Refer to figures.

A katabatic wind is caused by a flow of cold air down a hill or mountain side at night, when the highlands radiate heat and are cooled.

• The air in contact with the higher-level ground is also cooled, and it becomes denser than the air at the same elevation but away from the slope; it, therefore, begins to flow downhill.
• The katabatic effect is most marked if the mountain side is snow covered, if the sky is clear to assist radiation and if the PG is slack.

In valleys, inversions are also called cold-air pools and form as a result of both cooling of the ground due to long-wave radiation and nocturnal down-slope winds (Katabatic winds). The colder air which flows down the valley walls may “collect” at the bottom of the valley, creating a cold pool of air near the surface and low temperatures on the ground. They usually extend from the bottom of the valley up to the boundary-layer top. In winter, inversions can last for a few days to several weeks, being triggered and maintained by anticyclonic conditions at synoptic scale