Mammatus are most often associated with the anvil cloud that extends from a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds.[citation needed] In the United States, sky gazers may be most familiar with the very distinct and more common cumulonimbus mammatus. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or maybe even a tornadic storm. These tend to form more often during warm months and over the midwest and eastern portions of the United States, and more infrequently over the west and southwest. Due to the intensely sheared environment in which mammatus form, avi...

Mammatus are most often associated with the anvil cloud that extends from a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds.[citation needed] In the United States, sky gazers may be most familiar with the very distinct and more common cumulonimbus mammatus. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or maybe even a tornadic storm. These tend to form more often during warm months and over the midwest and eastern portions of the United States, and more infrequently over the west and southwest. Due to the intensely sheared environment in which mammatus form, aviators are strongly cautioned to avoid cumulonimbus with mammatus.