Mammatus clouds are unusual and distinctive cloud formations characterized by a series of bulges or pouches emerging from the base of a cloud. They are typically associated with large cumulonimbus clouds and often form in the presence of thunderstorms due to their huge mass of unstable air. Mammatus clouds are primarily composed of ice and can extend for hundreds of miles in each direction. These clouds are formed by sinking cold air, which is the opposite of how most clouds are formed.
Mammatus clouds are often seen as harbingers of a coming storm or other extreme weather system, but they are not the cause or indicators of severe thunderstorms and tornadoes. They can form in association with cumulonimbus clouds, which can bring thunderstorms, hail, heavy rain, and lightning. Mammatus clouds can also form under other cloud types, such as cirrus, stratocumulus, and altostratus clouds.