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.[1]

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

Formation

Cold-air pools mostly form with a night time temperature inversion on clear nights and calm winds. The ground is losing 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 into a lake. Above this pool that can reach many hundred 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, railways or roads in a landscape with a slight slope, embankments can be high enough to capture cold air over a significant area.

Impact

Cold-air pools have impacts on vegetation and agriculture, in particular by an increased risk of frost, have been the subject of studies.[3]

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 in altitude by an approaching weather system will become freezing rain at surface if the temperature in the pool is below freezing. Wind being light, wind farms in the area will have a lower electrical production.[4]

References
  1. "Cold-air pool". Glossary of Meteorology. American Meteorological Society. Retrieved 2023-04-05.
  2. "Cold-Air Pools in Mountain Valleys". Mont Washington Observatory. Retrieved 2023-04-12.
  3. Quenol, Hervé; Beltrando, Gérard; Bridier, Sébastien (2007-06-01). "Risques de gel accru liés à la présence d'ouvrages linéaires en remblai: Le cas des lignes nouvelles du TGV Méditerranée et de la future ligne du TGV Est européen". EchoGéo (1). doi:10.4000/echogeo.1052. ISSN 1963-1197. Retrieved 2023-02-22.
  4. J. D. Amick. "Predicting persistent cold pool events". Argonne National Laboratory. Retrieved 2023-04-12.
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