Inversions occur during the winter months when normal atmospheric conditions (cool air above, warm air below) become inverted. Inversions trap a dense layer of cold air under a layer of warm air. The warm layer acts much like a lid, trapping pollutants in the cold air near the valley floor. The Wasatch Front valleys and their surrounding mountains act like a bowl, keeping the cold air in the valleys. The snow-covered valley floors reflect rather than absorb the heat from the sun, preventing the normal vertical mixing of warm and cold air.
In the Salt Lake Valley, inversions typically occur following a snowstorm. The new snow pack enhances colder temperatures near the surface. At the same time, clear skies lead to warmer temperatures above. Fog and freezing rain can also occur during inversion periods. Fog facilitates chemical reactions that create even more particles and higher pollutant concentrations.
The lowered visibility that accompanies inversions doesn’t necessarily signal high pollution levels because visibility often deteriorates well in advance of harmful concentrations of pollutants. The longer the inversion lasts, the higher the levels of pollution trapped under it. The warm inversion air layer is usually displaced by a strong storm system, which restores air quality to healthy levels.