Wall cloud - A local and often abrupt lowering of a rain-free cumulonimbus base,
either rotating or non-rotating, from 1 to 4 miles in diameter, and usually situated in
the southwest portion of the storm. Wall clouds are found in the rear of the storm; NEVER
on the leading edge.
Researchers have shown that wall clouds probably develop when some rain-cooled
air is pulled upward, along with the more buoyant air, as the strengthening
updraft attempts to replace ever-growing volumes of rising air.
The rain-cooled air is very humid, and upon being lifted it quickly saturates
to form the lowered cloud base. Thus, the wall and tail cloud probably develop
sometime after an intense supercell or multicell storm begins to precipitate.