Mammatus cloud
From Freepedia
Mammatus (also known as mamma or mammatocumulus) is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud, often a cumulus or cumulonimbus. These pouches, typically measuring about half a kilometer in diameter, are often ragged, but may appear smooth. 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.
The mechanism of their production is poorly understood and has been little studied. The presence of very dry air beneath the cloud base is known to be a factor.
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 [1].
Sometimes thunderstorms manifest cumulonimbus with mammatus. The cumulonimbus is a dark, tall, dense vertical cloud; the presence of cumulonimbus with mammatus indicates severe atmospheric turbulence. 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.
