Page:Physical Geography of the Sea and its Meteorology.djvu/392

366 instead of blowing from that quarter for twelve months, as in other seas, they blow only for six. During the remaining six months they are turned back, as it were; for, instead of blowing towards the equator, they blow away from it, and instead of N.E. trades we have S.W. monsoons.

683. A low barometer in Northern India.—If the N.E. trade-winds blow towards the equator by reason (§ 657) of the lower barometer of the calm belt there, we should—seeing them turned back and blowing in«the opposite direction as the S.W. monsoon—expect to find towards the north, and at the place where they cease to blow, a lower barometer than that of the equatorial calm belt. The circumstances which indicate the existence of a lower summer barometer—the period of the S.W. monsoon—in the regions about northern India are developed by the law which (§ 657) requires the wind to blow towards that place where there is least atmospheric pressure.

684. The S.W. monsoons "backing down."—The S.W. monsoons commence at the north, and " back down," or work their way towards the south. Thus they set in earlier at Calcutta than they do at Ceylon, and earlier at Ceylon than they do at the equator. The average rate of travel, or "backing down to the south," as seamen express it, is from fifteen to twenty miles a day. It takes the S.W. monsoons six or eight weeks to "back down" from the tropic of Cancer to the equator. During this period there is a sort of barometric ridge in the air over this region, which we may call the monsoon wave. In this time it passes from the northern to the southern edge of the monsoon belt, and as it rolls along in its invisible but stately march, the air beneath its pressure flows out from under it both ways—on the polar side as the S.W. monsoon, on the equatorial as the N.E.

686. How they begin.—As the vernal equinox approaches, the heat of the sun begins to play upon the steppes and deserts of Asia with power enough to rarefy the air, and cause an uprising sufficient to produce an indraught thitherward from the surrounding regions. The air that is now about to set off to the south as the N.E. monsoon is thus arrested, turned back, and drawn into this place of low barometer as the S.W. monsoon. These plains become daily more and more heated, the sun more and more powerful, and the ascending columns more and more active; the area of inrushing air, like a circle on the water, is widened, and thus the S.W. monsoons, "backing down" towards the equator,