evaporative cooling: ice crystals and precipitation particles falling from the anvil into drier air below evaporate, cooling the air and causing it to become negatively buoyant — it sinks in rounded pockets rather than as a flat layer. Alternative theories invoke radiation cooling, wind shear instabilities, or simply the weight of precipitation-laden anvil material.
While often associated with severe weather, mammatus themselves are not dangerous — they typically appear on the trailing edge of a storm, after the most severe phase has passed. However, they indicate powerful convective activity in the vicinity and should be treated with respect. Mammatus can also form on the undersides of altostratus, altocumulus, and even volcanic ash clouds. Their appearance — especially illuminated by golden sunset light — produces some of the most spectacular and photographed cloud displays in nature.