Isothermal lines in anthropo-geography.
Despite some purely climatological objections, anthropo-geography finds the division of climatic zones according to certain isothermal lines of mean annual temperature the most expedient one for its purpose. The hot zone may be taken as the belt north and south of the equator enclosed between the annual isotherms of 20° C. (68° F.) These hold a course generally far outside the two tropics, and in the northern continents frequently reach the thirty-fifth parallel. The temperate climatic zones extend from the annual isotherm of 20° C. to that of 0° C. (32° F.), which bears little relation to the polar circles forming the limits of the solar Temperate Zone. The north temperate climatic zone has been further sub-divided along the annual isotherm of 5° C. (41° F.), distinguishing thus the warmer southern belt, which forms preëminently the zone of greatest historical intensity. The areas beyond the annual isotherms of 0°C. belong to the barren cold zones. [See map page 612.]
Historical effect of compressed isotherms.
This isothermal division of the climatic zones is abundantly justified, because the duration of a given degree of heat or cold in any region is a dominant factor in its human, animal, and plant life. A map of the mean annual isotherms of the earth is therefore eloquent of the relation between historical development and this one phase of climate. Where the lines run far apart, they enclose extensive areas of similar temperature; and where they approach, they group together regions of contrasted temperatures. The compression of climatic differences into a small area enlivens and accentuates the process of historical development. It produces the same sort of effect as the proximity of contrasted reliefs. Nowhere else in the world do the tropical and frigid climatic areas, as defined on the north and south by the annual isothermal lines of 20°C. and 0°C. respectively, lie so near together as in Labrador and northern Florida. Separated here by only twenty degrees of latitude, on the opposite side of the Atlantic they diverge so sharply as to include the whole western face of Europe, from Hammerfest and the North Cape down to the Canary Islands and the crest of the Atlas Mountains in Africa, a stretch of forty-two degrees of latitude. This approximation of contrasted climatic districts in North America was an immense force in stimulating the early economic development of the Thirteen Colonies, and in maturing them to the point of political autonomy. It gave New England commerce command of a nearby tropical trade in the West Indies, of sub-tropical products in the southern colonies, in close proximity to all the contrasted products of a cold climate—dense northern forests for naval stores and lumber, and an inexhaustible supply of fish from polar currents, which met a strong demand in Europe and the Antilles. The sudden southward drop of the 0°C. annual isothermal line toward the St. Lawrence and the Great Lakes brought the northwestern fur trade to the back gate of New York, where it opened on the Mohawk and upper Hudson, and brought prosperity to the young colony. Even to-day the center of collection for the Canadian fur fields is Quebec, located at 47° north latitude, while the corresponding point of concentration in Europe for the furs of Russia and Siberia is Nizhni-Novgorod, which lies ten degrees farther north.[1426]
Effect of slight climatic differences.
This compression of the isotherms emphasizes the differences of national characters produced in part by dissimilar climatic conditions. Contrasts in temperament, manner of life, and point of view, like that between the New Englander and Virginian, Chilean and Bolivian in the Americas, Breton and Provençal in France, Castilian and Andalusian in Spain, Gurkha and Bengali in India, seem to bleach out when they are located far apart, owing to many grades of transition between; but they become striking, stimulating, productive of important economic and political results, when close juxtaposition enables them to react sharply one upon the other. In effecting these nice differentiations of local types, climate is nearly always one of the factors at work, emphasizing perhaps an existing ethnic difference. Even the slight variations of temperature to be found in the same zone or the same climatic region produce distinct results, especially where they are harnessed, as is usually the case, with some other geographic condition of relief, area or soil, pulling in the same direction. Mexico, Peru, Italy, Switzerland, Greece and Asia Minor, with its high plateau interior and its contrasted Euxine and Aegean coasts, represent each a complex of climatic differences, which, reinforced by other geographic factors, have made in these regions a polychrome picture of national life.