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The Climate extends from subtropical to polar latitudes. The eastern portion of the Continent shows a strong latitudinal influence in the sequence of humid subtropical, humid continental, sub arctic and polar climates. Northward from the Gulf of Mexico the climate changes from the shorter and cooler summer to the longer and more severe winter. The interior west is lying on the leeward side of the major highlands and remains dry. Along the coast from California to Alaska is a subtropical to high latitude West coast sequence dry summer, subtropical marine west coast, sub arctic and polar climates. The Anglo-American highlands effectively prevent the extension of these humid climates to great distances into the interior.

There are two humid regions:

One region extends from Gulf coast to Hudson Bay and eastward to the Atlantic. Maritime air masses from the Gulf of Mexico or the Atlantic Ocean are the sources of moisture for areas, far to the interior. The second covers a smaller area along the west coast from California to Southern Alaska and the Aleutian Islands. A region of low rainfall is the region of low elevation. Rainfall less than 50 cm extends from the American southwest to the northwest territories of Canada. The deficiency of moisture over the Great Plains is affecting the agricultural potential of the region.

In recent years in United States less than 10% of water tapped is used for domestic use. More than half (50%) is for industrial and the remainder approximately 40% has been used for irrigation. The length of the growing season decreases with the latitude. A growing season in excess of 200 days is in the subtropical region. Much of Canada and Alaska are not suitable for agricultural production. Latitude, altitude and maritime influences are felt on temperature distribution. Growing season is reduced as latitude, altitude and distance from the sea are increasing.

Natural Vegetation The chief vegetation zones of North America are well marked, but it should be remembered that these zones gradually merge into one another, and that over extensive areas the natural vegetation has been greatly modified by Man.

Vegetation reflects the impact of topography, soil and climate. A narrow band of treeless Tundra extends across far into Northern America from Alaska to Greenland. In Alaska and northwest Canada the Tundra extends southward in highlands, from Newfoundland to Alaska is a vast coniferous forest.

This coniferous (boreal) forest is the largest, where spruce, fir and pine dominate. In the area adjacent to the waterways, the forest is intensively exploited. A deciduous forest belt is a broad transitional zone of white and yellow birch, poplars and maples in the humid eastern half of the continent. The original deciduous forest, which covered most of the northeastern U.S.A., consisted of oak, elms, hickory, beech and maples. The present deciduous forest is the most modified form of vegetation. Southern extension of this deciduous forest has disappeared because of exploitation and today pine trees have replaced the hardwood species as in the Piedmont zone. The grasslands extend from tall prairie grasses on the eastern margin of the Great Plains to short grasses on the drier western margin. The Prairie extended eastward beyond the Great Plains into Iowa and Illinois at the time of European settlement but now replacement is by oak trees. Grasslands are indicative of moist ure deficiency and its association with the best soils. Recurring droughts and over grazing replace the grasses with the growth of woody shrubs.

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