A rare cloud formation called a mammatus in Regina, Saskatchewan.

Mammatus clouds above Big Cottonwood Canyon near Salt Lake City, Utah

Mammatus, also known as mammatocumulus (meaning "mammary cloud" or "breast cloud"), is a term cloud. The name mammatus is derived from the Latin mamma (meaning "udder" or "breast").
applied to a cellular pattern of pouches hanging underneath the base of a meteorological.

Characteristics

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.^{[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. Due to the intensely sheared environment in which mammatus form, aviators are strongly cautioned to avoid cumulonimbus with mammatus.

Mammatus clouds over New York City, 2009

Mammatus may appear as smooth, ragged or lumpy lobes and may be opaque or translucent. Because mammatus occur as a grouping of lobes, the way they clump together can vary from an isolated cluster to a field of mammae that spread over hundreds of kilometers to being organized along a line, and may be composed of unequal or similarly-sized lobes. The individual mammatus lobe average diameters of 1–3 km and lengths on average of 0.5 km. A lobe can last an average of 10 minutes, but a whole cluster of mamma can range from 15 minutes to a few hours. They usually are composed of ice, but also can be a mixture of ice and liquid water or be composed of almost entirely liquid water.
True to their ominous appearance, mammatus clouds are often harbingers of a coming storm or other extreme weather system. Typically composed primarily of ice, they can extend for hundreds of miles in each direction and individual formations can remain visibly static for ten to fifteen minutes at a time. While they may appear foreboding they are merely the messengers - appearing around, before or even after severe weather.

Mammatus Clouds in San Francisco, California

Mammatus clouds and crepuscular rays over San Francisco Bay

Mammatus clouds in Milan, Italy, in July 2005 on a very hot, humid day without wind

Cumulus Mammatus clouds between Hamilton and Missoula, Montana

Aerial photo of mammatus clouds over central New South Wales, Australia,

Mammatus Clouds Over Sierras de Córdoba Mountains, Argentina

Mammatus Clouds over the Pacific Coast San Francisco

Mammatus clouds over the University of Central Florida in Orlando, Florida, 2011.

Mammatus clouds in Tulsa, Oklahoma, 1973

Mammatus clouds over Squaw Valley Ski Resort, Olympic Valley, California

Just before a large thunderstorm in Altomuenster, Bavaria, Germany