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52 From the Atlantic Coast to the Rocky Mountains the general westerly direction of the winds prevails pretty steadily, except as it is upset occasionally by cyclonic storm winds.

In the plateau region and the basin, the upper winds are westerly — southwest to northwest; but the surface winds in many instances are deflected by mountain ranges and become either southerly or northerly.

The surface winds of the Pacific Coast are westerly; but in the Sacramento and San Joaquin valleys they become northerly or southerly, being deflected by the mountain ranges. When the temperature of the interior is high, strong westerly winds are the rule along the coast. This is especially noticeable in the vicinity of San Francisco.

Monsoons.—The monsoons are seasonal winds which blow from the sea over the land during summer months and in an opposite direction during winter months. The name, meaning “season,” was first applied to the seasonal winds of the Indian Ocean Coast; subsequently it was applied to various seasonal winds of ocean coasts. In southern Asia the crop yield depends very largely on the rainfall which accompanies the southwest monsoon; hence its importance in the economic history of a considerable part of southern Asia.

The advance and recession of the Trade Wind belt along the Gulf Coast of the United States seems to emphasize a similar alternation of sea wind and land wind; but the monsoon characteristics extend as far north as Long Island Sound on the north and far into Mexico on the south. In the latitude of New York City, about eight weeks of southwesterly winds prevail in summer, while northwest winds prevail the rest of the year.

Calm Belts.—Along the narrow belt where the northeast and the southeast Trade Winds meet, the easterly components of the winds disappear, the only movement being an updraught. In the days of sailing vessels ships sometimes lay becalmed for many days—hence the expressive name, Doldrums. This calm belt lies north of the equator and practically covers the thermal equator. It is a region of low barometer, very moist air, cumulus clouds and excessive rains. It is practically coincident with the tropical rain-belt. Its detrimental effects on marine transportation ceased with the advent of steam navigation. Years ago it was a terror to sailing craft.