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 (Lapworth (2003) Qly J. Royal Met. Soc. 129); when cold air flows into valley floors, displacing warmer air (LeMone et al. (20030 J. Atmos. Scis 60); where a stream of warm air crosses the cool air over a cold ocean current (Vihma et al. (2003) Boundary-Layer Met. 107); where warm air rises over a cold front (Ross and Orlanski (1978) J. Atmos. Scis 35, 3); when air from the upper troposphere, subsiding in a warm anticyclone, is compressed and adiabatically warmed. The boundary between the top of the cold air and the beginning of the inversion is an inversion lid. See also trade wind inversion.