On the whole, the upper austral province may be said to have a warm temperate climate, in which the summer season is longer and more pronounced in its characteristics
than the winter season. It presents sufficient seasonal variations, however, to favour in a high degree both the physical and intellectual development of man.
The Transition Province (Plate III).—This, the transition region between the austral and boreal provinces, includes the cool temperate portions of North America. Like the austral provinces, its outlines are irregular, and in places it occupies detached or island-like areas, on account of the influence of mountains on climatic conditions. Its largest continuous area is situated along the northern border of the United States, but includes the southern portions of Assiniboia, Ontario, and Quebec, and extends from the Atlantic to the Pacific Ocean. From this main belt there is a marked extension southward along the Appalachian Mountains, which carries a cool temperate climate into northern Georgia, and another and much greater southward extension along the Pacific mountains, which reaches central Mexico. In the northern portion of the United States and adjacent parts of Canada, the region under consideration has, in general, an elevation of 1,000 feet or even less above the sea, but it rises when followed southward along the mountains, and in the southwestern portion of the United States and on the table-land of central Mexico attains an elevation of some 8,000 or 9,000 feet. On the higher portions of the mountains of Central America and the West Indies the climatic conditions are similar to those of the regions farther north here included in the transition climatic province, but these isolated areas are not sufficiently well known to be indicated on the accompanying map.
The mean annual temperature of the transition province is lower than that of the upper austral, and, although a precise average is not at present attainable, may be taken at about 45° F. The winters are long and cold, especially in the northern part of the United States and adjacent portions of Canada, the mean temperature being approximately 20° or 25°, but sudden and great variations are not uncommon. At times, and frequently for continuous periods of several days, the temperature falls to 20° or more below zero of the Fahrenheit scale, then again rises above
freezing, and the frozen soil thaws and possibly becomes entirely freed from ice.
In common with the austral provinces, the one under consideration is divided into two portions, an eastern and a western, in reference to precipitation, the dividing line being a little to the eastward of the one hundredth meridian. While the western portion of the province is characterized by its small rainfall, precipitation is greater, mainly on account of greater elevation, than in the adjacent portions of the austral provinces, and in Oregon and Washington there is a markedly humid area.
Snow falls throughout the entire transition province, at least during exceptional winters, and in general during every winter. There is great variation in the depth of the snow from winter to winter, and also with geographical position. It is greatest in the northeast—that is, from the Atlantic coast westward to Wisconsin and Minnesota—and least on the Pacific coast and the southern portion of the Pacific mountains. Over the northeastern portion of the province the snow frequently whitens the ground for weeks, and even for two or three months continuously. A deep accumulation which remains for a long time unmelted is welcome, as it protects the roots of plants from sudden changes of temperature and prevents alternate freezing and thawing of their sap, which is injurious to their tissues in numerous instances. Great variation in the amount of snow that falls annually in a given locality is of common occurrence. A large proportion of the yearly accumulation frequently occurs during one or two great storms. For example, in January and February, 1898, there were two severe storms, accompanied by an unusual depth of snow, the first being most pronounced over New England, and the second in the region of the Great Lakes. Other storms increased the amount of snow so that at the beginning of March the average depth in Maine was 40 to 70 inches, in New Hampshire and Vermont 10 to 40 inches, and in Massachusetts 10 to 20 inches, while in Michigan and Wisconsin the general depth on level ground was 1 to 2 feet, with many drifts 10 to 15 feet deep.
