A wall cloud is a persistent, often rotating lowering that hangs beneath the rain-free base of a supercell's updraft. It marks the place where the mesocyclone is drawing moist, rain-cooled air upward and condensing it well below the ambient cloud base.
How wall clouds form
The updraft of a supercell ingests warm, moist inflow air from the storm's forward-flank side. As rain from the forward-flank downdraft evaporates into that inflow, it cools and humidifies the near-surface air. When this cooler, more saturated parcel is lifted into the updraft, it condenses at a lower height than the drier ambient inflow, producing a lowering that sits below the surrounding rain-free base.
The lowering starts to rotate when the mesocyclone's circulation extends downward toward the ground. This descent is often driven by the rear-flank downdraft as it wraps rain and cool air around the updraft's south and west side, tightening the low-level circulation and stretching its vertical spin.
Signs that a wall cloud matters
Spotters and chasers pay attention to persistence, rotation, motion, and organization. A wall cloud that persists for five to ten minutes or longer, rotates visibly, rises rather than sags, and is fed by an obvious inflow tail is showing all the ingredients of a tornado-favorable mesocyclone.
A tail cloud running from the wall cloud back toward the forward-flank rain is a strong signal that inflow is being drawn into the base. A clear slot cutting in from the southwest, often carrying the leading edge of the rear-flank downdraft, is a sign that occlusion is beginning and that the tornado threat is elevated.
What a wall cloud does not confirm
Wall clouds are common features of supercells. Many form, wobble, and dissipate without ever spinning up a tornado. Ragged, non-rotating lowerings under multicell storms are often just scud responding to updraft turbulence, and they should not be reported as wall clouds.
The reverse is also true. High-precipitation supercells and rain-wrapped tornadoes can hide the wall cloud behind curtains of rain. Landspouts and QLCS mesovortices produce tornadoes with little or no supercell wall cloud at all. A missing wall cloud never rules out a tornado, and a beautiful wall cloud never guarantees one.
Reporting from the field
A useful spotter report describes the wall cloud's location relative to the storm, its rotation rate, its behavior over the last minute or two, and the presence of any associated features such as a tail cloud, funnel, clear slot, or ground contact. This lets warning forecasters combine the visual picture with radar velocity data and decide whether the storm is trending toward tornadogenesis.