Cold-air pool
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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.[1]
Cold-air pools mostly form with a nighttime 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.[2]
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.