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A cold, damp air mass over Ketchikan, Alaska. Nobody ever expects warm winters in Alaska, but sometimes conditions are even more severe than usual. This was particularly true in late January and early February of 1999, when a prolonged period of extreme cold produced the worst cold snap in a decade. Galena, in the northern part of the state, just missed an all-time low temperature when the reading dipped to -53 °C (-64 °F) in February. Conditions were so extreme that the city government officially closed down, maintaining only emergency services. If that wasn't cold enough, consider the -75 °C (-103 °F) wind chill factor at Kotzebue along the northwestern coast! Things weren't exactly balmy in Fairbanks, either, when on February 15 the low temperature dipped below -37 °C (-35 °F) for a record-breaking nineteenth consecutive day. Needless to say, the air was also extremely dry, as is always the case under cold conditions. Contrast those conditions with those experienced in Texas half a year earlier. College Station, by way of example, had its warmest May, June, and July average temperatures ever, as maximum temperatures exceeded 38 °C (100 °F) 51 times between May 31 and September 3. The heat was further compounded by high humidities and unusually sparse rainfall throughout the period. Have you ever wondered about episodes such as the one above, in which large areas experience more or less similar weather? At times like these, broadcasters use phrases such as "throughout the Midwest," or "across the eastern seaboard," or "Today the Pacific Northwest experienced . . . ." By way of contrast, it often happens that places within an hour's drive of each other have very different weather, with essentially nothing in common. What's the explanation for this behavior? Why does the atmosphere sometimes organize itself into broad uniform patches, and other times show extreme variation over short distances? This chapter addresses these and related questions using some very simple, but nevertheless powerful, concepts. The Alaska and Texas situations cited above represent two extreme instances in which large regions are covered by a body of air having more or less uniform temperature and moisture. These large volumes of air are called air masses. Often an area the size of North America will be covered by several air masses at the same time so that, for example, the northeastern United States and southeastern Canada may experience cold, dry conditions, while the southern United States is dominated by warm, moist air. Thus a person might board an airplane in Nashville feeling perfectly comfortable in shirt sleeves, only to end up shivering in Boston. Moreover, these air masses are commonly separated from each other by fairly narrow boundary regions, called fronts, across which conditions change rapidly. The passage of these fronts are significant weather events because they often bring abrupt changes in temperature, humidity, and wind. They also provide a lifting mechanism that can lead to the formation of clouds and precipitation. In this chapter we describe the formation and nature of air masses, the fronts that separate them, and their influences on local weather.
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