Wall cloud

Wall clouds (or pedestal clouds) are localized, large and persistent lowering cloud formations that develop below the rain-free region of thunderstorms. Rotating wall clouds are indicative of mesocyclones in thunderstorms. Wall clouds do not always rotate, though.
Wall clouds are formed by a process known as entrainment, when an inflow of warm, moist air rises and overpowers rain-cooled air during a downwind downdraft.