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How Big Cities' Bad Air Pollutes the Sierras
N early every afternoon, winds from the ocean blow pollution through three major passes in the coastal ranges -- the Carquinez Strait, Altamount Pass, and Pacheco Pass -- into the Central Valley and up against the Sierra. The streams of air carrying Bay Area emissions mix with locally generated pollution from automobile traffic, small engine exhaust, industry, and agriculture in the Valley and are diverted both north and south.
The Valley's geography is like a giant bathtub
The Valley's geography is like a giant bathtub -- with a lid on top in the form of inverted layers of cool and warm air that cannot mix. This inversion layer traps both local and transported dirty air, sometimes for weeks or even months. Organized wind patterns in the summer help create an eddy or swirl-like pattern that circulates around the Valley "tub."

Winds move south in the daytime, transporting pollution toward Fresno and Bakersfield. At night, the process reverses, taking it back north. The next day, the cycle begins again and continues until weather patterns change. This collection of trapped pollutants rises up into the Sierra on a daily basis-giving large areas of the mountains some of the worst air quality in the nation.
The cycle begins again and continues until weather patterns change
In the winter, the winds are slower and don't blow in a prevailing direction so there is no eddy pattern. But because of the low angle of the sun, inversions hang around even longer trapping pollution until the weather changes.

Next Next - The largest emitters of air pollution...

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Eye-opening Fact...
Because of unique topography, a large population and nearly one car per person, California still has eight of the 10 smoggiest cities in the U.S.

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