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 In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation. Radiative cooling has b ...
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 In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...
or daytime heating breaks the
temperature inversion In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...
. Since the cold-air pool can persist for long periods, it leads to poor air quality and fog.


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 A katabatic wind (named ) is a downslope wind caused by the flow of an elevated, high-density air mass into a lower-density air mass below under the force of gravity. The spelling catabatic is also used. Since air density is strongly dependent o ...
. 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 Freezing rain is rain maintained at temperatures below melting point, freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike rain and snow mixed, a mixture of rain and snow or ice pellets, freezing rain is made en ...
at the surface if the temperature in the pool is below freezing. The stagnation also reduces the electrical production of
wind farm A wind farm, also called a wind park or wind power plant, is a group of wind turbines in the same location used to produce electricity. Wind farms vary in size from a small number of turbines to several hundred wind turbines covering an exten ...
s in the area.{{cite web , url=https://www.anl.gov/article/predicting-persistent-cold-pool-events#:~:text=Cold%20pools%20can%20trap%20pollutants,in%20heavily%20populated%20urban%20areas. , author=J. D. Amick , title=Predicting persistent cold pool events , publisher=
Argonne National Laboratory Argonne National Laboratory is a Federally funded research and development centers, federally funded research and development center in Lemont, Illinois, Lemont, Illinois, United States. Founded in 1946, the laboratory is owned by the United Sta ...
, access-date=2023-04-12


References

Atmospheric circulation Atmospheric thermodynamics Mountain meteorology