The Humid Pacific Coast
The western humid region differs from its eastern counterpart in so many ways as to be a world apart. Much smaller, it is crammed into a narrow littoral belt to the windward of the Sierra–Cascade summit, dominated by mild Pacific air, and chopped by irregular topography into an intricate mosaic of climatic and biotic habitats. Throughout the region rainfall is extremely seasonal, falling mostly in the winter half of the year. Summers are droughty everywhere, but the main regional differences come from the length of drought—from about two months in humid Seattle, Washington, to nearly five months in semiarid San Diego, California.
Western Washington, Oregon, and northern California lie within a zone that climatologists call Marine West Coast. Winters are raw, overcast, and drizzly—not unlike northwestern Europe—with subfreezing temperatures restricted mainly to the mountains, upon which enormous snow accumulations produce local alpine glaciers. Summers, by contrast, are brilliantly cloudless, cool, and frequently foggy along the West Coast and somewhat warmer in the inland valleys. This mild marine climate produces some of the world’s greatest forests of enormous straight-boled evergreen trees that furnish the United States with much of its commercial timber. Mature soils are typical of humid midlatitude forestlands, a moderately leached gray-brown podzol.
Toward the south, with diminishing coastal rain the moist marine climate gradually gives way to California’s tiny but much-publicized Mediterranean regime. Although mountainous topography introduces a bewildering variety of local environments, scanty winter rains are quite inadequate to compensate for the long summer drought, and much of the region has a distinctly arid character. For much of the year, cool, stable Pacific air dominates the West Coast, bringing San Francisco its famous fogs and Los Angeles its infamous smoggy temperature inversions. Inland, however, summer temperatures reach blistering levels, so that in July, while Los Angeles expects a normal daily maximum of 83 °F (28 °C), Fresno expects 100 °F (38 °C) and is climatically a desert. As might be expected, Mediterranean California contains a huge variety of vegetal habitats, but the commonest perhaps is the chaparral, a drought-resistant, scrubby woodland of twisted hard-leafed trees, picturesque but of little economic value. Chaparral is a pyrophytic (fire-loving) vegetation—i.e., under natural conditions its growth and form depend on regular burning. These fires constitute a major environmental hazard in the suburban hills above Los Angeles and San Francisco Bay, especially in autumn, when hot dry Santa Ana winds from the interior regularly convert brush fires into infernos. Soils are similarly varied, but most of them are light in colour and rich in soluble minerals, qualities typical of subarid soils.
The Dry West
In the United States, to speak of dry areas is to speak of the West. It covers an enormous region beyond the dependable reach of moist oceanic air, occupying the entire Intermontane area and sprawling from Canada to Mexico across the western part of the Great Plains. To Americans nurtured in the Humid East, this vast territory across the path of all transcontinental travelers has been harder to tame than any other—and no region has so gripped the national imagination as this fierce and dangerous land.
In the Dry West nothing matters more than water. Thus, though temperatures may differ radically from place to place, the really important regional differences depend overwhelmingly on the degree of aridity, whether an area is extremely dry and hence desert or semiarid and therefore steppe.
Americans of the 19th century were preoccupied by the myth of a Great American Desert, which supposedly occupied more than one-third of the entire country. True desert, however, is confined to the Southwest, with patchy outliers elsewhere, all without exception located in the lowland rain shadows of the Cordillera. Vegetation in these desert areas varies between nothing at all (a rare circumstance confined mainly to salt flats and sand dunes) to a low cover of scattered woody scrub and short-lived annuals that burst into flamboyant bloom after rains. Soils are usually thin, light-coloured, and very rich with mineral salts. In some areas wind erosion has removed fine-grained material, leaving behind desert pavement, a barren veneer of broken rock.
Most of the West, however, lies in the semiarid region, in which rainfall is scanty but adequate to support a thin cover of short bunchgrass, commonly alternating with scrubby brush. Here, as in the desert, soils fall into the large family of the pedocals, rich in calcium and other soluble minerals, but in the slightly wetter environments of the West, they are enriched with humus from decomposed grass roots. Under the proper type of management, these chestnut-coloured steppe soils have the potential to be very fertile.
Weather in the West resembles that of other dry regions of the world, often extreme, violent, and reliably unreliable. Rainfall, for example, obeys a cruel natural law: as total precipitation decreases, it becomes more undependable. John Steinbeck’s novel The Grapes of Wrath describes the problems of a family enticed to the arid frontier of Oklahoma during a wet period only to be driven out by the savage drought of the 1930s that turned the western Great Plains into the great American Dust Bowl. Temperatures in the West also fluctuate convulsively within short periods, and high winds are infamous throughout the region.
The Humid–Arid Transition
East of the Rockies all climatic boundaries are gradational. None, however, is so important or so imperceptibly subtle as the boundary zone that separates the Humid East from the Dry West and that alternates unpredictably between arid and humid conditions from year to year. Stretching approximately from Texas to North Dakota in an ill-defined band between the 95th and 100th meridians, this transitional region deserves separate recognition, partly because of its great size, and partly because of the fine balance between surplus and deficit rainfall, which produces a unique and valuable combination of soils, flora, and fauna. The native vegetation, insofar as it can be reconstructed, was prairie, the legendary sea of tall, deep-rooted grass now almost entirely tilled and planted to grains. Soils, often of loessial derivation, include the enormously productive chernozem (black earth) in the north, with reddish prairie soils of nearly equal fertility in the south. Throughout the region temperatures are severely continental, with bitterly cold winters in the north and scorching summers everywhere.
The western edge of the prairie fades gradually into the shortgrass steppe of the High Plains, the change a function of diminishing rainfall. The eastern edge, however, represents one of the few major discordances between a climatic and biotic boundary in the United States, for the grassland penetrates the eastern forest in a great salient across humid Illinois and Indiana. Many scholars believe this part of the prairie was artificially induced by repeated burning and consequent destruction of the forest margins by Indians.
The Western mountains
Throughout the Cordillera and Intermontane regions, irregular topography shatters the grand bioclimatic pattern into an intricate mosaic of tiny regions that differ drastically according to elevation and exposure. No small- or medium-scale map can accurately record such complexity, and mountainous parts of the West are said, noncommittally, to have a “mountain climate.” Lowlands are usually dry, but increasing elevation brings lower temperature, decreased evaporation, and—if a slope faces prevailing winds—greater precipitation. Soils vary wildly from place to place, but vegetation is fairly predictable. From the desert or steppe of intermontane valleys, a climber typically ascends into parklike savanna, then through an orderly sequence of increasingly humid and boreal forests until, if the range is high enough, one reaches the timberline and Arctic tundra. The very highest peaks are snow-capped, although permanent glaciers rarely occur outside the cool humid highlands of the Pacific Northwest. Peirce F. Lewis