Wall cloud nearly touching surface. No location available. A wall cloud (or pedestal cloud) is a large, lowering, and rotating base of a cumulonimbus cloud that potentially forms tornadoes. It is typically beneath the rain-free base (RFB) portion of a deep cumulus cloud (normally cumulonimbus but on rare occasion cumulus congestus), and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of the ambient cloud base. Most strong tornadoes form from wall clouds.