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Air pollution episode over western U.S.

By: Dr. Jeff Masters, 2:19 PM GMT on December 14, 2005

Particle pollution in the western U.S. has reached Unhealthy for Sensitive Groups and Unhealthy levels on the U.S. EPA's Air Quality Index (AQI) due to a stagnant weather pattern impacting the region. Some regions of California have reported unhealthful levels of particle pollution ten straight days; on Tuesday, Bakersfield, Fresno, and Sacramento all reported Unhealthy AQI levels. The unhealthful conditions are expected to continue today, and Bakersfield, Fresno, Hanford, and Visalia, in California, and Phoenix, Arizona have all declared air quality Action Days.

Figure 1. Winds over the western U.S. have been light the past ten days, generally less than 10 mph.

Figure 2. Visibility has been reduced below seven miles in many regions of Califonia due to air pollution and fog today.

The jet stream has been locked in place the past two weeks with a ridge of high pressure over the western U.S., and a trough of low pressure over the eastern part of the country. This pattern has steered cold air and storms into the eastern U.S., but left the western U.S. under high pressure with light winds and no rain. With no wind to flush the stagnant air out, and no rain to deposit the pollutants to the ground, particles of air pollution have built up to unhealthful levels. This weather pattern is expected to continue for at least another week. Beginning December 22 or 23, long range computer models are showing a major shift in the jet stream pattern, which may allow rain and wind to return once more to the western U.S.--and bring the eastern U.S. out of its deep freeze.

Check your local air quality forecasts and conditions by visiting the EPA AIRNow website at http://www.airnow.gov and selecting Local Forecasts and Conditions. If you are under an air pollution action advisory, limit prolonged or heavy exertion, reduce your activity time or substitute another activity that involves less exertion. Plan outdoor activities for days on which particle levels are lower. Reduce particle levels in the atmosphere by reducing your use of electricity and other particle sources such as automobiles, wood-burning stoves, and fireplaces.

Jeff Masters

Air and Water Pollution

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.