Cumulonimbus, along with cumulus, are the most dramatic types of cloud. Rather than spreading out in bands at a fairly narrow range of elevations, like other clouds, they can span 6,500 to 20,000 feet (2,000 to 6,000 meters)—sometimes well above the level of jetliner flights.
"Mammatus clouds are most often associated with the anvil cloud and 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. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or perhaps even a tornadic storm."
A "Skypunch" in Switzerland. This rare phenomenon occurs when ice crystals form above the clouds and fall downward.
Storm forming one late May afternoon in the town of Terry eastern Montana