moving into the wall cloud, as it is also an inflow feature. Most movement is horizontal, but some rising motion is often apparent as well. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base; this feature is a collar cloud. Another accessory cloud is the flumen, commonly known as the beaver's tail. It is formed by the warm, humid inflow of a strong thunderstorm, and is often mistaken for tornadoes. Although the presence of a flumen is associated with tornado risk, the flumen, similar to scud clouds, does not rotate. Wall cloud vs. shelf cloud Many storms contain shelf clouds, which are often mistaken for wall clouds, since an approaching shelf cloud appears to form a wall made of cloud and may contain turbulent motions. 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. Also, shelf clouds tend to move outward away from the precipitation area of a storm. Shelf clouds most often appear on the leading edge of a thunderstorm as they are formed by condensation from cool outflow of the storm that lifts warmer air in the ambient environment (at the outflow boundary). When present in a supercell thunderstorm these shelf clouds on the leading edge of a storm are associated with the forward flank downdraft (FFD). Shelf clouds in supercells also form with the rear flank downdraft (RFD), although these tend to be more transitory and smaller than shelf clouds on the forward side of a storm. A wall cloud will usually be at the rear of the storm, though small, rotating wall clouds (a feature of a mesovortex) can occur within the leading edge (typically of a quasi-linear convective system (QLCS) or squall line) on rare occasion. Supercell and tornado significance The wall cloud feature was first identified by Ted Fujita and as associated with tornadoes in tornadic storms following a detailed site investigation of the 1957 Fargo tornado. In the special case of a supercell thunderstorm, but also occasionally with intense multicellular thunderstorms such as the aforementioned QLCS, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm that is most likely