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 sinks 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 air mass or daytime heating breaks the temperature inversion. Since the cold-air pool can persist for long periods, it leads to poor air quality and fog.

The air cools down at night and sinks (top) while daytime heating breaks the temperature inversion (bottom).

Formation

Cold-air pools mostly form with a night time temperature inversion on clear nights and calm winds. The ground loses energy by radiation and the air in contact with it cools down by conduction until sunrise. When this process occurs on mountain slopes, the cooling air becomes denser than the air further afield and sinks downslope producing a katabatic wind. When the cold air reaches a relatively flat area or a valley, it slows down and accumulates, like water entering a lake. Above this pool (which can reach many hundreds of feet thick, depending on the geography), the air remains warmer.

Man-made barriers can also promote the formation of a cold-air pools. For example, when railways or roads traverse a slight slope horizontally, embankments can be high enough to capture cold air over a significant area.

Impact

Cold-air pools have impacts on vegetation and agriculture by increasing the risk of frost.

Because of the stagnant nature of the cold-air pools, they will trap pollutants, causing serious health risks in urban areas. With the temperature inversion, rain produced at altitude by an approaching weather system will become freezing rain at the surface if the temperature in the pool is below freezing. The stagnation also reduces the electrical production of wind farms in the area.