Ask A Scientist General Science Archive

Author:     Bryan Spray
Text:       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?

Response #:  1 of 3
Author:      Robert Topper
Text:        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.

Response #:  2 of 3
Author:      Ronald Winther
Text:        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.

Response #:  3 of 3
Author:      Mark Fernau
Text:        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.