Mammatus is one of the most spectacular and distinctive of all cloud formations, making it a favorite with weather-watchers and photographers alike. It consists of pendulous globules of cloud (mamma is the Latin word for breast) that hang from the underside of the anvil of a thundercloud. Mammatus is always associated with mature cumulonimbus clouds and is therefore an indicator of severe weather conditions.

The formation occurs as a result of a process that can be described as reverse-direction convection. During a thunderstorm, warm, moist updrafts rise to the top of the troposphere. Here the temperature levels off and the air stabilizes. This causes the rising cloud to expand horizontally over areas of cooler, cloud-free air. The temperature difference between the two air masses creates instability under the anvil, which causes pockets of warm, moist air in the cloud to convect downward. This reverse-direction convection is enhanced by the effects of gravity and by precipitation from the cloud. The process produces near-symmetrical protuberances on the underside of the anvil, known as the mammatus, which may cover large areas.

Tornado Alert

As the anvil of a mature cumulonimbus cloud can spread out over hundreds of square miles, the center of the storm may be some distance away from the mammatus formation. However, mammatus normally occurs soon after the cumulonimbus cloud has reached maximum growth and intensity and is usually a sign of a particularly vigorous thunderstorm. In the United States, this formation is regarded as a clear warning sign of possible tornado development.

Airline pilots will normally take action to avoid any cumulonimbus clouds, but particularly those bearing mammatus formations, as these indicate especially severe turbulence within the cumulonimbus. Mammatus may be observed wherever cumulonimbus clouds occur, but it is particularly common in areas where thunderstorms are severe, such as tropical and subtropical areas. Because the formation is associated with mature cumulonimbus clouds, it is most likely to be seen from mid-afternoon to early evening, when ground heating and associated convective activity have reached a maximum.