During the winter months the high pressure area over the con tinent causes the winds to blow outward. In the east, the heaviest rainfall is along the south part of the Atlantic coastal plain and round the Gulf, where, owing to the relatively high temperature, there is a considerable amount of water vapour in the atmosphere, which is drawn inland by local storms and precipitated. To the Mexican coast, also, the trade winds bring a considerable amount of rainfall. On the Pacific coast, precipitation is heavy as far south as San Francisco, the heat equator having moved southwards and the whole region being brought within the influence of the south westerly winds. Between Vancouver and San Francisco precipita tion takes place only during the autumn and winter months, but south of the latter place the winds are off-shore, and the surrounding region is as dry in winter as it is in summer.
The annual distribution of precipitation over North America as determined by these conditions is somewhat as follows : the Atlantic and Gulf Coastal Plains and the interior of the country as far north as the fortieth parallel and as far west as the ninety-fifth meridian (with the exception of the north-west corner of this region), have a rainfall of between 40 and 60 inches per year, except around, and for some distance east of, the Mississippi delta where the latter amount is exceeded. The shores of the Gulf of St Law rence, and the country south of the Great Lakes and east of a line running from their western extremity to the Gulf of Mexico, have at least 30 inches annually. A third region is that which lies to the south of Hudson Bay and east of a line which oscillates between the 95th and 100th meridians, and over which the mean annual precipitation is between 20 and 30 inches ; this amount is also received by a great part of the Mexican Plateau, especially in the south. On the eastern slope of the Mexican tableland, the pre cipitation is much greater, and rises in places to over 80 inches per year. On the Pacific coast there is a heavy rainfall which, as a rule, does not extend far inland, except in the Puget Sound region. Along the coast from Sitka to Vancouver it is over 80 inches per year, and as far south as San Francisco there are at least 40 inches. The greater part of the western slope of Mexico has at least 20 inches, while a more restricted coastal strip has over 40 inches. Over the remainder of North America the total precipi tation falls between 10 and 20 inches annually except in two regions. The first of these is in the extreme north of Canada, where a tri angular area,with its base along the Arctic shores, receives less than 10 inches ; and the second, which receives a like amount, is situated in the south-west of the United States and the north-west of Mexico.
The first region lies in an area of constant high pressure, while the second lies on the west' side of a continent within the belt of constant trade winds.
VEGETATION.—In this section the natural vegetation regions of the continent will alone be described, changes introduced by man, such as the substitution of agricultural land for woodland, being deferred until later.
The forests of the Atlantic and Pacific slopes differ greatly in their essential characteristics, the result of differences both in their past and present climatic environment ; but they are connected by a great belt of woodland called the Northern Forest, which stretches across the continent from the one ocean to the other. Its northern boundary is coincident with the limit of tree growth, which runs along the coast of Labrador near the sixtieth parallel, across north-west Canada from the mouth of the Churchill to that of the Mackenzie, and along the north-eastern and north-western slopes of Alaska. The southern limit on the Atlantic coast may be taken as the fiftieth parallel, and on the Pacific as the fifty-eighth. On these coasts the Northern Forest gradually merges into the Atlantic and Pacific Forests respectively ; on the south-west it is limited by drought, and on the north by high winds during cold weather. The characteristic trees are the black and the white spruce, but the pine, larch, birch, and poplar are also found. This forest is not of great value ; the trees grow in open formation, and, owing to the shortness and low temperature of the vegetative season, seldom reach a considerable size. On the southern margin some are suitable for timber, but over the greater part of the area they are at best fit only for pulp. The Northern Pine Belt is the continuation to the south, on the Atlantic side, of the Northern Forest. It extends inland as far as the ninety-sixth meridian, and runs south wards along the coast to the fortieth, and along the higher parts of the Appalachians to the thirty-seventhCparallel ; west of these mountains it does not extend beyond.; the forty-third parallel, where it merges into the deciduous Mississippi Forest. The white pine is the characteristic, and perhaps the most valuable tree of the region, while the black spruce, sugar maple, birch, and cedar are all extensively found.