Popular Science Monthly/Volume 7/August 1875/Distribution of Atmospheric Moisture
ALL over the earth, the more largely where its beams reach the surface with the least diminution of heat, the sun is continually engaged in evaporating moisture from all exposed surfaces of water; this remains suspended in the atmosphere, and is carried about by the winds in the form of impalpable vapor or of clouds, till the point of saturation is reached, and the moisture falls again to the earth's surface in the form of rain, or snow, or hail. Air becomes lighter, and consequently expands and ascends, when it grows hotter, and becomes heavier and falls with cold. The hotter it is the more moisture it is able to hold in solution. Between the equator and the poles there is a difference of 80° of average annual temperature. In the torrid zone the light, warm, vapor-laden air is ascending continually to the upper regions of the atmosphere, and there flowing outward north and south toward the poles, and the cold, heavy air from the polar regions comes rushing along the surface to fill its place. As the seasons change, the line of the greatest heat in the world gradually moves its position. At the equinoxes of spring and autumn it runs along the actual equator, or near it. In winter it lies south of the earth's equator, about midway between the equator and tropic of Capricorn. Not more than half as much of the tropic of Capricorn as of the tropic of Cancer runs over land, and this makes a material difference, because the more sea the more the intense heat is deadened and absorbed. In summer the great continental area traversed by the tropic of Cancer, a long line of which is removed from the ameliorating influence of the sea, becomes excessively heated, and from the great African Sahara, through Nubia and Arabia to the north of India, runs a tract of intense heat, in which the July average in the shade rises to 90°.
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 sun from the broad expanse of the Indian Ocean, we find that in the eastern Himalayas the rainfall varies from 200 to 600 inches a year, and that at Mahabaleshwar, where the clouds drift against tlie high ridge that lines the west side of the peninsula, it is 248 inches, but that at Courtallum it is only 40 inches, at Bangalore 35 inches, at Cape Comorin 30 inches, and at Bellary in Mysore 22 inches, which is as low as in any part of England,
Zone of Periodic Winds without Rain.—Outside the zone of periodic winds and rains comes a double belt, one girdling the world in the northern, and the other in the southern hemisphere, the breadth and area of which are greatly modified by local circumstances, within which no rain ever falls. These belts are estimated to include altogether an area of 5,000,000 square miles, but it is impossible to make any calculation that is at all precise, because round the tracts that are entirely rainless are regions in which rain falls but rarely, which again pass gradually into the two rainy zones, through countries like Southern Palestine and the Gangetic plain, which, though usually rainy, are liable at intervals to years of drought. These belts of rainless land near the tropics contain some of the most hopelessly dreary country which the world can show. Beginning with the west of the old continent, we have along the tropic of Cancer in Africa the Sahara or great desert, on the southern border of which the rains cease at 16° north latitude, and begin again on the north at 28°. Passing farther east, the southern rains cease in the countries on the banks of the Nile between 18° and 19°, and the northern begin between 27° and 28°. Passing into Asia, there is a great rainless tract in Arabia of which we do not know the exact bounds, and it reaches through Beloochistan over into the delta of the Indus, where it does not cover more than 4° of latitude. From this point the rainless zone turns to the northeast and extends to 30° north latitude. Crossing the great Himalayan chain it includes the high table-land of Thibet, but does not appear to reach into the Chinese Empire. In South Africa there is a sandy, desert, rainless tract on the north of the Orange River, between 24° and 28° south latitude, and a great part of the interior of Australia seems to be nearly or quite rainless. In North America the rainless belt includes the Californian peninsula, and extends round the northern end of the Sierra Madre chain past Chihuahua and Monterey to the shores of the Gulf of Mexico between latitudes 24° and 26°. In South America it includes between latitudes 23° and 27° the northern province of Chili, and, through an extensive low tract in the interior of the continent belonging to the territory of the Argentine Confederation, rain is very unfrequent and small in quantity.
Zone of Variable Winds and Rains.—From about latitude 80° on each side of the equator to the poles extends a region of everchanging and variable winds, and of rain that is irregularly distributed throughout the whole year. Sometimes in these middle latitudes, in Britain, for instance, we fall within the sway of the south-rushing polar current deflected to the east by the earth's rotation, and sometimes within that of the north-rushing current from the equator, deflected to the west by the rotation which it shared with the earth at the zone from which it started. In Britain this southwest wind comes to us laden with vapor from the great mass of the Atlantic, and makes Ireland and our western shores unusually damp and rainy. The relative temperatures of sea and land in the temperate zones are continually changing with the seasons. In summer and autumn the Atlantic is colder than the European Continent, and this has a tendency to produce a west current at the surface. In winter and spring the Atlantic is warmer than the continent, and this has a tendency to produce an east wind. Sometimes one of these varying tendencies gains the predominance and sometimes another, and the result is constant and often rapid change and variety. The heat and moisture which the wind brings with it depend entirely upon where it comes from, and what it has passed in its way. A west wind blows to us from the Atlantic, and usually brings rain; an east wind brings up the fog of the German Ocean; and in winter and spring the prevalent northeaster brings the cold and often the snow of Russia and Norway. At the sea-side, unless it be overpowered by a general current, there is a breeze from off the sea during the day, and a breeze from off the land during the night. The quantity of rain that falls in this zone at different points is extremely variable, and depends upon the position of a place with regard to mountain-masses and the seas from which the vapors come. In England the rainfall is greatest on the west side of the island, and smallest on the east. The difference within a short distance is sometimes very striking. There are 140 inches a year at Borrowdale, in the lake district, and not more than 20 inches at Shields and Sunderland, which are directly opposite on the east coast. But the habitual humidity of the atmosphere often varies but little between places the rainfall of which is very different. The number of days upon which more or less rain falls, varies in England from 100 to 300, but in the Mediterranean region the number of days is fewer, the quantity is smaller, and there is an almost regular period of entirely dry weather in summer. Taking the north temperate zone as a whole, there is, as a rule, least rain in places away from hills in the interior of continents, and most in insular and mountainous situations.—Gardener's Chronicle.