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Mammatus cloud
Mammatus, also known as mammatocumulus (meaning "mammary cloud"),〔http://weatherwing.com/Cumulonimbus-Mammatus.html〕〔http://www.erh.noaa.gov/er/pit/mam98.htm〕 is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name ''mammatus'' is derived from the Latin ''mamma'' (meaning "udder" or "breast"). According to WMO International Cloud Atlas ''mamma'' is a cloud supplementary feature.〔http://library.wmo.int/pmb_ged/wmo_407_en-v1.pdf〕
== 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. 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. They also attach to the bottom of other clouds.
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 are usually 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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