Wednesday, January 19, 2011


What is a temperature inversion?

A temperature inversion is a thin layer of the atmosphere where the normal decrease in temperature with height switches to the temperature increasing with height. An inversion acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion.

This can cause several weather-related effects. One is the trapping of pollutants below the inversion, allowing them to build up. If the sky is very hazy, or is sunsets are very red, there is likely an inversion somewhere in the lower atmosphere. This happens more frequently in high pressure zones, where the gradual sinking of air in the high pressure dome typically causes an inversion to form at the base of a sinking layer of air.

Another effect that an inversion has is to make clouds just below the inversion to spread out and take on a flattened appearance. For instance, marine stratocumulus clouds over cold ocean waters; or the tops of thunderstorms when they reach the base of the stratosphere, which also forms a temperature inversion.

Still another effect is to prevent thunderstorms from forming. Even in an air mass that is hot and humid in the lowest layers, thunderstorms will be prevented if an inversion in the lower atmosphere is keeping this air from rising. The conceptual opposite of a temperature inversion is an unstable air layer.

Normal atmospheric conditions

Usually, within the lower atmosphere (the trophosphere) 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 thermals (convective heat transfer).

How and why inversions occur

Height (y-axis) versus Temperature (x-axis) under normal atmospheric conditions (black line). When the layer from 6–8 kilometers (designated A-B) is descended dry adiabatically, the result is the inversion seen near the ground at 1–2 kilometers (C-D).
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 fronts, and also in areas of oceanic upwelling such as along the California coast. 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 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. 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. A stable marine layes 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

Fata Morgana of a ship is due to an inversion
Winter smoke in Shanghai with a clear border-layer for the vertical air-spread.
Temperature inversion in Bratislava, viewing the top of Nový Most bridge
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, India, Los Angeles, California; Mexico City; Sao Paolo, Brazil; Santiago, Chile; and Tehran, Iran but also in smaller cities like Oslo, Norway; Prague, Czech Republic; Ljubljana, Slovenia; Salt Lake City, Utah;Logan, Utah; Vancouver, British Colombia; Chiang Mai and Boise, Idaho 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 oof 1952, one of the most serious examples of such an inversion, occurred in London and was blamed for thousands of deaths.
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.

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