In this general view of the climatic zones and the normal changes they undergo we may note that the torrid zone is characterized by its simplicity and monotony of climatic conditions, although disturbed at times, especially in the West Indies, by occasional great cyclonic storms, termed hurricanes, which occur, however, at quite definite seasons. The temperate belt is equally well marked by its complex and frequently changing atmospheric conditions, the winds being subject to numerous and great variations, and storms of diverse character being frequent. The frigid zone, again, is without conspicuous variations except during the change from its monotonous summer to its still more uniform winter weather, and the reverse change six months later. The disturbances in the balance of atmospheric conditions at the far north, or the storms, are of a much less varied character than in the fickle temperate zone—thunder-storms and tornadoes, for example, being unknown.
It is the summer migration of a heated belt from the south northward across the temperate zone, and the equally conspicuous winter advances of cold from the north southward across the same broad region, which gives to the United States and the southern portion of Canada a conspicuously changeable climate. The temperate zone, so far
at least as North America is concerned, deserves its name only when the mean of the yearly changes in temperature is considered, as much of it is hotter in summer than equal areas between the tropics, and in winter over all of its northern half the cold is, at times, nearly or quite as intense as during the same season in the far north. As a whole, the portion of the continent embraced in the temperate zone is characterized by its pronounced seasonal changes, including wide extremes of heat and cold over large areas, and by its frequently sudden and strongly marked weather changes during short periods of time. It is a highly suggestive fact that of all the great climatic zones the one having the most changeable climate, the greatest extremes of heat and cold, and the most frequent storms should be the one in which man has reached the highest development both of body and mind. Evidently it is the struggle for existence, when not too severe, which insures advancement. The part of North America most densely inhabited by descendants of Europeans, and the portion of the continent where intellectual development has made the greatest advance, is the east-central portion, where not only the variation of climate from season to season, but the weather changes from week to week and day to day are the most conspicuous.
Secondary Conditions influencing Climate.—While the primary conditions controlling the climate of North America in common with all other portions of the earth's surface depend on the relation of the earth to the sun, there are many secondary conditions to be considered. First in importance among these, so far as the broader features of the climate of the continent are concerned, is the unequal heating of land and water areas. During summer, more especially in the temperate zone, the land becomes more highly heated than the adjacent oceans, and an inflow of the cooler and moister air from the sea over the land occurs. In winter the land cools more quickly and to a greater degree than the adjacent waters, and the tendency of the heavier air over the land is to flow outward as surface winds. Continental winds are thus generated, similar in their origin to the familiar land and sea breezes of the ocean shore in
summer, but on a large scale, which have an important bearing on the seasonal changes. The influence of the continental winds is sufficiently well marked to give North America two general classes of climate. One pertains to inland regions, is characterized by great contrasts in temperature and humidity between summer and winter, and is termed continental. The other pertains to the border of the land where, on account of the equalizing influence of large water borders, the contrast between the climate of summer and winter is less pronounced, and has received the general title of oceanic climate. The climate of the Dakotas, for example, is of the continental type, while that of New Jersey is of the oceanic type.
The unequal heating and cooling of adjacent portions of land areas also produces important atmospheric movements, as, for instance, when broad, treeless plains become more highly heated in summer than adjacent forested areas; or on account of rapid radiation become excessively cold in winter and lower the temperature of the air above them. In the first instance an inflow of cooler and heavier air from adjacent regions would be established; and in the second example the chilled air would tend to flow outward, thus, in each instance, establishing winds which usually acquire a more or less well-pronounced circular motion. The Prairie plains and the Great plateaus to the east of the Rocky Mountains become highly heated in summer, and together with several other similar regions in North America, meet the first of the conditions just considered; while the higher portions of the Great plateau, especially at the north, and the still more elevated mountains of Montana, Colorado, etc., become excessively cold in winter and illustrate the other extreme.
Mountains serve to deflect the winds blowing against them either to one side or upward, the former frequently producing important changes in direction of the surface air-currents, and the latter, by causing the air to rise, permits of its expansion and consequent cooling, thus favouring precipitation. For this and other reasons precipitation increases with elevation, at least until an altitude of many
thousands of feet is reached, and the mountains are cooler and more humid than the adjacent valleys. The air-currents on passing over a mountain range and descending are warmed by compression, and having lost a part and in many observed instances a large percentage of the moisture they previously contained, become warm, drying winds. The chinook winds, as they are termed in America, are marked examples of the influence exerted by mountains on climatic conditions.
What are termed above the secondary conditions, tending to modify climate, produce such great changes in the distribution of rainfall, temperature, etc., and in the influence of the planetary winds, that the subdivision of the northern hemisphere into torrid, temperate, and frigid zones, while based on astronomical data, does not serve to represent actual conditions, except in a general way, in reference even to the single element of temperature expressed in these names. A comparison of the isotherms and of the distribution of precipitation as indicated on the preceding maps, with the parallels of latitude, shows at once that these two most important elements of climate are conspicuously independent of distance from the equator. A logical basis for subdividing the continent into climatic provinces must therefore be sought in other directions.
CLIMATIC PROVINCES