The 2003 fire season was another very active one for western North America, especially in the Canadian province of British Columbia where some 620,000 acres of forest were consumed in almost 2,500 fires. Here the Rocky Mountains have both lofty, snow-capped peaks and long, narrow valleys that create special conditions and problems with air quality from these smoky fires.
This image taken by the crew of the International Space Station on August 20, 2003, illustrates how smoke has become trapped in valleys. Normally air temperature decreases with altitude; in other words, the higher up you are, the colder it is. Warmer, more buoyant air near the surface of the Earth usually rises into the atmosphere, carrying away air pollutants such as smoke.
However, sometimes the “higher equals colder” relationship breaks down, for example, here in the northern Rockies, where light winds and cold air drainage from the higher elevations have created “temperature inversions,” making the air in the valley colder and denser than the air at the mountain peaks. The cold dense air does not rise, but intsead stays trapped—along with the smoke—in the valleys. Note how the snowy peaks of the mountains are relatively smoke-free while the long, north-south valleys of Kootenay Lake and Columbia River are filled with trapped aerosols from the plumes of the large fires situated to the southwest. Meanwhile shifting winds have now swept the bulk of the plumes southeastward over the Columbia River Basin of Washington.