Contrasting Climates in N. W. America.—An even more striking contrast, showing the effects of the warm ocean and air currents, when intercepted by mountain chains, exists on the Pacific coast farther northward, as already mentioned. In Oregon and Washington first the low Coast ranges, and then the higher Cascade mountains, obstruct the eastward progress of the westerly ocean winds. The result is a very heavy rainfall to coastward of and within the Coast ranges, and an almost equally heavy precipitation on the western slope of the Cascades. Standing on the crest of the latter in summer, one may see to westward a rolling sea of clouds, causing almost daily rains; while to eastward the eye ranges over brown or whitish, dusty plains or rolling lands, almost destitute of tree growth and quivering with heat, under a deep blue sky untroubled by clouds for months.
A somewhat similar contrast is seen in the Hawaiian islands, which are in the sweep of the subtropical northeast trade winds, and on their windward (eastern) slopes have abundant rains; while on the leeward slopes an almost arid climate prevails, calling for extended irrigation.
Continental, Coast and Insular Climates.—From what has been said above, the striking differences of climate caused by the position of any region with reference to the sea or other large bodies of water on the one hand, and to mountain chains on the other, can be readily understood; provided of course that the direction of the winds and the trend of the mountain chains be properly taken into consideration. Western coasts in the temperate and subtropical regions will have a relatively even, temperate and moist climate as compared with the interior of continents, from which the tempering influence of the sea is cut off by mountain chains. Where no such chains intervene the coast climate may extend far inland. The latter case is that of Europe, where the prevailing westerly winds, warmed by the Gulf Stream, temper the climate as far east as the borders of Russia, and northward to Norway; while to southward the warm waters of the Mediterranean and Black seas temper both heat and cold in Spain, southern France, Italy and the Mediterranean border generally. But to eastward, in Russia and Siberia, the climate becomes “continental” to an extreme degree, with very cold winters and very hot summers. The same is true of interior North America, wherever the continental divide cuts off the tempering influence of the westerly winds; Montana, the Dakotas and the Great Plains states generally being examples. The climate of the Mississippi valley, as stated before, is tempered by the winds blowing from the Gulf of Mexico, but with occasional irruptions of the continental climate (sometimes reaching as far east as the South Atlantic coast) in the forms of cold “blizzards,” from which the coast climates of the Pacific and of western Europe are practically free. The Atlantic coast of North America (including the coast of the Gulf of Mexico), moreover, not unfrequently suffers from the violent cyclonic storms that originate in the Antilles and follow more or less the direction of the Gulf Stream.
Islands, differing from continents mainly in their extent, and having a relatively large proportion of coast, naturally have climates controlled essentially by the surrounding ocean. The insular or oceanic climates are therefore, as a rule, more temperate and even than are those of the nearest mainland. It is often said that the climate of western Europe is “insular”; and owing to its position under the lee of the Gulf Stream, this is eminently true of Great Britain.
Subtropic Arid Belts.—Where the surface features of the land in relation to the ocean and prevailing winds do not interpose special obstacles, we find to poleward of both tropics a climatic belt of greater or less width, in which the annual, or at least the summer rainfall is too small to maintain annual herbaceous vegetation throughout the season, even when the temperature is favorable. These two “arid” belts are best defined in Africa, where the northern one is represented by the Sahara desert, lower Egypt and Arabia, while the southern one is exemplified in the Kalahari desert, to northward of the Cape of Good Hope. The northern belt is continued into Asia Minor, Palestine, Syria and Persia, and is again manifest in northwestern India; but to eastward is stopped by the influence of the great Himalaya range. The plateau countries beyond, in Central Asia, are extremely arid, largely by reason of their high elevation.
In Australia the southern arid belt is very strongly defined. In North America, the arid belt is characteristically defined on the Pacific Coast. It embraces all but the southernmost point of the peninsula of Lower California, with about two-thirds of the State of California; thence eastward across Sonora and Arizona to New Mexico and western Texas. But here the influence of the mountain ranges and high plateaus obscures the subtropical belt as such, the arid climate continuing, east of the great Pacific ranges, through Nevada, Utah, Wyoming, Montana, Idaho, and eastern Oregon and Washington nearly to the line of British Columbia on the north, and with gradually decreasing aridity, into Colorado, Kansas, Nebraska, and the Dakotas.
In South America the rainless seaward slopes of southern Peru and northern Chile indicate the southern arid belt; but here, the great chain of the Andes intervening, the dry pampas of Argentina, and the Gran Chaco of southwestern Brazil, like the Nevada basin, though arid would naturally be referred to the moisture-condensing influence of the Andes chain, under the lee of which they lie. From this cause the region of deficient rainfall, which on the western coast ends to northward of Santiago de Chile, is east of the Andes continued much farther poleward, as in North America; reaching into Patagonia.
Utilization of the Arid Belts.—While, as already explained, the distribution of the rainfall through the year is nearly as important as its total amount, yet it is evident that even with the minimum of twenty inches of total precipitation as the measure for crop production, a very large proportion of the land of the arid region cannot, even with the most elaborate system of water conservation, be supplied with sufficient water for ordinary crops, and must be otherwise utilized, mainly for pasture purposes. This is rendered practicable to a much greater extent than might be expected, because the rapid transition from the rainy to the permanent dry season cures the standing herbage into hay, which affords good grazing during the rainless season. Moreover, the use of drought-resistant, browsing forage plants, both shrubs and trees, serves to supplement materially any deficiency in the supply of “standing hay,” especially in case the rains should toward the end be unduly delayed. The same is true of the dried pods and seeds of native herbage, which in some cases (bur clover, lupins, etc.,) afford highly nutritious additions to the leafy forage.[104]
CHAPTER XVII.
RELATIONS OF SOILS AND PLANT
GROWTH TO HEAT.
The Temperature of Soils.—The rapid germination of seeds, as well as the development of plants to maturity, is essentially dependent upon the maintenance of the appropriate temperature. The temperature most favorable to germination or growth, as well as the degree of tolerance of high and low temperatures, varies greatly with different plants, governing mainly what is known as their climatic adaptation. A knowledge of these points with reference to the several crops is therefore of no mean importance to the farmer; for, to a certain extent, he can control the temperature in the soil itself, and he can mostly choose for sowing and planting, the time when the soil shall have the proper temperature for rapid germination or maturity. As a rule, it is not desirable to have either seeds or seedling plants in the ground for any length of time when the temperature is too low for active vegetation; for while they rest, other, lower organisms (fungi and bacteria), adapted to low temperatures, may continue in full activity at the expense of the vitality of the crop plant.