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 notoriously result in freezing rain in 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 thermals (convective heat transfer).
How and why inversions occur
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 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, 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
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 Paulo, Brazil; Santiago, Chile; and Tehran, Iran, but also in smaller cities like Oslo, Norway, Salt Lake City, Utah, 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, one of the most serious examples of such an inversion, occurred in London in 1952 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.
Similarly, very-high frequency (VHF - 30 to 300 MHz) radio waves (being part of the electromagnetic spectrum, like light) can be refracted by such inversions. This is why it i...
