Thermal inversion is a natural phenomenon that occurs when a layer of warm air traps cool air near the Earths surface, which is the inverse of what normally happens. The temperature inversion can be caused by different factors, including:

• Radiational cooling: The earths surface cools at night by longwave radiation emission to space, which is maximized on clear nights with light wind and dry air. Air in the lower planetary boundary layer (PBL) cools much more rapidly than air at the top of the PBL at night, causing an inversion that can be quite impressive.

• Warm air flowing over a large cold water body: This can create an inversion.

• High pressure subsidence: A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result, the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes.

• Frontal inversion: A shallow cold front can create an inversion. The inversion created from a cold front is especially evident when a shallow layer of polar air moves into lower latitudes. The air associated with the shallow air mass is colder than the air aloft, thus creating an inversion.

• Tropopause inversion: Warming by absorption of shortwave radiation by ozone can create an inversion.

The presence of a temperature inversion can have several effects on the environment, including cloud formation, fog, smog, lack of precipitation, and visibility disruption. For example, cooler air gets trapped within a layer of warmer air, and the moisture condenses and forms clouds called smog. But since these clouds cannot escape the level of inversion, they cause poor visibility.