Puget Sound Clean Air Agency

Photo: Smoke rising in Lochcarron is stopped by an overlying layer of warmer air.  Courtesy http://en.wikipedia.org/wiki/Thermal_inversionHow Do Temperature Inversions Affect
Air Quality?

Temperature inversions are relatively common in the Puget Sound area, and refer to reversals of normal temperature patterns seen in our lower atmosphere. In wintertime, a temperature inversion occurs when cold air close to the ground is trapped by a layer of warmer air. As the inversion continues, air becomes stagnant and pollution becomes trapped close to the ground. Since our area lies in a basin, it takes an extra push of marine air to flush the pollution out of our area.

Inversions also occur during the summer months, but are a product of even hotter upper air trapping warm air close to the ground. The result is the same: we are unable to rid ourselves of the everyday pollution that we create. Air pollution will continue to accumulate until the weather pattern changes. Just how much air quality degrades depends on our everyday, pollution-causing actions, such as driving, using gasoline-powered yard and recreational equipment, or burning wood. Voluntarily refraining from, or limiting, these actions during inversions can help keep air quality within healthy levels.

The following two graphs show the difference between a normal weather day and a temperature inversion day. The vertical axis on the graph shows distance or height from the ground's surface up into the lower atmosphere. The horizontal axis shows temperature. The colored lines show air characteristics at different times of the morning hours. The black line is a measurement at 6 a.m.

On a normal day, the temperature is warmest at the ground's surface and the air gets cooler at increasing altitudes. The above graph shows that the temperature is about 7 degrees Celsius (or about 40 degrees Fahrenheit) at ground level. The temperature decreases the farther up we measure. This means that air is mixing and that air pollution is able to rise and be dispersed. In this situation, our air pollution has a lot of room in which to disperse upward into the lower atmosphere.

The graph below depicts the opposite scenario.


Warm air has formed a layer on top of cooler surface air. This layer is called the temperature inversion. This means that temperatures actually rise as we measure farther up into the air. Little or no mixing occurs and concentrations of atmospheric particulate pollution and vapors are trapped below the level where temperatures again decrease with altitude. This point is referred to as the top of the inversion. Inversions can occur at the surface or aloft in the atmosphere. Those close to the surface have the greatest influence on air quality because air pollution has little room to be dispersed.

In this graph, the temperature initially is increasing with altitude. At about the 300 meter mark, the top of the inversion, the temperature pattern is beginning to revert to normal. This inversion means that instead of having virtually endless amounts of air in which to dissipate, our daily air pollution might be confined to a space only 1500 feet above the ground's surface.