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Question:
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?


Replies:
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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