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Inversion Layers
Graphic showing inversion layer

The warm inversion layer acts like a blanket on the smog layer, preventing it from dissipating higher in the atmosphere. Because of high pressure, the Central Valley regularly experiences these thermal inversions. The Valley, which is nearly at sea level, often fills at night with cool heavy air underneath a layer of warmer air. The cool air layer grows through the night reaching up to 3000 feet thick. Pollutants are trapped beneath the blanket of warmer air until the sun warms the cool air layer equalizing the temperature and allowing the inversion to break up and disperse.

Graphic showing air movement into Central Valley from San Francisco Bay Area

Pollutants are then carried into the mountains on upslope winds on an almost daily basis during the summer months. In the winter when the sun angle is lower in the sky, inversions are stronger and longer lasting. It isn't until a low pressure system moves in with wind and rain that the stagnant air clears out, lifting the veil of pollution away from the valley and mountain slopes. Rain and snowfall can clear the air, but they can also carry pollutants in the form of nitric and sulfuric acid, dropping them into lakes, rivers, and soil. Hence, air pollution affects not only the air we breathe, but many terrestrial and aquatic environments as well.

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