Inversion (meteorology)
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In meteorology, an 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.[2]
An inversion traps air pollution, such as smog, close to the ground. 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. Temperature inversion can cause freezing rain in cold climates.
Normal atmospheric conditions
Usually, within the lower atmosphere (the
Description
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 fronts, and also in areas of oceanic upwelling such as along the 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.[4]
Subsidence inversion
An inversion can develop aloft as a result of air gradually sinking over a wide area and being warmed by
Atmospheric consequences
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
Sometimes the inversion layer is at a high enough altitude that cumulus clouds can condense but can only spread out under the inversion layer. This decreases the amount of sunlight reaching the ground and prevents new thermals from forming. As the clouds disperse, sunny weather replaces cloudiness in a cycle that can occur more than once a day.
Wave propagation
Light
As the temperature of air increases, the 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 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.[7] 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.[8]
Radio waves
Sound
When an inversion layer is present, if a sound or explosion occurs at ground level, the sound wave is 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.[9]
Shock waves
The
See also
References
- ^ Smoke Filled Canyons, Arizona
- ^ Service, NOAA's National Weather. "Glossary – NOAA's National Weather Service". w1.weather.gov. Retrieved January 19, 2017.
- ^ Nagle, Garrett, and Paul Guinness. Cambridge International A and AS Level Geography. Hodder Education, 2011. 41. Print.
- ISBN 978-0-521-84950-0. Retrieved June 21, 2022.
- ^ Wallace and Hobbs (2006) Atmospheric Science: An Introductory Survey
- PMID 14698923.
- S2CID 125714499. Retrieved June 21, 2022.
- S2CID 125714499. Retrieved June 21, 2022.
- ^ Dean A. Pollet and Micheal M. Kordich, 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
- ^ Johnston, Wm. Robert. "RDS-37 Nuclear Test, 1955". Retrieved April 11, 2014.
- ^ "RDS-37: The Soviet Hydrogen Bomb". Retrieved December 26, 2015.
External links
- 'Fire inversions' lock smoke in valleys
- The dictionary definition of temperature inversion at Wiktionary