In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inversion. An inversion traps air pollution, such as smog, near the ground. An inversion can also suppress convection by acting as a "cap". If this cap is broken for any of several reasons, convection of any humidity can then erupt into violent thunderstorms. Temperature inversion can cause freezing rain in cold climates.
Normal atmospheric conditions Usually, within the lower atmosphere (the troposphere) the air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth's surface, which in turn then warms the layer of the atmosphere directly above it, e.g., by thermals (convective heat transfer). Air temperature also decreases with an increase in altitude because higher air is at lower pressure, and lower pressure results in a lower temperature, following the ideal gas law and adiabatic lapse rate.
Description
Under the right conditions, the normal vertical temperature gradient is inverted so that the air is colder near the surface of the Earth. This can occur when, for example, a warmer, less-dense air mass moves over a cooler, denser air mass. This type of inversion occurs in the vicinity of warm fronts, and also in areas of oceanic upwelling such as along the California coast in the United States. With sufficient humidity in the cooler layer, fog is typically present below the inversion cap. An inversion is also produced whenever radiation from the surface of the earth exceeds the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the sun is very low in the sky. This effect is virtually confined to land regions as the ocean retains heat far longer. In the polar regions during winter, inversions are nearly always present over land. An example of this is the town of Fairbanks, Alaska, which in December 2025 experienced a prolonged inversion that led to the town's eighth coldest December on record. Temperatures in Fairbanks remained over 20 degrees below normal for nearly the entire month. A warmer air mass moving over a cooler one can "shut off" any convection which may be present in the cooler air mass: this is known as a capping inversion. However, if this cap is broken, either by extreme convection overcoming the cap or by the lifting effect of a front or a mountain range, the sudden release of bottled-up convective energy—like the bursting of a balloon—can result in severe thunderstorms. Such capping inversions typically precede the development of tornadoes in the Midwestern United States. In this instance, the "cooler" layer is quite warm but is still denser and usually cooler than the lower part of the inversion layer capping it.
Subsidence inversion An inversion can develop aloft as a result of air gradually sinking over a wide area and being warmed by adiabatic compression, usually associated with subtropical high-pressure areas. A stable marine layer may then develop over the ocean as a result. As this layer moves over progressively warmer waters, however, turbulence within the marine layer can gradually lift the inversion layer to higher altitudes, and eventually even pierce it, producing thunderstorms, and under the right circumstances, tropical cyclones. The accumulated smog and dust under the inversion quickly taints the sky reddish, easily seen on sunny days.
Atmospheric consequences
Temperature inversions stop atmospheric convection (which is normally present) from happening in the affected area and can lead to high concentrations of atmospheric pollutants. Cities especially suffer from the effects of temperature inversions because they both produce more atmospheric pollutants and have higher thermal masses than rural areas, resulting in more frequent inversions with higher concentrations of pollutants. The effects are even more pronounced when a city is surrounded by hills or mountains since they form an additional barrier to air circulation. During a severe inversion, trapped air pollutants form a brownish haze that can cause respiratory problems. Temperature inversions create stable atmospheric stratification that inhibits vertical mixing between air near the surface and the air above it, allowing pollutants to accumulate near the ground; this effect is often more pronounced in valleys, basins, or areas surrounded by mountains, where terrain further restricts airflow and promotes the retention of cold air and pollutants near the surface. The Great Smog of 1952 in London, England, is one of the most serious examples of such an inversion. It was blamed for an estimated 10,000 to 12,000 deaths. Sometimes the inversion layer is at a high enough altitude that cumulus clouds can condense but can only spread out under the inversion layer. This decreases the amount of sunlight reaching the ground and prevents new thermals from forming. As the clouds disperse, sunny weather replaces cloudiness in a cycle that can occur more than once a day. In winter, an inversion can lead to the development of ice pellets and freezing rain. Both these phenomena occur when snow melts in a warm layer aloft and falls into a colder layer near the surface. If the layer of cold air near the surface is thick enough, it will lead to the development of ice pellets as the raindrops re-freeze. A shallow layer does not give the raindrops enough time to freeze as they quickly fall through it, leading to the development of freezing rain.
Wave propagation
Light As the temperature of air increases, the index of refraction of air decreases, a side effect of hotter air being less dense. Normally this results in distant objects being shortened vertically, an effect that is easy to see at sunset when the sun is visible as an oval. In an inversion, the normal pattern is reversed, and distant objects are instead stretched out or appear to be above the horizon, leading to the phenomenon known as a Fata Morgana or mirage.
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