Mammatus (also known as mamma or mammatocumulus, meaning "breast-cloud") is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud, often a cumulus or cumulonimbus.
Their color is normally a bluish gray, the same as that of the host cloud, but direct illumination from the setting sun and other clouds may cause a gold or reddish cast.
Mammatus can persist anywhere from minutes to hours, diffusing and disappearing over time.
Mammatus only occur where cumulonimbus are present; however, they can drift up to 25 miles away from a thunderstorm. The atmosphere must also meet certain conditions, which include a moist and unstable middle to upper atmosphere over a very dry, lower layer of the atmosphere. An updraft then must occur, which shapes the mammatus into the pouch-like shape.
Mammatus clouds tend to form more often during warm months. In the United States, they tend to occur more often over the midwest and eastern portions of the country, though they can and do occur more infrequently over the west and southwest.
Mammatus has often been linked with the occurrence of tornadoes, particularly during the 1950s and 1960s. Though tornadic storms often produce mammatus under their downwind anvil, many weak storms and even stratiform clouds also produce mammatus. Contrary to common misconceptions, mammatus are not precursors to tornadoes, but are a possible byproduct.
It is very common for storms producing mammatus clouds also to produce wind shear, and possibly - though less likely - ball lightning; therefore, aviators are strongly cautioned to avoid cumulonimbus with mammatus.
Mammatus Clouds Over Saskatchewan NASA - October 23, 2012
Normal cloud bottoms are flat. This is because moist warm air that rises and cools will condense into water droplets at a specific temperature, which usually corresponds to a very specific height. As water droplets grow, an opaque cloud forms. Under some conditions, however, cloud pockets can develop that contain large droplets of water or ice that fall into clear air as they evaporate. Such pockets may occur in turbulent air near a thunderstorm. Resulting mammatus clouds can appear especially dramatic if sunlit from the side. These mammatus clouds were photographed over Regina, Saskatchewan, Canada during the past summer.
Mammatus Clouds Over Olympic Valley NASA - February 20, 2011
These mammatus clouds were photographed last August over Olympic Valley, California, USA. Normal cloud bottoms are flat because moist warm air that rises and cools will condense into water droplets at a very specific temperature, which usually corresponds to a very specific height. After water droplets form that air becomes an opaque cloud. Under some conditions, however, cloud pockets can develop that contain large droplets of water or ice that fall into clear air as they evaporate. Such pockets may occur in turbulent air near a thunderstorm, being seen near the top of an anvil cloud, for example. Resulting mammatus clouds can appear especially dramatic if sunlit from the side.
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