With increasing height, air temperature within the troposphere and mesosphere drops uniformly with altitude at a rate of approximately 6.5 degrees Celsius per 1000 meters - known as the environmental lapse rate. However, sometimes this normal overall decline is reversed. A point or layer at which the temperature increases with height is called inversion or inversion layer.
Temperature inversions frequently occur in anticyclones, but are also common in depressions when air in the middle troposphere subsides. Inversions may occur at any height, but the large temperature inversions at the tropopause and mesopause are stable and permanent features of the Earth's atmosphere.
In the troposphere, the weather-sphere, inversions cause an increase in stability and tend to limit the upward growth of cloud, preventing further upward convection. When particularly strong, with high potential temperatures that suppress small-scale convection in the layers beneath them, they are often termed capping inversion. The lowermost layer of air frequently becomes an inversion layer known as surface inversion. The condition results, for example, from radiation cooling of the ground and the air above. This usually occurs when there is strong nocturnal radiation, after a clear, dry and starry night, called radiation night. Or from advection of warm air over cold surfaces.
In wintertime, a temperature inversion occurs when cold air close to the ground is trapped by a layer of warmer air. As the inversion continues, air becomes stagnant and pollution becomes trapped close to the ground. Therefore inversions often cause the formation of smog. However, inversions and smog do also occur in summer and might turn into a serious respiration hazard over densely populated areas.