Climate
Climate affects human habitats both directly and indirectly through its influence on vegetation, soils, and wildlife. In the United States, however, the natural environment has been altered drastically by nearly four centuries of European settlement, as well as thousands of years of Indian occupancy.
Wherever land is abandoned, however, “wild” conditions return rapidly, achieving over the long run a dynamic equilibrium among soils, vegetation, and the inexorable strictures of climate. Thus, though Americans have created an artificial environment of continental proportions, the United States still can be divided into a mosaic of bioclimatic regions, each of them distinguished by peculiar climatic conditions and each with a potential vegetation and soil that eventually would return in the absence of humans. The main exception to this generalization applies to fauna, so drastically altered that it is almost impossible to know what sort of animal geography would redevelop in the areas of the United States if humans were removed from the scene.
Climatic controls
The pattern of U.S. climates is largely set by the location of the conterminous United States almost entirely in the middle latitudes, by its position with respect to the continental landmass and its fringing oceans, and by the nation’s gross pattern of mountains and lowlands. Each of these geographic controls operates to determine the character of air masses and their changing behaviour from season to season.
The conterminous United States lies entirely between the tropic of Cancer and 50° N latitude, a position that confines Arctic climates to the high mountaintops and genuine tropics to a small part of southern Florida. By no means, however, is the climate literally temperate, for the middle latitudes are notorious for extreme variations of temperature and precipitation.
The great size of the North American landmass tends to reinforce these extremes. Since land heats and cools more rapidly than bodies of water, places distant from an ocean tend to have continental climates; that is, they alternate between extremes of hot summers and cold winters, in contrast to the marine climates, which are more equable. Most U.S. climates are markedly continental, the more so because the Cordillera effectively confines the moderating Pacific influence to a narrow strip along the West Coast. Extremes of continentality occur near the centre of the country, and in North Dakota temperatures have ranged between a summer high record of 121 °F (49 °C) and a winter low of −60 °F (−51 °C). Moreover, the general eastward drift of air over the United States carries continental temperatures all the way to the Atlantic coast. Bismarck, North Dakota, for example, has a great annual temperature range. Boston, on the Atlantic but largely exempt from its influence, has a lesser but still-continental range, while San Francisco, which is under strong Pacific influence, has only a small summer–winter differential.
In addition to confining Pacific temperatures to the coastal margin, the Pacific Coast Ranges are high enough to make a local rain shadow in their lee, although the main barrier is the great rampart formed by the Sierra Nevada and Cascade ranges. Rainy on their western slopes and barren on the east, this mountain crest forms one of the sharpest climatic divides in the United States.
The rain shadow continues east to the Rockies, leaving the entire Intermontane Region either arid or semiarid, except where isolated ranges manage to capture leftover moisture at high altitudes. East of the Rockies the westerly drift brings mainly dry air, and as a result, the Great Plains are semiarid. Still farther east, humidity increases owing to the frequent incursion from the south of warm, moist, and unstable air from the Gulf of Mexico, which produces more precipitation in the United States than the Pacific and Atlantic oceans combined.
Although the landforms of the Interior Lowlands have been termed dull, there is nothing dull about their weather conditions. Air from the Gulf of Mexico can flow northward across the Great Plains, uninterrupted by topographical barriers, but continental Canadian air flows south by the same route, and, since these two air masses differ in every important respect, the collisions often produce disturbances of monumental violence. Plainsmen and Midwesterners are accustomed to sudden displays of furious weather—tornadoes, blizzards, hailstorms, precipitous drops and rises in temperature, and a host of other spectacular meteorological displays, sometimes dangerous but seldom boring.
The change of seasons
Most of the United States is marked by sharp differences between winter and summer. In winter, when temperature contrasts between land and water are greatest, huge masses of frigid, dry Canadian air periodically spread far south over the midcontinent, bringing cold, sparkling weather to the interior and generating great cyclonic storms where their leading edges confront the shrunken mass of warm Gulf air to the south. Although such cyclonic activity occurs throughout the year, it is most frequent and intense during the winter, parading eastward out of the Great Plains to bring the Eastern states practically all their winter precipitation. Winter temperatures differ widely, depending largely on latitude. Thus, New Orleans, Louisiana, at 30° N latitude, and International Falls, Minnesota, at 49° N, have respective January temperature averages of 55 °F (13 °C) and 3 °F (−16 °C). In the north, therefore, precipitation often comes as snow, often driven by furious winds; farther south, cold rain alternates with sleet and occasional snow. Southern Florida is the only dependably warm part of the East, though “polar outbursts” have been known to bring temperatures below 0 °F (−18 °C) as far south as Tallahassee. The main uniformity of Eastern weather in wintertime is the expectation of frequent change.
Winter climate on the West Coast is very different. A great spiraling mass of relatively warm, moist air spreads south from the Aleutian Islands of Alaska, its semipermanent front producing gloomy overcast and drizzles that hang over the Pacific Northwest all winter long, occasionally reaching southern California, which receives nearly all of its rain at this time of year. This Pacific air brings mild temperatures along the length of the coast; the average January day in Seattle, Washington, ranges between 33 and 44 °F (1 and 7 °C) and in Los Angeles between 45 and 64 °F (7 and 18 °C). In southern California, however, rains are separated by long spells of fair weather, and the whole region is a winter haven for those seeking refuge from less agreeable weather in other parts of the country. The Intermontane Region is similar to the Pacific Coast, but with much less rainfall and a considerably wider range of temperatures.
During the summer there is a reversal of the air masses, and east of the Rockies the change resembles the summer monsoon of Southeast Asia. As the midcontinent heats up, the cold Canadian air mass weakens and retreats, pushed north by an aggressive mass of warm, moist air from the Gulf. The great winter temperature differential between North and South disappears as the hot, soggy blanket spreads from the Gulf coast to the Canadian border. Heat and humidity are naturally most oppressive in the South, but there is little comfort in the more northern latitudes. In Houston, Texas, the temperature on a typical July day reaches 93 °F (34 °C), with relative humidity averaging near 75 percent, but Minneapolis, Minnesota, more than 1,000 miles (1,600 km) north, is only slightly cooler and less humid.