In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs.
An inversion can lead to pollution such as smog being trapped close to the ground, with possible adverse effects on health. An inversion can also suppress convection by acting as a "cap". If this cap is broken for any of several reasons, convection of any moisture present can then erupt into violent thunderstorms.
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.
Under certain conditions, the normal vertical temperature gradient is inverted such 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. 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 is inferior to the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the angle of 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.
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 actually quite warm, but is still denser and usually cooler than the lower part of the inversion layer capping it.
With the ceasing of convection, which is normally present in the atmosphere, a number of phenomena are associated with a temperature inversion. The air becomes stiller, hence the air becomes murky because dust and pollutants are no longer lifted from the surface.
This can become a problem in cities where many pollutants exist. Inversion effects occur frequently in big cities such as Mumbai, India; Los Angeles, California; Mexico City ; Sao Paulo, Brazil; Santiago, Chile; and Tehran, Iran, but even also in smaller cities like Oslo, Norway and Salt Lake City, Utah which is closely surrounded by hills and mountains that together with the inversion effect bottle-caps the air in the city. During a severe inversion, trapped air pollutants form a brownish haze that can cause respiratory problems. The Great Smog, one of the most serious examples of such an inversion, occurred in London in 1952 and was blamed for thousands of deaths.
Sometimes the inversion layer is higher so that the cumulus clouds can condense but then they spread out under the inversion layer. This cuts out sunlight to the ground and prevents new thermals from forming. A period of cloudiness is followed by sunny weather as the clouds disperse. This cycle can occur more than once in a day.
The index of refraction of air decreases as the air temperature increases, 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 (where the sun is "squished" into an oval). In an inversion the normal pattern is reversed, and distant objects are instead stretched out or appear to be above the horizon. This leads to the interesting optical effects of Fata Morgana or mirage.
Similarly, very-high frequency (VHF - 30 to 300 MHz) radio waves (being part of the electromagnetic spectrum, like light) can be refracted by such inversions. This is why it is possible to sometimes hear FM radio (or watch VHF-LO band TV) broadcasts from otherwise impossible distances as far as a few hundred miles distant on foggy nights. The signal, still powerful enough to be received even at hundreds or rarely, thousands, of miles, would normally be refracted up and away from the ground-based antenna, is instead refracted down towards the earth by the temperature-inversion boundary layer. This phenomenon is called tropospheric ducting. It is also referred to as skip by small radio operators and Ham operators.
In addition, when an inversion layer is present (for example early in the morning when ground-level air temperatures are cool, and high-level air temperatures are warmer), if a sound or explosion occurs at ground level, the sound wave can get totally reflected from the warmer upper layer (in which the sound travel faster, i.e. the air has lower acoustic refractive index, so the sound can undergo total internal reflection) and return back to ground level; the sound is therefore heard much further than normal.
Inversions can magnify the so called "green flash": a phenomenon occurring at sunrise/sunset, usually visible for a few seconds, in which the sun's green light is isolated due to dispersion - the shorter wavelength is refracted most, so it is the first/last light from the upper rim of the solar disc to be seen.
The shockwave from a nuclear explosion will bounce off 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 three people in the RDS-37 nuclear test.
In an inversion, vertical motion in the atmosphere is suppressed because the atmosphere is stable. Hence vertical heat transport by eddies is suppressed; this reduced (downwards) heat transport leads to further cooling of the lower surface. This can lead to an effective decoupling of the atmosphere from the surface in extreme conditions, such as may be found in Antarctica during the polar night, where inversions greater than 25 °C commonly occur.