"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."
Polar stratospheric clouds or PSCs, also known as nacreous clouds, occur in the winter polar stratosphere at altitudes of 15,000–25,000 meters (49,000–82,000 ft). Though beautiful, they are implicated in the formation of ozone holes.