A Murus, or wall cloud, is a localized, persistent, and often abrupt lowering of cloud from the base of a Cumulonimbus (Cb), associated with a supercell or severe multicell storm; typically develops in the rain-free portion of a Cumulonimbus and indicates an area of strong updrafts, and can indicate the presence, or impending development, of a tornado. It is a supplementary feature of cloud.

A rain-free base with a wall cloud lowering in the foreground and precipitation in the background. Source:NOAA

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