A mammatus cloud is not a cloud characteristic of tornadoes, but it is often associated with severe thunderstorms. Mammatus clouds consist of small, smooth puffs of cloud that hand down from the base of a larger cloud, often the anvil of a thunderstorm.

On Jupiter, Saturn, Uranus, and Neptune.

They form on the under side of mountains.

Mammatus. Mammatus is cloud that hangs from an anvil of a mature storm cloud and brings severe weather especially tornadoes. Another one is called Cumulonimbus. These clouds bring VERY heavy rain or thunder storms.

Mammatus clouds form on the underside of a storm cloud and are most often associated with the anvil cloud that extends from a cumulonimbus. They have also been seen under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds.