TEMPERATURE INVERSION

During day light the sun warms the air near the earth’s surface. Normally, this warm air and most of the pollutants it contains rises to mix with the cooler air above it. This mixing of warm and cold air creates turbulence, which disperses the pollutants. However, under certain atmospheric conditions a layer of warm air can lie atop a layer of cooler air nearer the ground, a situation known as a temperature inversion. Because the cooler air is denser than the warmer air above it, the air near the surface does not rise and mix with the air above it. Pollutants can concentrate in this stagnant layer of cool air near the ground.

There are two types of temperature inversion. (a) Subsidence temperature inversion: occurs when a large mass of warm air moves into a region at high altitude and floats over a mass of colder air near the ground. This keeps the air over a city stagnant and prevents vertical mixing and dispersion of air pollutants. Normally such conditions do not last long. (b) Radiation temperature inversion: occurs typically at night as the air near the ground cools faster than the air above it. As the sun rises and warms the earth’s surface, a radiation inversion normally disappears by noon and disperses the pollutants built up during the night.

However, under certain conditions, the temperature inversion can last for several days and allow pollutants to build up to dangerous concentrations. Areas with two types of topography and weather conditions are especially susceptible to prolonged temperature inversion.

i)       Town or city located in a valley surrounded by mountains that experiences cloudy and cold weather during part of the year.

Highly populated coastal (sea) city surrounded by mountains on three sites with sunny climate.