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{{More citations needed|date=May 2020}}
{{Technical|date=July 2022}}
[[File:Thermal Inversion in Urban Environment.png|thumb|Temperature inversion in an urban environment]]▼
[[File: Temperature inversion in the Lake District.jpg|alt=Temperature inversion in the Lake District|thumb|Temperature inversion in the [[Lake District]], [[England]], forms clouds at a low level beneath clear skies.]]▼
[[Image: SmokeCeilingInLochcarron.jpg|thumb|Smoke rising in [[Lochcarron]], [[Scotland]], is stopped by an overlying layer of warmer air (2006).]]▼
[[Image: Smog_over_Almaty.jpg|thumb|Smog trapped over the city of [[Almaty]], [[Kazakhstan]] during a temperature inversion.]]▼
[[File: Smoke-filled Canyons in AZ from ISS.jpeg|thumb|Smoke-filled canyons in [[Northern Arizona|northern]] [[Arizona]], 2019. During morning and evening hours, dense smoke often settles in low-lying areas and becomes trapped due to temperature inversions—when a layer within the lower atmosphere acts as a lid and prevents vertical mixing of the air. Steep canyon walls act as a horizontal barrier, concentrating the smoke within the deepest parts of the canyon and increasing the strength of the inversion.<ref>[https://earthobservatory.nasa.gov/images/145692/smoke-filled-canyons-arizona?src=eoa-iotd Smoke Filled Canyons, Arizona]</ref>]]▼
▲[[File:Thermal Inversion in Urban Environment.png|thumb|Temperature inversion in an urban environment]]
In [[meteorology]], an '''[https://www.weather.gov/media/lzk/inversion101.pdf inversion]''' (or "temperature inversion") is a layer of warmer air held above colder air. Normally, air temperature gradually decreases as altitude increases, but in an inversion this relationship is reversed.<ref>{{Cite web|url=http://w1.weather.gov/glossary/index.php?word=inversion|title=Glossary – NOAA's National Weather Service|last=Service|first=NOAA's National Weather|website=w1.weather.gov|language=EN-US|access-date=2017-01-19}}</ref>▼
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▲In [[meteorology]], an '''
An inversion traps [[air pollution]], such as [[smog]], close to the ground. An inversion can also suppress [[Atmospheric convection|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 [[thunderstorm]]s. Temperature inversion can cause [[freezing rain]] in cold climates. ▼
▲An inversion traps [[air pollution]], such as [[smog]],
==Normal atmospheric conditions==
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[[Image:Absinkinversion.png|thumb|Height ([[y-axis]]) versus temperature ([[x-axis]]) under normal atmospheric conditions (black line). When the layer from {{convert|6|-|8|km|abbr=off|0}} (designated A-B) descends dry [[Adiabatic process|adiabatically]] <!-- see also Lapse_rate#Dry_adiabatic_lapse_rate, but not sure if a link is warranted -->, the result is the inversion seen near the ground at {{convert|1|-|2|km|abbr=off|0}} (C-D).]]
[[File:Reifgrenze.JPG|thumb|Klagenfurter Becken ([[Austria]]) in December 2015: on Mount {{Proper name|Goritschnigkogel}} there is a distinct inverse [[hoarfrost]] margin.]]
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 front]]s, and also in areas of oceanic [[upwelling]] such as along the [[Coastal California|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.
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.<ref>{{cite book |last1=Oke |first1=Tim |last2=Mills |first2=Gerald |last3=Christen |first3=Andrea |last4=Voogt |first4=James |title=Urban Climates |date=2017 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-84950-0 |pages=30–35 |doi=10.1017/9781139016476 |edition=1st |url=https://doi.org/10.1017/9781139016476 |access-date=21 June 2022}}</ref>
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===Radio waves===
{{main|Super refraction}}
[[Very high frequency]] [[radio waves]] can be refracted by inversions, making it possible to hear [[FM radio]] or watch [[VHF low]]-band television broadcasts from long distances on [[fog]]gy nights. The
In higher frequencies such as [[microwave]]s, such refraction causes [[multipath propagation]] and [[fading]].
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