TRADE WIND, in meteorology, in the plural, certain ocean winds, blowing constantly in one direction or very near ly so, can be calculated on beforehand by the mariner and are therefore bene ficial to trade. They exist on all open oceans to a distance of about 30° N. and S. of the equator, blowing from about the N. E. in the Northern and from the S. E. in the Southern Hemisphere. Where they meet they neutralize each other, creating a region of calm N., and the same distance S. of the equator. Atmospheric air expands by heat, and, expanding, naturally ascends, its place being supplied by a rush of colder and consequently of denser air beneath. The process is continually in progress, to a great extent, everywhere throughout the tropics, but especially above the land. If the globe consisted solely of land, or solely of water, and had no rotation, the cold currents would travel directly from the N. and S. poles to the equator; but the rotation of the earth deflects them from their course. The atmosphere lags behind the moving planet, especially at the equator, where the rotation is about 1,000 miles an hour. Neither the direc tion nor the area of the trade winds re mains fixed. Since they supply the place of rarefied air, which is ascending, they must follow the movement of the sun, blowing to the point of greatest rare faction, as a cold current coming through a keyhole goes to the fire. Hence, the
area of the trade winds extends from two to four degrees farther N. than usual when the sun is at the Tropic of Cancer, and the same number of degrees farther S. than usual when he is at the Tropic of Capricorn. In the former case the S. E. trade wind declines further from the E. from its N. limit, sometimes passing the equator, while the N. E. trade wind approaches an E. direction more than at other times. The region of calms also changes its position. As the difference of pressure is not great, the trade wind is generally moderate in strength, espe cially in the opposite hemisphere from that in which the sun is at the time. The trade winds were not known till Columbus' first voyage. They are most marked on the Atlantic and Pacific Oceans, where they occur between 9° and 30° N., and between 4° and 22° S. in the former, and between 9° and 26° N., and between 4° and 23° S. in the latter ocean, but become modified in the vicinity of land, so as to lose their dis tinctive character. in the Indian Ocean and in Southeastern Asia they become altered into monsoons.