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Inmeteorology, aninversion (ortemperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally,air temperature gradually decreases asaltitude increases, but this relationship is reversed in an inversion.[2]
An inversion trapsair pollution, such assmog, near the ground. An inversion can also suppressconvection by acting as a "cap". If this cap is broken for any of several reasons, convection of anyhumidity can then erupt into violentthunderstorms. Temperature inversion can causefreezing rain incold climates.
Usually, within the lower atmosphere (thetroposphere) the air near the surface of theEarth 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., bythermals (convective heat transfer).[3] 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 theideal gas law andadiabatic lapse rate.
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 ofwarm fronts, and also in areas of oceanicupwelling such as along theCalifornia 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 thepolar regions during winter, inversions are nearly always present over land. An example of this is the town ofFairbanks, Alaska, which in December 2025 experienced a prolonged inversion that led to the town's eighth coldest December on record.[4] Temperatures in Fairbanks remained over 20 degrees below normal for nearly the entire month.[5][6]
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 acapping 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 theMidwestern 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.[7]
An inversion can develop aloft as a result of air gradually sinking over a wide area and being warmed byadiabatic compression, usually associated withsubtropicalhigh-pressure areas.[8] A stablemarine 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.
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 brownishhaze that can cause respiratory problems. TheGreat Smog of 1952 inLondon, England, is one of the most serious examples of such an inversion. It was blamed for an estimated 10,000 to 12,000 deaths.[9]
Sometimes the inversion layer is at a high enough altitude thatcumulus clouds can condense but can only spread out under the inversion layer. This decreases the amount of sunlight reaching the ground and prevents newthermals 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 ofice pellets andfreezing 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.[10]
As the temperature of air increases, theindex 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 thephenomenon known as aFata Morgana ormirage.
Inversions can magnify the so-called "green flash"—a phenomenon occurring at sunrise or sunset, usually visible for a few seconds, in which the sun's green light is isolated due to dispersion.[11] The shorter wavelength is refracted most, with the blue component of sunlight "completely scattered out byRayleigh scattering", making green the first or last light from the upper rim of the solar disc to be seen.[12]
Very high frequencyradio waves can be refracted by inversions, making it possible to hearFM radio or watchVHF low-band television broadcasts from long distances onfoggy nights. The signal, which would normally berefracted up and away into space, is instead refracted down towards the earth by the temperature-inversion boundary layer. This phenomenon is calledtropospheric ducting. Along coastlines during Autumn and Spring, due to multiple stations being simultaneously present because of reduced propagation losses, many FM radio stations are plagued by severe signal degradation disrupting reception.In higher frequencies such asmicrowaves, such refraction causesmultipath propagation andfading.
When an inversion layer is present, if a sound or explosion occurs at ground level, the sound wave isrefracted by the temperature gradient (which affects sound speed) and returns to the ground. The sound, therefore, travels much better than normal. This is noticeable in areas around airports, where the sound of aircraft taking off and landing often can be heard at greater distances around dawn than at other times of day, and inversion thunder which is significantly louder and travels further than when it is produced by lightning strikes under normal conditions.[13]
Theshock wave from an explosion can be reflected by an inversion layer in much the same way as it bounces off the ground in an air-burst and can cause additional damage as a result. This phenomenon killed two people in theSovietRDS-37nuclear test when a building collapsed.[14][15]
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The dictionary definition oftemperature inversion at Wiktionary