inversion

The increase of air temperatures with height. (This is the reverse of the more common situation in which air cools with height.) Inversions occur: when strong, nocturnal, terrestrial radiation cools the earth's surface and therefore chills the air which is in contact with the ground; when cold air flows into valley floors, displacing warmer air (See also frost pockets); where a stream of warm air crosses the cool air over a cold ocean current; where warm air rises over a cold front; when air from the upper troposphere, subsiding in a warm anticyclone, is compressed and adiabatically warmed. A subsidence inversion is a stable layer in the low troposphere of an anticyclone, caused by the subsidence of warm, dry air. It forms where the descending air meets small-scale upward-rising convection currents. In spite of the name, an inversion is not always present.

The boundary between the top of the cold air and the beginning of the inversion is an inversion lid. Inversions are very stable and damp or polluted air is often trapped below them. See also trade wind inversion.