Whenever an anomaly exists in the atmosphere in which an increase in temperature or humidity occurs where a decrease would be expected, there is an inversion, or reversal. An atmospheric inversion most commonly refers to temperature.
Normally air temperature decreases with altitude at an average rate of about 3.6° F per 1000 ft (6.5° C per 1000 m). There are three factors that alter this rate, causing the temperature to rise within the first few hundred meters of the ground. First, inversions can occur as a result of cooling from the Earth's surface. This occurs at night when the ground cools more rapidly than the air above it. The effects of an inversion are thus greatest during early morning, usually the coolest part of the day. Second, inversions occur as a result of subsidence (sinking) of air in an anticyclone, or high pressure system, where the descending air warms adiabatically, that is, within itself, while the ground remains cool. High pressure systems have the stability that inversion layers require. Finally, movement of air can create an advective inversion. For instance, if a warm air mass moves over a relatively cold surface, such as a body of water or over snow cover, an inversion will occur.
Inversion layers block the upward movement of air, trapping moisture and natural and man-made pollutants near the ground. The result is fog and smog. The lower the inversion ceiling, the more concentrated the accumulation of moisture and particulates. Some of the most serious episodes of smog or fog occur in mountainous areas, especially where a city or industrial site is located. In places like the San Fernando Valley in California, polluted air may be trapped both vertically and horizontally.
The mere presence of a city or factory can create a microclimate of its own, creating a pocket of warm air within the cool ground layer. Smoke from a stack, instead of escaping upward or laterally, may descend to the ground, delivering a direct dose of pollution to residents of the area.
The contrast between the warm and cool layers of air can best be observed during early morning fog when the upper reaches of a tall building or monument rise above the top of the fog layer. The Eiffel Tower in Paris offered a good observation point for early inversion research for measurements could be made at the base and top of the tower. A major research effort on atmospheric inversions was made by G. I. Taylor in 1915. Taylor spent six months on the S.S. Scotia sailing the Grand Banks off Newfoundland using kites and balloons to measure and analyze fogs. Prior to this, fog had been perceived as nothing more than precipitation that was falling at an imperceptibly slow rate.
Inversion layer research was particularly active in Europe beginning in the 1920s. The Industrial Age had brought with it a relatively new problem. Pollution from coal-burning factories was extremely critical in areas such as the Ruhr Basin. German meteorologists such as Gustav Hellmann and Richard Assmann researched and wrote on the subject. The advancement of atmospheric measurement and the air mass theories and models of Jacob Bjerknes, Vilhelm Bjerknes and the Bergen school increased understanding of how inversions occur.
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