Wall cloud seen here indicates a Mesocyclone - a rapidly rotating air mass within a thunderstorm that often gives rise to a tornado A wall cloud (or pedestal cloud) is a large, lowering cloud formation that develops beneath the base of a cumulonimbus cloud that often 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. Wall Cloud Vs. Shelf CloudSome storms contain shelf clouds, which are often mistaken for wall clouds, since an approaching shelf cloud appears to form a wall made of cloud. Generally, a shelf cloud appears on the leading edge of a storm, and a wall cloud is usually at the rear of the storm, though small, rotating wall clouds (a feature of a mesovortex) can occur within the leading edge on rare occasion. Wall clouds are inflow clouds and tend to slope inward, or toward the precipitation area of a storm. Shelf clouds, on the other hand, are outflow clouds that jut outward from the storm, often as gust fronts.