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Wind can have a direct impact on how we feel even under the mildest of weather conditions. A gentle breeze can make a hot afternoon more comfortable, or it can make a winter night bitter cold. But occasionally, the movement of air can impact our lives in far more substantial ways. Residents of eastern Washington and Oregon were reminded of this on September 25, 1999, when winds up to 135 km/hr (85 mph) created a blinding dust storm that triggered a spate of accidents. Over the course of the day, 6 people were killed and 23 injured, and miles of highway were completely shut down for hours. The worst of the multiple-vehicle accidents occurred along Interstate Highway 84 in northeastern Oregon. Fifty-eight-year-old Harold Fell described his experience: "An 18-wheeler passed us and the next thing I knew, it was stopped dead in the road. It just loomed up in front of me." Fell and his wife suffered minor injuries, but they undoubtedly realized how lucky they were when they surveyed the wreckage and realized that 4 people were dead amid the wreckage of the 16 vehicles involved.
As concerned as we sometimes are with wind conditions, few of us pay much attention to a closely associated component of weather--atmospheric pressure. After all, how many times have you canceled a picnic because the pressure was too low? Or how many people do you know who have special clothes they wear only on days of high pressure?
Although seldom considered in everyday life, air pressure deeply affects other weather variables that have much more immediate impact. For example, horizontal variations in atmospheric pressure are directly responsible for the motion of the wind. And because air descends in areas of high surface pressure and rises in regions of low surface pressure, differences in air pressure strongly influence the likelihood of cloud formation and precipitation.
This chapter introduces the basic concepts of atmospheric pressure and its vertical and horizontal distributions. We discuss the relationship between pressure and other atmospheric variables and the processes that create horizontal and vertical variations in pressure. With this foundation we can go on to discuss storm patterns in later chapters.