Inversion (meteorology)
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 (inversion layer) within which such an increase occurs.

An inversion can lead to pollution such as smog
Smog is a type of air pollution; the word "smog" is a portmanteau of smoke and fog. Modern smog is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine...

 being trapped close to the ground, with possible adverse effects on health. An inversion can also suppress convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 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. Temperature inversion can notoriously result in freezing rain
Freezing rain
Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...

 in cold climates.

Normal atmospheric conditions

Usually, within the lower atmosphere (the troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

) the air near the surface of the Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 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 thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...

s (convective heat transfer
Convective heat transfer
Convective heat transfer, often referred to as convection, is the transfer of heat from one place to another by the movement of fluids. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid. Convection is usually the dominant form of heat...


How and why inversions occur

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 front
Warm front
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient...

s, and also in areas of oceanic upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

 such as along the California coast. With sufficient humidity in the cooler layer, fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 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 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
Capping inversion
A capping inversion is an elevated inversion layer that caps a convective boundary layer.The boundary layer is the part of the atmosphere which is closest to the ground. Normally, the sun heats the ground, which in turn heats the air just above it. Thermals form when this warm air rises into the...

. 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.

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
An anticyclone is a weather phenomenon defined by the United States' National Weather Service's glossary as "[a] large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere"...

. A stable marine layer
Marine layer
A marine layer is an air mass which develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect of the water on the surface layer of an otherwise warm air mass...

 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, leading to tropical cyclones.
The accumulated smog and dust under the inversion quickly taints the sky reddish, easily seen on sunny days.

Consequences of a thermal inversion

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
Mumbai , formerly known as Bombay in English, is the capital of the Indian state of Maharashtra. It is the most populous city in India, and the fourth most populous city in the world, with a total metropolitan area population of approximately 20.5 million...

, India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

; Los Angeles, California
Los Angeles, California
Los Angeles , with a population at the 2010 United States Census of 3,792,621, is the most populous city in California, USA and the second most populous in the United States, after New York City. It has an area of , and is located in Southern California...

Mexico City
Mexico City
Mexico City is the Federal District , capital of Mexico and seat of the federal powers of the Mexican Union. It is a federal entity within Mexico which is not part of any one of the 31 Mexican states but belongs to the federation as a whole...

; São Paulo, Brazil; Santiago, Chile
Santiago, Chile
Santiago , also known as Santiago de Chile, is the capital and largest city of Chile, and the center of its largest conurbation . It is located in the country's central valley, at an elevation of above mean sea level...

; and Tehran, Iran but also in smaller cities like Oslo, Norway; Prague, Czech Republic; Ljubljana, Slovenia; Salt Lake City, Utah
Salt Lake City, Utah
Salt Lake City is the capital and the most populous city of the U.S. state of Utah. The name of the city is often shortened to Salt Lake or SLC. With a population of 186,440 as of the 2010 Census, the city lies in the Salt Lake City metropolitan area, which has a total population of 1,124,197...

; Logan, Utah
Logan, Utah
-Layout of the City:Logan's city grid originates from its Main and Center Street block, with Main Street running north and south, and Center east and west. Each block north, east, south, or west of the origin accumulates in additions of 100 , though some streets have non-numeric names...

; Vancouver, British Columbia; Chiang Mai
Chiang Mai
Chiang Mai sometimes written as "Chiengmai" or "Chiangmai", is the largest and most culturally significant city in northern Thailand. It is the capital of Chiang Mai Province , a former capital of the Kingdom of Lanna and was the tributary Kingdom of Chiang Mai from 1774 until 1939. It is...

; Launceston, Tasmania
Launceston, Tasmania
Launceston is a city in the north of the state of Tasmania, Australia at the junction of the North Esk and South Esk rivers where they become the Tamar River. Launceston is the second largest city in Tasmania after the state capital Hobart...

 and Boise, Idaho
Boise, Idaho
Boise is the capital and most populous city of the U.S. state of Idaho, as well as the county seat of Ada County. Located on the Boise River, it anchors the Boise City-Nampa metropolitan area and is the largest city between Salt Lake City, Utah and Portland, Oregon.As of the 2010 Census Bureau,...

 which are 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 of 1952
Great Smog of 1952
The Great Smog of '52 or Big Smoke was a severe air pollution event that affected London, England, during December 1952. A period of cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants mostly from the use of coal to form a thick layer of smog over the...

, one of the most serious examples of such an inversion, occurred in London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 and was blamed for an estimated 11,000 to 12,000 deaths.

Sometimes the inversion layer is higher so that the cumulus
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

 clouds can condense but then they spread out under the inversion layer. This cuts out sunlight to the ground and prevents new thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...

s 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
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 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
Fata Morgana (mirage)
A Fata Morgana is an unusual and very complex form of mirage, a form of superior mirage, which, like many other kinds of superior mirages, is seen in a narrow band right above the horizon...

 or mirage
A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at"...


Electromagnetic Radiation (Radio-TV)

Similarly, very-high frequency (above ca. 90 MHz) radio waves (being part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, 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 on fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

gy 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
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 is called tropospheric ducting. It is also referred to as skip by small radio operators and ham radio operators. Along coast lines during Autumn and Spring many FM radio stations are plagued by severe signal degradation causing them to sound like "scrambled eggs".

Inversions can magnify the so called "green flash
Green flash
Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena...

": 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.


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 travels faster, i.e. the air has lower acoustic refractive index, so the sound can undergo total internal reflection
Total internal reflection
Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...

) and return back to ground level; the sound, therefore, travels much farther than normal. This is noticeable in areas around airports, when the sound of aircraft taking off and landing often can be heard at greater distances around dawn than at other times of day.

Shock Waves

The shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 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 three people in the Russian RDS-37
RDS-37 was the Soviet Union's first "true" hydrogen bomb, first tested on November 22, 1955. The weapon had a nominal yield of approximately 3 megatons. It was scaled down to 1.6 megatons for the live test....

 nuclear test when a building collapsed.

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. When it happens the sky is a reddish color.
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