Inversion is a short name for temperature inversion. A temperature inversion is a high stability layer in the atmosphere where temperature, rather than showing its normal decrease with height, increases with height. Such layers are often surface-based, with coldest temperatures at the surface and temperatures increasing with height above the surface. Such surface-based inversions are usually formed at night and in winter when the ground cools faster than the air above. In fact, inversions are a common nighttime feature year around, since cold temperatures near the ground are produced commonly after sunset when the ground loses heat through outgoing long-wave radiation.
Temperature inversion layers can also form in the free atmosphere above the surface. This often happens when warm air is advected (i.e., blown in) above a layer of cold air.
The word 'inversion' is used in the local newspapers to refer to the mid-winter pollution events that affect the Salt Lake Valley. Inversions and pollution layers are, in fact, not synonymous. Inversions are a meteorological phenomenon; pollution is produced by a variety of human activities. Pollution can be emitted into and stored in a layer of high atmospheric stability, such as a temperature inversion, but the atmosphere, even when temperature does not increase with height, has sufficient stability to store pollutants and keep them from mixing vertically. In fact, the atmosphere in the Salt Lake Valley in winter, when polluted, is often isothermal, with temperature being more or less constant with height. Further, non-polluted inversions can form in areas where pollution sources are not present. The Salt Lake Basin is a topographic basin where stable layers (including inversions) often form, but the air pollution problem is not caused by inversions, it is caused by pollutants that are emitted into the stable layers. The solution to our pollution problem must come from reducing pollutant emissions. In the meantime, our PCAPS meteorological research program is gaining a better understanding to the meteorological processes that lead to the formation, maintenance and destruction of stable layers within the Salt lake Valley, and we hope this will, with effort, lead to improvements in forecasting stable layer and pollution events.