such small fruits as currants, huckleberries, raspberries, blackberries, cranberries, etc., grow wild and yield abundant returns when cultivated. In favoured localities white potatoes, turnips, beets, and certain varieties of the apple, as well as the more hardy cereals, are cultivated with moderate success.
The Arctic Province (Plate III) comprises the cold, treeless plains sloping to the Arctic Ocean and the summits of the higher mountains at the south which rise above the transition province. The one controlling climatic feature is the low temperature, the mean for each year being 32° F. or lower. The winters are longer and more severe than in the boreal province, and the summers short and hot. Insolation, on account of the length of the days in summer of the main area of the province and the free exposure on the mountain summits to the southward, is intense, but its beneficial effect on vegetation is largely counterbalanced by the influence of the lingering snow and ice. In the mountainous regions of North America the arctic province is the birthplace of numerous glaciers. Although destitute of trees, the arctic, or arctic-alpine province, as it may be termed, is rendered glorious in numberless localities by the profusion and brilliancy of its flowering annuals.
SECONDARY DISTURBANCES OF THE ATMOSPHERE
In the broad, general movements of the atmosphere over North America embraced in what are termed the planetary and continental winds there are many disturbances due to more or less local changes in conditions, the most conspicuous of which are whirlwinds, chinook winds, thunder-storms, tornadoes, cyclones, and hurricanes. While some of these disturbances are local, as the whirlwind and tornadoes, and may not extend beyond the boundaries of the particular climatic provinces where they originate, others, as the cyclones and hurricanes, may affect the climate of several provinces.
Whirlwind.—A conspicuous, although minor feature in the atmospheric phenomena of the hot, dry plains and
valleys, especially of the Mexican plateau and the Great Basin, and less markedly of the Great plateau to the east of the Rocky Mountains, is the occurrence of small whirlwinds which carry dust and light objects into the air in spiral columns that are not infrequently 2,000 or 3,000 feet high, and have a diameter of perhaps 50 to 100 feet. These small whirls of the air, in which some of the characteristic features of the intensely active tornadoes and widely destructive tropical hurricanes can be studied on a small scale, occur most commonly during hot summer afternoons, when from a commanding station half a dozen or more swaying columns may be seen moving in various directions over the parched valleys and sun-scorched plains. These columns not only move in various directions, showing that they are not due to the same immediate cause, but have different internal motions, some whirling from right to left, and others in the opposite direction.
The generally accepted explanation of these small whirlwinds is that the air over the surface of the deserts, which are frequently almost bare of vegetation and perhaps white with saline incrustations, becomes locally highly heated, especially when there is little or no wind, and is forced upward by the inflow of the surrounding cooler and heavier air. The inflowing currents have different velocities, and on meeting the strongest one gives a rotary or spiral motion to the ascending column, which acts like a chimney in allowing the escape upward of the hot air from below. A central vertical line frequently seen in the dust columns shows that a core of comparatively still air is present, about which the dust-charged air rises in a spiral course. If the conditions just outlined should be greatly increased in magnitude some of the leading features of tornadoes and even of hurricanes would be produced. In short, all of the winds cited above, except the chinook, are concentric, swirling movements in ascending air, due primarily to a local increase in temperature at the lower portion of the atmosphere.
Chinook Winds.—On the Great plateaus adjacent to the Rocky Mountains, and in similar situations to the eastward
of the Sierra Nevada and Cascade Mountains, warm, drying winds frequently occur, especially in winter, when they bring a balminess as of spring. The remarkable feature of these interesting winds is that they come from the snow-clad mountains, but are warm and dry in contrast with the preceding condition of the air on the plains. The capacity of the air brought by these winds for moisture is so great that evaporation is active, and the snow in the valleys and over the broad plains disappears without visible melting. The change in the previously winter aspect of a region within the influence of these chinook winds, as they are termed, is truly surprising, and to their influence is due to a marked extent the value of the Great plateaus as stock-ranges, for the reason that the snow is removed from them so as to allow cattle to feed on the naturally dried grasses.
The chinook winds are the counterpart of the foehn winds of Switzerland, and are explained on the principle that descending air is made more dense by the increased pressure to which it is subjected, and its temperature correspondingly raised, its capacity for moisture being at the same time increased on account of its rise in temperature. The apparent anomaly of a warm, dry wind blowing from a snow-clad mountain range is no longer a mystery, if we consider that the air is drawn over the mountains towards a centre of low barometrical pressure owing to the wide-reaching influence of a cyclonic storm or other large atmospheric movement. The air as it rises in order to cross a mountain is cooled, largely on account of relief of pressure, and parts with a portion, possibly a large portion, of its moisture, which condenses on the mountain commonly as snow; on passing the mountain the air descends and is warmed by compression, and having less moisture than before, becomes a drying wind, which produces the sudden and surprising changes on the plains and valleys to the leeward.