From AMS Glossary
A region of reverse flow immediately downwind of an obstacle or mountain, typically found when the static stability of the air is near neutral (Froude number greater than about 1.5).
The reverse flow indicates a circulation or permanent eddy downwind of the obstacle where the surface wind is opposite to the prevailing wind direction upstream of the obstacle. The circulation continues partway up the mountain slope and then breaks away from the mountain and flows in the prevailing wind direction, finally descending to make a closed circulation. Such a cavity circulation causes air pollutants emitted near the downwind base of the mountain or building to be drawn back toward the obstacle and remain somewhat trapped within the circulation, leading to much larger pollutant concentration values than would be predicted by typical pollutant dispersion models.