United States

Since the Gulf air is unstable as well as wet, convectional and frontal summer thunderstorms are endemic east of the Rockies, accounting for a majority of total summer rain. These storms usually drench small areas with short-lived, sometimes violent downpours, so that crops in one Midwestern county may prosper, those in another shrivel in drought, and those in yet another be flattened by hailstones. Relief from the humid heat comes in the northern Midwest from occasional outbursts of cool Canadian air; small but more consistent relief is found downwind from the Great Lakes and at high elevations in the Appalachians. East of the Rockies, however, U.S. summers are distinctly uncomfortable, and air conditioning is viewed as a desirable amenity in most areas.

Again, the Pacific regime is different. The moist Aleutian air retreats northward, to be replaced by mild, stable air from over the subtropical but cool waters of the Pacific, and except in the mountains the Pacific Coast is nearly rainless though often foggy. In the meanwhile, a small but potent mass of dry hot air raises temperatures to blistering levels over much of the intermontane Southwest. In Yuma, Arizona, for example, the normal temperature in July reaches 107 °F (42 °C), while nearby Death Valley, California, holds the national record, 134 °F (57 °C). During its summer peak this scorching air mass spreads from the Pacific margin as far as Texas on the east and Idaho to the north, turning the whole interior basin into a summer desert.

Over most of the United States, as in most continental climates, spring and autumn are agreeable but disappointingly brief. Autumn is particularly idyllic in the East, with a romantic Indian summer of ripening corn and brilliantly coloured foliage and of mild days and frosty nights. The shift in dominance between marine and continental air masses, however, spawns furious weather in some regions. Along the Atlantic and Gulf coasts, for example, autumn is the season for hurricanes—the American equivalent of typhoons of the Asian Pacific—which rage northward from the warm tropics to create havoc along the Gulf and Atlantic coasts as far north as New England. The Mississippi valley holds the dubious distinction of recording more tornadoes than any other area on Earth. These violent and often deadly storms usually occur over relatively small areas and are confined largely to spring and early summer.

The bioclimatic regions

Three first-order bioclimatic zones encompass most of the conterminous United States—regions in which climatic conditions are similar enough to dictate similar conditions of mature (zonal) soil and potential climax vegetation (i.e., the assemblage of plants that would grow and reproduce indefinitely given stable climate and average conditions of soil and drainage). These are the Humid East, the Humid Pacific Coast, and the Dry West. In addition, the boundary zone between the Humid East and the Dry West is so large and important that it constitutes a separate region, the Humid–Arid Transition. Finally, because the Western Cordillera contains an intricate mosaic of climatic types, largely determined by local elevation and exposure, it is useful to distinguish the Western Mountain Climate. The first three zones, however, are very diverse and require further breakdown, producing a total of 10 main bioclimatic regions. For two reasons, the boundaries of these bioclimatic regions are much less distinct than boundaries of landform regions. First, climate varies from year to year, especially in boundary zones, whereas landforms obviously do not. Second, regions of climate, vegetation, and soils coincide generally but sometimes not precisely. Boundaries, therefore, should be interpreted as zonal and transitional, and rarely should be considered as sharp lines in the landscape.

For all of their indistinct boundaries, however, these bioclimatic regions have strong and easily recognized identities. Such regional identity is strongly reinforced when a particular area falls entirely within a single bioclimatic region and at the same time a single landform region. The result—as in the Piedmont South, the central Midwest, or the western Great Plains—is a landscape with an unmistakable regional personality.

The Humid East

The largest and in some ways the most important of the bioclimatic zones, the Humid East was where the Europeans first settled, tamed the land, and adapted to American conditions. In early times almost all of this territory was forested, a fact of central importance in American history that profoundly influenced both soils and wildlife. As in most of the world’s humid lands, soluble minerals have been leached from the earth, leaving a great family of soils called pedalfers, rich in relatively insoluble iron and aluminum compounds.