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In the Northern Hemisphere, the western portions of con¬tinents are especially favored by the prevailing winds. This is because the western lands gather the rains as they come off the ocean, blown by storms that circle from west to east.
Unfortunately, the Cascade Mountains and the Sierra Nevada Mountains, so dose to the west coast, catch the largest share of the rain off the Pacific Ocean before it can go further Inland. As a result, there is too little rain for almost the whole western half of the United States, which lies in the ,,rain shadow" of the mountains. In a great part of that territory, therefore, farmers must depend on irrigation water from the snows or rains that are trapped by the mountains.
One of the most important geographic boundaries in the United States 15 the 50-centimeter rainfall line, which runs north and south almost through the middle of the country. East of the line, farming is relatively easy, and the popula¬tion is relatively large. West of the line, one finds man-made irrigation systems, dry-farming, grazing, and fewer people. West of the Rocky Mountains, running all the way from the Canadian border to Mexico, there are vast areas where almost no trees grow. In this section of the country are the deserts which receive as little as 12.7 centimeters of rainfall a year. Yet, west of the Sierra Nevada Mountains, there are places in which 250 centimeters of ram falls annually.
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