Mammatus are most often associated with the anvil cloud and also severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds.
Mammatus clouds
By mlc1us @ 2012-05-17 14:39:48
WacoKidd110
@ 2012-05-17 14:53:00
Science lesson of the day :)