Editing Inversion (meteorology)

{{short description|Deviation from the normal change of an atmospheric property with altitude}}
{{use mdy dates|date=July 2012}}
{{More citations needed|date=May 2020}}

[[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 [[sea of clouds|clouds at a low level]] under clearer air.]]
[[File:SmokeCeilingInLochcarron.jpg|thumb|Smoke rising in [[Lochcarron]], [[Scotland]], is stopped by an overlying layer of warmer air (2006).]]
[[File: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>{{cite web |url=https://earthobservatory.nasa.gov/images/145692/smoke-filled-canyons-arizona?src=eoa-iotd |title=Smoke Filled Canyons, Arizona |publisher=[[NASA Earth Observatory]] |website=earthobservatory.nasa.gov}}</ref>]]

In [[meteorology]], an '''inversion''' (or '''temperature inversion''') is a phenomenon in which a layer of warmer air overlies cooler air. Normally, [[atmospheric temperature|air temperature]] gradually decreases as [[altitude]] increases, but this relationship is reversed in an inversion.<ref>{{cite web |url=https://forecast.weather.gov/glossary.php?word=inversion |title=Glossary |publisher=[[National Oceanic and Atmospheric Administration|NOAA]]'s [[National Weather Service]] |website=forecast.weather.gov |language=en-US |access-date=2024-07-30}}</ref>

An inversion traps [[air pollution]], such as [[smog]], near 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 [[humidity]] can then erupt into violent [[thunderstorm]]s. Temperature inversion can cause [[freezing rain]] in [[polar climate|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 [[thermal]]s ([[Convection (heat transfer)|convective heat transfer]]).<ref>Nagle, Garrett, and Paul Guinness. Cambridge International A and AS Level Geography. Hodder Education, 2011. 41. Print.</ref> 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==
[[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>

==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]] [[anticyclone|high-pressure areas]].<ref>Wallace and Hobbs (2006) Atmospheric Science: An Introductory Survey</ref> 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 cyclone]]s. The accumulated smog and dust under the inversion quickly taints the sky reddish, easily seen on sunny days.

==Atmospheric consequences==
[[Image:Superior mirage of a boat 2.jpg|220px|A [[Fata Morgana (mirage)|Fata Morgana]] (or [[mirage]]) of a ship is due to an inversion (2008).|thumb]]
[[Image:Sha1993 smog wkpd.jpg|thumb|right|Winter smoke in [[Shanghai]], [[China]], with a clear border-layer for the vertical air-spread (1993).]]
[[Image:Bratislava Temperature inversion1 2005-Nov-11.jpg|thumb|right|A temperature inversion in [[Bratislava]], [[Slovakia]], viewing the top of [[Nový Most]] (2005).]]
[[File:2017 Smog nad Nową Rudą.jpg|thumb|Inversion-created smog in Nowa Ruda, Poland, 2017|alt=A valley in low, partially forested mountains seen in wintertime, covered with snow. At the bottom is a village, almost obscured by a layer of grayish-brown air]]
[[File:Smoke from oil palm mill Tawau Sabah Malaysia.jpg|thumb|Temperature inversion phenomenon in the early morning near [[Tawau]], [[Sabah]], [[Malaysia]] where smoke that was emitted from an oil palm mill stayed close to the ground. The wind carried the smoke in the direction of the nearby settlement to the middle-right of the photo (August 2023).]]

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. 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.<ref name="EHP_112_1">{{cite journal |author1= Bell, M.L. |author2=Davis, D.L. |author3=Fletcher, T. |date=2004 | title = A Retrospective Assessment of Mortality from the London Smog Episode of 1952: The Role of Influenza and Pollution | journal=[[Environmental Health Perspectives|Environ Health Perspect]] | volume = 112 | issue = 1; January | pages = 6–8 | doi = 10.1289/ehp.6539 | pmid=14698923 | pmc=1241789}}</ref>

Sometimes the inversion layer is at a high enough altitude that [[cumulus cloud]]s can condense but can only spread out under the inversion layer. This decreases the amount of sunlight reaching the ground and prevents new [[thermal]]s 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. <ref name="U-Ill">{{cite web |date=1999-10-04 |title=Cyclones and Fronts: the development of freezing rain |url=http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/cond/cyc.rxml |access-date=2022-02-21 |work=WW2010: The Weather World 2010 Project |publisher=University of Illinois}}</ref>

==Wave propagation==
[[File:mirage_principle.svg|thumb|Mirages due to differing air refractive indices, ''n'']]
===Light===
As the temperature of air increases, the [[refractive index|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 (mirage)|Fata Morgana]] or [[mirage]].

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.<ref>{{cite journal |last1=ben Aroush |first1=Tomer |last2=Boulahjar |first2=Saber |last3=Lipson |first3=Stephen G |title=Observing the green flash in the laboratory |journal=European Journal of Physics |date=13 December 2017 |volume=39 |issue=1 |page=2 |doi=10.1088/1361-6404/aa90f5 |s2cid=125714499 |url=https://iopscience.iop.org/article/10.1088/1361-6404/aa90f5/pdf |access-date=21 June 2022|url-access=subscription }}</ref> The shorter wavelength is refracted most, with the blue component of sunlight "completely scattered out by [[Rayleigh scattering]]", making green the first or last light from the upper rim of the solar disc to be seen.<ref>{{cite journal |last1=ben Aroush |first1=Tomer |last2=Boulahjar |first2=Saber |last3=Lipson |first3=Stephen G |title=Observing the green flash in the laboratory |journal=European Journal of Physics |date=13 December 2017 |volume=39 |issue=1 |page=2 |doi=10.1088/1361-6404/aa90f5 |s2cid=125714499 |url=https://iopscience.iop.org/article/10.1088/1361-6404/aa90f5/pdf |access-date=21 June 2022|url-access=subscription }}</ref>{{Clear|left}}

===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 signal, which would normally be [[refract]]ed up and away into space, is instead refracted down towards the earth by the temperature-inversion boundary layer. This phenomenon is called [[tropospheric 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 as [[microwave]]s, such refraction causes [[multipath propagation]] and [[fading]].

===Sound===
When an inversion layer is present, if a sound or explosion occurs at ground level, the sound wave is [[Refraction|refracted]] 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.<ref>Dean A. Pollet and Micheal M. Kordich, [https://apps.dtic.mil/sti/pdfs/ADA477220.pdf User's guide for the Sound Intensity Prediction System (SIPS) as installed at the Naval Explosive Ordnance Disposal Technology Division (Naveodtechdiv)]. Systems Department February 2000. DTIC.mil</ref>

====Shock waves====
The [[shock 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 the [[Soviet Union|Soviet]] [[RDS-37]] [[nuclear test]] when a building collapsed.<ref>{{cite web| first=Wm. Robert| last=Johnston| title=RDS-37 Nuclear Test, 1955| url=http://www.johnstonsarchive.net/nuclear/tests/1955USSR-1.html| access-date=11 April 2014}}</ref><ref>{{cite web| title=RDS-37: The Soviet Hydrogen Bomb| url=http://www.atomicarchive.com/History/coldwar/p7_image.shtml| access-date=26 December 2015}}</ref>

==See also==
{{portal|Weather}}
* [[Aerosol]]
* [[Particulates]]
* [[Index of meteorology articles]]

==References==
{{reflist|30em}}

==External links==
*[https://phys.org/news/2019-09-inversions-valleys.html 'Fire inversions' lock smoke in valleys]
*{{wiktionary-inline|temperature inversion}}

{{Commons category|Meteorological inversion}}

[[Category:Atmospheric thermodynamics]]
[[Category:Radio frequency propagation]]