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.
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Wall clouds are caused by the ascending and converging inflow air of the updraft ingesting moist, rain cooled air from the normally downwind downdraft. In supercells, this is the forward flank downdraft (FFD). Since temperature tends to decrease and dew point (moisture content) to increase as the updraft entrains this air, saturation occurs sooner as the air rises. Wall clouds may form as a descending of the cloud base or may form as rising scud consolidates and organizes.
Wall clouds can be anywhere from a fraction of a mile (0.25 km) wide to over five miles (8 km) across, and in the Northern Hemisphere typically form at the south or southwest end of a supercell. Wall clouds form in the inflow region, on the side of the storm coinciding with the direction of the steering winds (deep layer winds through the height of the storm). Rotating wall clouds are visual evidence of a mesocyclone.
Some wall clouds have a feature similar to an "eye". Attached to many wall clouds, especially in moist environments, is a tail cloud, a ragged band of cloud and cloud tags (fractus) extending from the wall cloud toward the precipitation.[1] It can be thought of as an extension of the wall cloud in that not only is it connected to the wall cloud but also that condensation forms for a similar reason. Cloud elements may be seen to be moving into the wall cloud, as it is 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.
Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud.[2] Generally, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm, though small rotating wall clouds associated with mesovortices can occur within the leading edge on rare occasion.[2] Wall clouds will tend to slope in, or toward the precipitation area, whereas shelf clouds as outflow clouds will jut outward from the storm. Wall clouds are inflow features with (often warm) air moving towards them whereas shelf clouds are an outflow feature with cool air moving away from the storm, often as a gust front.
The wall cloud feature was first identified by Ted Fujita associated with tornadoes in tornadic storms.[1][3] In the special case of a supercell thunderstorm but also occasionally with intense multicellular thunderstorms, the wall cloud will often be seen to be rotating. A rotating wall cloud is the area of the thunderstorm that is most likely to produce tornadoes, and the vast majority of intense tornadoes.
Tornadogenesis is most likely when the wall cloud is persistent with rapid ascension and rotation. The wall cloud typically precedes tornadogenesis by ten to twenty minutes but may be as little as one minute or more than an hour. Often, the degree of ascension and rotation increase markedly shortly before tornadogenesis, and sometimes the wall cloud will descend and "bulk" or "tighten". Tornadic wall clouds tend to have strong, persistent, and warm inflow air. This should be sensible at the surface if one is in the inflow region; in the Northern Hemisphere, this is typically to the south and southeast of the wall cloud. Large tornadoes tend to come from larger, lower wall clouds closer to the back of the rain curtain (providing less visual warning time if you're in the path of an organized storm).
Although it is rotating wall clouds that contain most strong tornadoes, many rotating wall clouds do not produce tornadoes. Absent a low-level boundary, tornadoes very rarely occur without a rear flank downdraft (RFD), which usually manifests itself visually as a drying out of clouds, called a clear slot or notch. The RFD initiates the tornado, occludes around the mesocyclone, and when it wraps completely around, cuts off the inflow causing death of the low-level mesocyclone and tornadolysis. Therefore, in most cases, the RFD is responsible for both the birth and the death of a tornado.
Usually, but not always, the dry slot occlusion is visible (assuming one's line of sight is not blocked by precipitation) throughout the tornado life cycle. The wall cloud withers and will often be gone by the time the tornado lifts. If conditions are favorable, then, often even before the original tornado lifts, another wall cloud and occasionally a tornado may form downwind of the old wall cloud, typically to the east or the southeast in the Northern Hemisphere (vice versa in the Southern Hemisphere).
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