Wall cloud

A wall cloud (or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which sometimes 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. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do rotate, however some do not.

Wall clouds are formed by a process known as entrainment, when an inflow of warm, moist air rises and converges, overpowering wet, rain-cooled air from the normally downwind downdraft. As the warm air continues to entrain the cooler air, the air temperature drops and the dew point increases (thus the dew point depression decreases). As this air continues to rise, it becomes more saturated with moisture, which results in additional cloud condensation, sometimes in the form of a wall cloud. Wall clouds may form as a descending of the cloud base or may form as rising scud comes together and connects to the storm's cloud base.

Wall clouds can be anywhere from a fraction of 1.6 km (1 mi) wide to over 8 km (5 mi) across. 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). In the Northern Hemisphere wall clouds typically form at the south or southwest end of a supercell. This is in the rear of the supercell near the main updraft and most supercells move in a direction with northeasterly components, thus for supercells forming in northwest flow situations and moving southeastward, the wall cloud may be found on the northwest or back side of such storms. Rotating wall clouds are visual evidence of a mesocyclone.

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