Climate

Climate is determined by a complex combination of geographic conditions and latitude. It is weather writ large, which is to say, weather patterns considered over a long time—years, decades, even centuries. Temperature and precipitation, wind and humidity, in all their seasonal variations, characterize different geographic regions in the same way personality characterizes a person. The most visible indicator of a particular climate region is the local vegetation, whether it be swamplands, prairie grass, or giant redwoods. According to the universally used system of classification devised around 1900 by the German meteorologist Wladimir Koppen (1846—1940), the U.S. landmass has a continental climate, which is characteristic of the temperate regions of the globe. Such a climate is marked by sharp temperature swings, including mild to cold winters, warm to hot summers, and a broad diurnal range of temperatures. Within this broad classification U.S. climate ranges from "moist" in the eastern half of the continent to "dry" over most of the western half. The climate can only be described in the most general terms to leave room for numerous variations within any given region.

Rainfall generally declines going westward from the humid eastern zone, beginning at about 40 inches annually on the southeast side of the Appalachians, dropping to between 30 and 40 inches across most of the Central Lowland, then still further to between 10 and 30 inches annually on the plains. The intermontane basin lying between the Rockies and the Sierra Nevada receives less than four inches of precipitation annually before the pattern is thrown completely off on the upper Pacific shores.

The West Coast benefits from mild Pacific weather. As the cool moist winds from the Pacific rise over the coast ranges, they shed much precipitation so that Oregon and Washington have the heaviest rainfall of any region of the country (often as much as 70 inches annually), including Louisiana and Florida. Southern California, on the other hand, tends to have a warm, dry Mediterranean climate. As a result of these different West Coast conditions, a strong lumbering industry developed in the Pacific Northwest while farmers in California depended on irrigation or, alternatively, crops (such as grapes) that do not require much water.

Weather patterns move from west to east, so as westerly winds cross California, they dump rain or snow over the Cascade mountains and Sierra Nevada, then become warm and dry on the eastern side of both ranges, thereby helping create the forbidding deserts of Nevada and Arizona. The Great Plains likewise suffer from scant rainfall. The Central Lowland on the east and south of that broad area benefit from hot, humid air flowing off the Gulf of Mexico and as a result receive plentiful rain in the summer, which benefits cotton and sugar cultivation in particular.

Homesteaders soon learned that weather patterns over the Great Plains can swing wildly between extremes as the humid air from the south meets the colder, dry air off the Rocky Mountains. The results are seasonal thunderstorms, tornadoes, hailstorms, and blizzards that can devastate both people and crops. A broad belt running through Kansas, Oklahoma, and Missouri has been dubbed tornado alley because of the violent twisters that vacuum wide swathes through the countryside, usually in the late spring. The spring and fall seasons tend to be short, with longer winters and summers dependent at least partly on latitudinal location.

The southeast corner of the United States, wedged between the Gulf of Mexico and the Atlantic seaboard, has warm, humid conditions year-round, approaching even subtropical close to the coast. The Seminole Indians used the Florida Everglades for years as a nearly impenetrable refuge in their war against the U.S. government. The humid, subtropical climate of the Deep South did more than aid agriculture; it also encouraged the slower pace of life usually identified with that region, as well as the dreaded epidemic seasons in summer and fall that caused the ruling classes to flee north.

The Appalachians receive plenty of rainfall during the year, although their elevation and more northern latitudes preclude the semitropical conditions of the coastal regions. As a result, when the first settlers arrived, they found them covered with lush forests virtually all the way to their crests.

In the Northeast, the familiar continental climate is only slightly modified by close proximity to the Atlantic Ocean. Because the prevailing westerly winds blow offshore, the region shows relatively low winter temperatures and some very heavy snowfalls most years.

Temperatures across the United States vary seasonally, with the greatest extremes occurring on the north-central plains. The frostfree period of the year declines from south to north; Gulf Coast residents experience more than 240 days per year, whereas those living near the Canadian border experience fewer than 120 frost-free days per year. Because the climate on both oceanic coasts is generally milder than in the interior, people moving westward from the eastern settlements had to learn to adapt early on, not just their farming methods but their very lifestyle. One simple invention that caused a revolution on the Great Plains was barbed wire, which made it possible for the first time to fence in the range and put homesteaders on a par with ranchers.

The latest research by climatologists at the University of Wisconsin points toward a general warming all over North America between 1845 and 1995 (the most recent year for which data are available). Their conclusion is based, not on computer models and statistical calculations, but on direct human observation over that past 150-year period of freezing and thawing trends on lakes and other bodies of water. Regular reports made by government weather observers in major cities all over the United States from 1850 to 1870 tend to support that conclusion, too (see Chapter 10). The gradual rise in the heat quotient was most noticeable in summers, when the triple-digit highs known today were still virtually unknown, but the mean temperature during the hottest months crept steadily upward toward the 90°+ Fahrenheit values as the decades rolled by. Recognizing a trend and knowing the reasons for it are two different things, however.

TABLE 1.1 HISTORICAL MONTHLY TEMPERATURE EXTREMES, HIGHS AND LOWS, BY STATE

(temperatures in °F)

TABLE 1.1 HISTORICAL MONTHLY TEMPERATURE EXTREMES, HIGHS AND LOWS, BY STATE

(temperatures in °F)

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