winds also are much affected by the changes of pressure due to the
strong continental changes of temperature. The warmed air of
summer produces an area of low pressure in the west-central United
States, which interrupts the belt of high pressure that planetary
conditions alone would form around the earth about latitude 30°;
hence there is a tendency of the summer winds to blow inward
from the northern Pacific over the Cordilleras toward the continental
centre, and from the trades of the torrid Atlantic up the Mississippi
Valley; conversely in winter time, the cold air over the lands produces
a large area of high pressure from which the winds tend to
flow outward; thus repelling the westerly winds of the northern
Pacific and greatly intensifying the outflow southward to the Gulf
of Mexico and eastward to the Atlantic. As a result of these
seasonal alternations of temperature and pressure there is something
of a monsoon tendency developed in the winds of the Mississippi
Valley, southerly inflowing winds prevailing in summer and northerly
outflowing winds in winter; but the general tendency to inflow
and outflow is greatly modified by the relief of the lands, to which we
next turn.
The climatic effects of relief are seen directly in the ascent of the higher mountain ranges to altitudes where low temperatures prevail, thus preserving snow patches through the summer on the high summits (over 12,000 ft.) in the south, and maintaining snowfields and moderate-sized glaciers on the ranges in the north. With this goes a general increase of precipitation with altitude, so that a good rainfall map would have its darker shades very generally along the mountain ranges. Thus the heaviest measured rainfall east of the Mississippi is on the southern Appalachians; while in the west, where observations are as yet few at high level stations, the occurrence of forests and pastures on the higher slopes of mountains which rise from desert plains clearly testifies to the same rule. The mountains also introduce controls over the local winds; diurnal warming in summer suffices to cause local ascending breezes which frequently become cloudy by the expansion of ascent, even to the point of forming local thunder showers which drift away as they grow and soon dissolve after leaving the parent mountain. Conversely, nocturnal cooling produces well-defined descending breezes which issue from the valley mouths, sometimes attaining an unpleasant strength toward midnight.
The mountains are of larger importance in obstructing and
deflecting the course of the general winds. The Pacific ranges, standing
transverse to the course of the prevailing westerlies near the
Pacific Ocean, are of the greatest importance in this respect; it is
largely by reason of the barrier that they form that the tempering
effects of the Pacific winds are felt for so short a distance inland
in winter, and that the heat centre is displaced in summer
so far towards the western coast. The rainfall from the strormy
westerly winds is largely deposited on the western slopes of the
mountains near the Pacific coast, and arid or desert interior plains
are thus found close to the great ocean. The descending winds
on the eastern slopes of the ranges are frequently warm and dry,
to the point of resembling the Fohn winds of the Alps; such winds
are known in the Cordilleran region as Chinook winds. The ranges
of the Rocky Mountains in their turn receive some rainfall from the
passing winds, but it is only after the westerlies are reinforced
by a moist indraft from the Gulf of Mexico and the Atlantic—the
result of summer or of cyclonic inflow—that rainfall increases to as
sufficient measure on the lower lands to support agriculture without
irrigation. The region east of the Mississippi is singularly favoured
in this way; for it receives a good amount of rainfall, well distributed
through the year, and indeed is in this respect one of the
largest regions in the temperate zones that are so well watered.
The Great Plains are under correspondingly unfavourable conditions,
for their scanty rainfall is of very variable amount. Along the
transition belt between plains and prairies the climate is peculiarly
trying as to rainfall; one series of five or ten years may have sufficient
rainfall to enable the farmers to gather good crops; but the next
series following may be so dry that the crops fail year after year.
The cyclonic inflows and anticyclonic outflows, so characteristic of the belt of westerly winds the world over, are very irregular in the Cordilleran region; but farther eastward they are typically developed by reason of the great extent of open country. Although of reduced strength in the summer, they still suffice to dominate weather changes; it is during the approach of a low pressure centre that hot southerly winds prevail; they sometimes reach so high a temperature as to wither and blight the grain crops; and it is almost exclusively in connexion with the cloudy areas near and south-east of these cyclonic centres that violent thunderstorms, with their occasional destructive whirling tornadoes, are formed. With the passing of the low pressure centre, the winds shift to west or north-west, the temperature falls, and all nature is relieved. In wintertime, the cyclonic and anticyclonic areas are of increased frequency and intensity; and it is partly for this reason that many meteorologists have been disposed to regard them as chiefly driven by the irregular flow of the westerly winds, rather than as due to convectional instability, which should have a maximum effect in summer. One of the best indications of actual winter weather, as apart from the arrival of winter by the calendar, is the development of cyclonic disturbances of such strength that the change from their warm, sirocco-like southerly inflow in front of their centre, to the “cold wave” of their rear produces non-periodic temperature changes strong enough to overcome the weakened diurnal temperature changes of the. cold season, a relation which practically never occurs in summer time. A curious feature of the cyclonic storms is that, whether they cross the interior of the country near the northern or southern boundary or along an intermediate path, they converge towards New England as they pass on toward the Atlantic; and hence that the north-eastern part of the United States is subjected to especially numerous and strong weather changes. (W. M. D.)
IV.—Fauna and Flora
Fauna.—Differences of temperature have produced in North America seven transcontinental life-zones or areas characterized by relative uniformity of both fauna and flora; they are the Arctic, Hudsonian and Canadian, which are divisions of the Boreal Region; the Transition, Upper Austral and Lower Austral, which are divisions of the Austral Region, and the Tropical. The Arctic, Hudsonian and Canadian enter the United States from the north and the Tropical from the south; but the greater part of the United States is occupied by the Transition, Upper Austral and Lower Austral, and each of these is divided into eastern and western subzones by differences in the amount of moisture. The Arctic or Arctic-Alpine zone covers in the United States only the tops of a few mountains which extend above the limit of trees, such as Mt Katahdin in Maine, Mt Washington and neighbouring peaks in the White Mountains of New Hampshire, and the loftier peaks of the Rocky, Cascade and Sierra Nevada Mountains. The larger animals are rare on these mountain-tops and the areas are too small for a distinct fauna. The Hudsonian zone covers the upper slopes of the higher mountains of New England, New York and North Carolina and larger areas on the elevated slopes of the Rocky and Cascade Mountains; and on the western mountains it is the home of the mountain goat, mountain sheep, Alpine flying-squirrel, nutcracker, evening grosbeak and Townsend's solitaire. The Canadian zone crosses from Canada into northern and north-western Maine, northern and central New Hampshire, northern Michigan, and north-eastern Minnesota and North Dakota, covers the Green Mountains, most of the Adirondacks and Catskills, the higher slopes of the mountains in Pennsylvania, West Virginia, Virginia, western North Carolina and eastern Tennessee, the lower slopes of the northern Rocky and Cascade Mountains, the upper slopes of the southern Rocky and Sierra Nevada Mountains, and a strip along the Pacific coast as far south as Cape Mendocino, interrupted, however, by the Columbia Valley. Among its characteristic mammals and birds are the lynx, marten, porcupine, northern red squirrel, Belding's and Kennicott's ground squirrels, varying and snowshoe rabbits, northern jumping mouse, white-throated sparrow, Blackburnian Warbler, Audubon Warbler, olive-backed thrush, three-toed woodpecker, spruce grouse, and Canada jay; within this zone in the North-eastern states are a few moose and caribou, but farther north these animals are more characteristic of the Hudsonian zone. The Transition zone, in which the extreme southern limit of several boreal species overlaps the extreme northern limit of numerous austral species, is divided into an eastern humid or Alleghanian area, a western arid area, and a Pacific coast humid area. The Alleghanian area comprises most of the lowlands of New England. New York and Pennsylvania, the north-east corner of Ohio, most of the lower peninsula of Michigan, nearly all of Wisconsin, more than half of Minnesota, eastern North Dakota, north-eastern South Dakota, and the greater part of the Appalachian Mountains from Pennsylvania to Georgia. It has few distinctive species, but within its borders the southern mole and cotton-tail rabbit of the South meet the northern star-nosed and Brewer's moles and the varying hare of the North, and the southern bobwhite, Baltimore oriole, bluebird, catbird, chewink, thrasher and wood thrush are neighbours of the bobolink, solitary vireo and the hermit and Wilson's thrushes. The Arid Transition life-zone comprises the western part of the Dakotas, north-eastern Montana, and irregular areas in Washington, Oregon, Idaho, Wyoming, California, Nevada, Utah, Colorado, Arizona, New Mexico and western Texas, covering for the most part the eastern base of the Cascade and Sierra Nevada Mountains and the higher parts of the Great Basin and the plateaus. Its most characteristic animals and birds are the white-tailed jack-rabbit, pallid vole, sage hen, sharp-tailed grouse and green-tailed towhee; the large Columbia ground-squirrel (Spermophilus columbianus) is common in that part of the zone which is west of the Rocky Mountains, but east of the Rockies it is replaced by another species (Cynomys) which closely resembles a small prairie dog. The Pacific Coast Transition life-zone comprises the region between the Cascade and Coast ranges in Washington and Oregon, parts of northern California, and most of the California coast region from Cape Mendocino to Santa Barbara. It is the home of the Columbia black-tail deer, western raccoon, Oregon spotted skunk, Douglas red squirrel, Townsend's chipmunk, tailless sewellel (Haplodon rufus), peculiar species of pocket gophers and voles, Pacific coast forms of the great-horned, spotted, screech and pigmy owls, sooty grouse, Oregon ruffed grouse, Steller's jay, chestnut-backed chickadee and Pacific winter wren. The Upper Austral
