Page:Physical Geography of the Sea and its Meteorology.djvu/113

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It thus appears that the south-east trade-winds of the Atlantic blow with most regularity between 10° and 5°, and with most force between 10° and 15°.

226. The air sloughed off from the counter trades, moist air.—On the polar side of 35°-40°, and in the counter trades (§ 204 [7]), a different process of sloughing off and turning back is going on. Here the winds are blowing towards the poles; they are going from parallels of large to parallels of smaller circumference, while the upper return current is doing the reverse; it is widening out with the increasing circumference of parallels, and creating room for more air, while the narrowing current below is crowding out and sloughing off air for its winds.

227. The air sloughed off from the upper trade current dry.—In the other case (§ 224), it was the heavy dry air that was sloughed off to join the winds below. In this case it is the moist and lightest air that is crowded out to join the current above.

228. The meteorological influences of ascending columns of moist air.—This is particularly the case in the southern hemisphere, where, entirely around the globe between the parallels of 40° and 60° or 65°, all, or nearly all, is water. In this great austral band the winds are in contact with an evaporating surface all the time. Aqueous vapour is very much lighter than atmospheric air: as this vapour rises, it becomes entangled with the particles of air, some of which it carries up with it, thus producing, through the horizontal flow of air with the winds, numerous little ascending columns. As these columns of air and vapour go up, the superincumbent pressure decreases, the air expands and cools, causing precipitation or condensation of the vapour. The heat that is set free during this process expands the air still farther, thus causing here and there in those regions, and wherever it may chance to be raining, intumescences, so to speak, from the wind stratum below; the upper current, sweeping over these protuberances, bears them off in its course towards the equator,