United States

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