Wall cloud

A wall cloud (or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which sometimes forms tornadoes. It is typically beneath the rain-free base (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do not rotate, however.