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Name:  Bryan Spray
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993


Question:
Please explain the causes of a thermal inversion. We are curious about how it is possible for this to occur for any length of time. Would the air underneath, cool air, not heat up and cause a convection current?



Replies:
Gee, my knowledge of atmospheric science is really pretty weak, especially when it comes to meteorological topics. One thing I do know is that in eastern Colorado, inversions are perpetually a problem because they tend to keep the smoke from people's chimneys close to the ground, as well as holding in CO from cars, and it is generally unpleasant. I was always told that this had to do with the fact that Denver lies right at the foot of the Rocky Mountains, on very flat land, and the cold air blowing over the city from the west during the winter tends to be colder than warm air near the city down in its basin, which is relatively stagnant (does not mix much with the cool air above). However I realize that this is not much of an explanation. All I can do is suggest that you hit the library and do some reading up on the subject....I am sorry I cannot help more.

Robert Topper


The meteorology texts I consulted list two main types of thermal inversion. One, called radiation inversion, develops during the night when the ground cools by giving off long wave radiation. The air near the ground is cooled by contact with the ground, and this layer eventually gets cooler than the air higher up. This is enhanced by the presence of a large high pressure system overhead, which usually is dry (clouds scatter long wave radiation, returning some heat energy back to the ground) and has only light winds (so there is not much horizontal or vertical mixing). The inversion layer may extend from a few feet to several hundred feet above the surface, and is usually dispersed by the next day's solar heating (there is your convection). So ordinarily it lasts only through the night. But occasionally the inversion can persist for several days. This happened in London in December of 1952. It led to respiratory problems for a sizable part of the population and resulted in the deaths of 4000 people, mostly elderly. (It was not the inversion per se, but rather the excessive pollution, a lot of it from coal-burning furnaces, which the inversion trapped near the ground.) There is an interesting description of what it was like in the Time-Life book "Planet Earth: Atmosphere" (this book also discusses the causes of inversions, and how the terrain may contribute to their formation, as in the case of Los Angeles). The inversion was able to persist for a couple of reasons: there was a big, slow-moving high pressure area present, and there was a persistent fog, which prevented sunlight (which was weak anyway, since it was late autumn) from reaching the ground and heating it to produce convection. Finally, a low-pressure system came in and pushed the stagnant, coal-dust- laden air away.

Ronald Winther


In cold, dark places such as Antarctica or Alaska in the winter, inversions can last for a long time because the sunlight never warms the very cold air.

Mark Fernau



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