Zone of Periodic Winds and Rains.—It is to this changing line of the greatest heat that the main currents of wind are directed. Within a zone extending for about 30° on each side of the equator the winds blow with great regularity. When they leave the polar regions the tendency of the surface-currents is due north and south, but in their course they become deflected longitudinally in consequence of the earth's motion, and reach the line of greatest heat as northeast and southeast currents. The tropic of Capricorn has its air rarefied by heat in our winter, and this produces within the torrid zone what is called the northeast trade-wind or monsoon. The tropic of Cancer has its air still more rarefied by heat in our summer, and this produces the south monsoon. Through these causes this central belt of the world has its winds and rains perfectly steady and regular, and within it there falls the greatest quantity of rain which there is in any part of the world. The rainy season begins some time before the sun reaches the zenith of a place, and continues for some time afterward. In a belt near the equator there are two rainy seasons, the main one, which lasts three or four months, beginning when the sun, in its progress to a vertical position, has crossed the equator, and a shorter one, which lasts four or six weeks, when the sun is coming again from the tropic to the equator. Nearer the two tropics the countries have only one rainy season, which begins when the sun approaches the tropic, and one dry season, the year being divided between the two. The rain pours down in torrents in a way of which we can form no notion from our experience in temperate countries. Our London rainfall is 2 inches a month, but in the tropics an inch a day is not an uncommon average for the whole rainy season. On the banks of the Rio Negro Humboldt collected as an ordinary rain 1¾ inch in five hours. In Cayenne Admiral Roussin collected, between the 1st and 24th of February, 12½ feet, and in one night, between 8 p. m. and 9 a. m., measured 10¼ inches. In the Himalayas of Khasia as much as 600 inches are said to fall in a single year. The rain, however, does not commonly pour down without intermission night and day, and day after day, as is sometimes the case in the English lake country. The ordinary succession of atmospheric phenomena is as follows: The sun rises in a cloudless sky. Toward noon some faint clouds appear on the horizon, which increase rapidly in density and extent, and are soon followed by thunder and violent gusts of wind, accompanied by heavy rains. Toward evening the rain abates, the clouds disappear, the sun sets in a serene sky, and during the night no rain falls. The annual quantity of rain which falls upon any particular place depends greatly upon local circumstances, just as it does in the temperate zones, and is greatest where hill-ridges are placed so as to catch the clouds, and smallest in tracts that lie to landward of such ridges. To take our illustrations from India, where the south monsoon blows laden with the copious vapors raised by the equatorial
Page:Popular Science Monthly Volume 7.djvu/500
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THE POPULAR SCIENCE MONTHLY.