. Elementary physical geography;. is scarcely more than halfas great. In higher latitudes, however, the diameter of the storm in-creases. The wind is more violent in tropical than in higher latitudes. The direction of the whirl probably results from the conflict of windsas they approach the updraught. Of all the currents setting towardthe storm centre, the northeast Trade Wind is the strongest. As itapproaches the storm centre it is opposed by weaker winds from thenorth, northwest, and west. The Trade Wind is bent therefore towardthe east and forced to rotate in the manner described. Tropical
. Elementary physical geography;. is scarcely more than halfas great. In higher latitudes, however, the diameter of the storm in-creases. The wind is more violent in tropical than in higher latitudes. The direction of the whirl probably results from the conflict of windsas they approach the updraught. Of all the currents setting towardthe storm centre, the northeast Trade Wind is the strongest. As itapproaches the storm centre it is opposed by weaker winds from thenorth, northwest, and west. The Trade Wind is bent therefore towardthe east and forced to rotate in the manner described. Tropical Cyclones.—Tropical cyclones usually origi-nate within a few degrees ofthe equator. They are thehurricanes of the West In-dies and the typhoons of theChina Sea. The storm areaextends over a surface vary-ing from a few hundred tomore than a thousand milesin diameter. The illustra-tion, p. 249, shows roughlythe track which, ordinarily, one of them follows. What isits direction in tropical latitudes? in latitudes beyond thetropics?. STREAMERS OF CIRRUS CLOUDS—THE FORECAST OF A CYCLONE CYCLONIC STORMS 251 The real beginning of the tropical cyclone is the deadcalm that for a few days precedes it, for a quiet atmosphereis a necessary condition to its formation. The first es-sential condition is the overheating of the air next thewater—precisely the same condition that formed the be-ginning of the desert whirl (p. 226). But while the stratumof air that causes the desert whirl is only a few hundredfeet in height and of very small area, the atmosphere dis-turbed by the tropical cyclone is, perhaps, several thousandfeet high and many thousand miles in extent. The longer the sun beats upon the glassy surface ofthe water, heating the air nearest to it, the greater will bethe energy of the storm when it begins. Moreover, thereis one element present in the tropical cyclone that is notfound in the case of the desert whirl—namely, the vaporof water. This is the most important distinctio
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