Cold-air pool

A cold-air pool is an accumulation of cold air in a topographic depression, such as a valley or basin. The cold air is produced by radiative cooling at night along the slopes and sink down, as it is denser than the surrounding air, settling at the bottom of the depression. The cold dome is trapped by the surrounding higher terrain until a change of airmass or daytime heating breaks the temperature inversion. Since the cold-air pool can persists long periods, it leads to poor air quality and fog.[1]

The air cool down at night and sink (top) while daytime heating break the temperature inversion (bottom).