Temperature inversion
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A temperature inversion is a meteorological phenomenon in which air temperature increases with height for some distance above the ground, as opposed to the normal decrease in temperature with height. This effect, which can be caused by a number of different factors, can lead to pollution such as smog being trapped close to the ground, with possible adverse effects on health.
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 by solar radiation absorbed at the surface. The air is expanded while rising with the dry adiabatic lapse rate of -10°C/km. As a result, the upper air is cooler and it is called "adiabatic cooling".
Hot air, however, rises (*). This is convection in which the warmer air rises up, to be replaced with cooler air which is then heated. It is this process that leads to cloud building, thermals, and other convection related atmospheric behaviour.
(*) The truth is that Colder air sinks and occupies the lower layers. It is the simple gravitational effect of a "heavier" (denser) mass winning over a lighter mass. Hot air has no intrinsic antigavitational properties!
How inversions occur
Sometimes the gradient is inverted, so that the air gets colder nearer the surface of the Earth: this is a temperature inversion. It can be created by the movement of air masses of different temperature moving over each other. A warm air mass moving over a colder one can "shut off" the convection effects, keeping the cooler air mass trapped below (see capping inversion). It commonly occurs at night: when solar heating ceases, the surface cools by radiation, and cools the immediately overlying atmosphere (see Surface temperature inversion). Over most of Antarctica, there is a near-permanent inversion.Subsidence inversion
A temperature inversion that develops 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.Consequences of an 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 a lot of pollutants exist. Inversion effects occur frequently in cities such as Mumbai, India; Los Angeles, California; and Santiago, Chile. During a severe inversion, trapped air pollutants form a brownish haze that can cause respiratory problems.
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, radio (being part of the electromagnetic spectrum, like light) can be redirected by such inversions. This is why it is common to hear radio (and sometimes TV) broadcasts from otherwise impossible distances on foggy nights. The signal, still more than powerful enough to receive even at hundreds or thousands of miles or kilometres, would normally be refracted up and away from the ground-based antenna, but is now refracted back down instead. This phenomenon is called tropospheric ducting, or a tropo duct.
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 travel much further than normal- the sound is refracted by the temperature change at the boundary and it undergoes total internal reflection. Much of the sound is thus trapped under the layer and the sound can travel much greater distances than normal.
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