Whenever an anomaly exists in the atmosphere in which an increase in temperature or humidity occurs where a decrease would be expected, there is an inversion, or reversal. An atmospheric inversion most commonly refers to temperature.

Normally air temperature decreases with altitude at an average rate of about 3.6° F per 1000 ft (6.5° C per 1000 m). There are three factors that alter this rate, causing the temperature to rise within the first few hundred meters of the ground. First, inversions can occur as a result of cooling from the Earth's surface. This occurs at night when the ground cools more rapidly than the air above it. The effects of an inversion are thus greatest during early morning, usually the coolest part of the day. Second, inversions occur as a result of subsidence (sinking) of air in an anticyclone, or high pressure system, where the...