The western or arid portion of the lower austral province embraces western Texas, a large area in northeastern Mexico, and circling about the southern extremity of the Pacific mountains in central Mexico, extends northward adjacent to the tropical border of the Pacific and the Gulf of Lower California, into Arizona and southern California. A detached area of this same province occupies the great Valley of California.
The leading feature in the climate of the extensive and irregular region just outlined is its aridity. The rainfall is too small to insure forest growths; the land is treeless, except along the streams, and irrigation is necessary for successful agriculture. With a sufficient amount of water for irrigation, a great variety of fruits, etc., may be raised, including many products usually considered as indicating tropical conditions, such as mangoes, dates, figs, citrus fruits, olives, pineapples, etc. Not only are the agricultural products numerous and varied, but the yield per acre under the most favourable conditions is far in excess of the best results reached in most regions where rain is relied on to furnish the requisite moisture. Under the prevailingly cloudless skies of the hot arid lands of the southwestern
portion of the continent insolation is intense and the growth of vegetation phenomenal when the necessary amount of water is supplied. The land in its present condition presents great contrasts, ranging from desolate, sun-burned tracts which are almost absolute deserts, to the vivid green of irrigated fields and the deep shade of heavily fruit-laden orchards.
The Upper Austral Province (Plate III).—The portion of North America embraced in this climatic province lies principally in the central part of the United States, but includes also a narrow strip in southern Ontario, adjacent to the north shore of Lake Erie, and a large irregular area in the central plateau of Mexico. A marked feature of its geography is its extreme irregularity in the portion occupied by the Pacific mountains in the United States and Mexico. The reasons for this lie mainly in the influence of the relief of the land on climate, the direction of the prevailing winds, and varying distances from the ocean. It is a familiar fact that boreal and even arctic climatic conditions are met with on high mountains. The attention that is given to changes in climate with increase in altitude is no doubt largely due to the fact that the mountains present conditions which are exceptional and more or less novel as seen from our accustomed point of view. A person living in an elevated region, on descending into a deep valley, would be impressed with the reverse order in which the climatic zones occur. In making such a descent he would pass in succession from a boreal or perhaps arctic climate, through a transitional or cold temperate, to the warm temperate or upper austral province, and might even reach the semitropical division of the lower austral. In the Pacific mountains within the border of the United States the valleys are sufficiently deep to have the climatic conditions here ascribed to the upper austral, and in the southwestern portion of the United States descents may be made—as in the Great Valley of California and in the arid basins of southern Nevada, Arizona, etc.—sufficiently great to reach the lower austral. The valleys amid the Pacific mountains, which fall in the upper austral province, are in
general low at the north in reference to sea-level, and become higher and higher at the south. For example, the upper austral region in central Washington is but 400 or 500 feet above the sea, while in Mexico it lies in general at an altitude of between 4,000 and 6,000 feet.
The upper austral province may be termed warm temperate, with a marked contrast between the heat of summer and the cold of winter. The summers are long, with an average temperature of 70° or 75°, while the winters are variable, with frequent cold periods when ice forms and snow-storms are not rare. The snow seldom remains on the ground for more than a few days at a time, however, except in the northeast, where the warm temperate climatic conditions of the province under review merge with those of the colder region to the northward embraced in the transition province.
The upper, like the lower austral, presents two well-marked divisions in reference to humidity—an eastern or humid and a western or semihumid portion; the dividing line is in the neighbourhood of the one hundredth meridian. In the eastern division the mean annual precipitation in the Piedmont region to the east of the Appalachians and on the coastal plain adjacent to the Atlantic in Maryland and New Jersey is from 40 to 80 inches, but decreases westward, and on the border of the Great plateaus in Kansas and Nebraska is about 20 inches. In the western division the annual precipitation is less than 20 inches, and agriculture without irrigation is uncertain and usually impossible. To the east of the one hundredth meridian the rain is somewhat evenly distributed throughout the year, although an increasing dryness of the summer is easily detected as one travels from east to west, but in the various upper austral valleys of the Pacific mountain region the precipitation is mostly during the winter, and the summers are practically rainless. The marked difference in precipitation between the humid and semihumid division of the upper austral province is recorded on the surface of the land by the vegetation. In the eastern division the entire region, with the exception of the prairies in the central part of the Mississippi
basin, was originally clothed with a varied and beautiful forest, consisting mainly of broad-leaved trees, such as the hickory, maple, oak, etc., while the semihumid western division is treeless, except in immediate proximity to streams.
In the southern portion of the humid division of the upper austral province cotton is one of the staple products, but the northern limit of the region in which it can be successfully cultivated is soon reached as one travels northward. Tobacco is grown extensively in the southeastern and eastern portions of the province. The principal crop of the great central area in the Mississippi Valley is corn (maize). Successful wheat culture begins in the northern portion of the province, but the conditions favouring its cultivation increase to the northward and it becomes the characteristic and most valuable crop of the transition province.
In the western or semihumid division of the upper austral the variety of agricultural products that can be successfully cultivated with the aid of irrigation is greater than in its eastern portion, where irrigation is not generally practised. The northern limit at which tobacco, fruits, the vine, etc., may be advantageously cultivated in the west is greater than in the east. For example, in the east the northern limit at which tobacco is raised on a commercial scale is in Connecticut, while in the west it reaches a large size and excellent quality in central Washington. Various fruits, such as the peach, pear, plum, grape, etc., have their northern limit of successful cultivation in the east in western New York, southern Ontario, and southern Michigan, a region favourably influenced in this connection by the proximity of the Great Lakes; in the west these same fruits reach a high degree of perfection, and are produced in great abundance, with the aid of irrigation, in north-central Washington, fully 5 degrees of latitude farther northward.