Stability of Temperature Inversions
An atmospheric temperature inversion happens when warm air
settles in a layer above cold air. When this condition occurs over
cities, it can create a very stable air mass that traps pollution
near the Earth's surface. Why is the air mass in a temperature
inversion so stable?
A temperature inversion is stable because cold air is denser than warm air.
Consequently, warm air rises and cold air sinks. If warm air is above cold air,
both the cold air and the warm air will stay where they are.
Things get a little more complicated in the atmosphere because air density depends
on pressure as well as temperature. What is more, the temperature of air decreases
as the air expands, and increases as the air is compressed. (This is a difficult
property to explain: it relates to what temperature means and to the principle of
conservation of energy.) The pressure decreases as altitude increases, so rising
air expands and cools, while sinking air compresses and warms. That is why all the
cold air at high altitudes does not just switch places with the warmer air near the
ground: at equilibrium, the air higher up is at just the right temperature lower
than the air lower down that if it came down to the lower altitude, it would
compress and become warmer than the air below it.
In an inversion, the air at high altitude is even warmer than necessary for
equilibrium. In an "unstable" atmosphere, the air at altitude is cooler than
necessary for equilibrium. (This happens when the air below is heated, usually by
the ground warming in the daylight.)
Richard Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012