Rainfall of the Subtropical Belts of High Pressure.—Poleward from the drying trade winds the air descends in the subtropical belts of high pressure and calms. Therefore, like the compressed air in an automobile pump this air grows warmer. Hence the amount of water vapor that can be mingled with it constantly increases and when the dry air reaches the earth's surface what moisture it finds on the ground is eagerly evaporated. Thus a dry belt is produced and the subtropical regions are among the driest parts of the world.
Rainfall of the Belt of Westerly Winds: Cyclonic Storms.—In the belts of westerly winds the air moves horizontally part of the 7time. This movement may cause a little rain because the air is mov ing from lower to higher latitudes and hence is growing cool. The chief reason for the abundant rainfall of the belts of westerly winds, -y however, is that they are regions of cyclonic storms. Such storms are areas of low pressure, 500 to 1000 miles or more in width. The winds blow toward the centers of such areas, but not directly, for they are deflected to the right in the northern hemisphere and to the left in the southern. This causes the air in such areas to move spirally in great whirls and hence gives rise to the term cyclonic which means wheel like. Sometimes cyclonic storms of small area and great severity occur in the trade wind belt. These tropical cyclones are called " hurricanes." Tornadoes, also, which are often wrongly called cyclones, are cyclonic in character, but cover only a small area and are very severe. Some thunderstorms are of the same kind, but less severe. By far the most important cyclonic storms, however, are those which cause the ordinary changes of weather in. the United States, Europe, Japan, Argentina, New Zealand, and other parts of the belts of prevailing westerlies.
Ordinary cyclonic storms, or cyclones, as they may properly be called, are always associated with areas or anti-cyclones.
/ These are areas of high presssure which may be 500 to 1000 miles in diameter or may sometimes cover a continent. The high pressure of anti-cyclonic areas causes the winds to blow outward in all directions. These out-blowing winds naturally move toward the low-pressure areas of the cyclones. Both anti-cyclones and especially cyclones are carried forward in the general drift of the westerly winds. Some times the center of a cyclonic storm moves 1000 miles in a day while at other times it moves only a few miles.
How Cyclones and Anti-Cyclones Influence the Weather.—The passage first of cyclones and then of anti-cyclones over a place gives 7 rise to frequent changes of weather. Since the cyclones are areas of low pressure their conditions resemble those of the equatorial belt of low pressure. Their passage is accompanied by a gentle upward movement of the air, and by clouds and rain. That is why the word cyclone means almost the same thing as storm. Because of the high pressure in anti-cyclones, on the other hand, the air there descends, just as in the subtropical high-pressure belt. Hence it becomes relatively warm and its power to hold moisture increases. There fore anti-cyclones are marked by clear weather. Anti-cyclones are often cooler than cyclones, especially in winter. This is because the clear skies permit heat to be radiated rapidly during the long nights, and because the air usually moves from higher to lower latitudes. Moreover, sometimes the loss of heat at high levels is so great that the air may come down cool in spite of the heat due to compression.
Cyclones and anti-cyclones are of the utmost importance to man. The cyclonic storms trouble the farmer in summer because they bring rain when he wants to get in his hay. They trouble the railroads in winter by piling up deep drifts of snow. They bring heavy rains which make floods like those which at various times have almost annihilated such towns as Johnstown, Pennsylvania, and Dayton, Ohio. Yet in spite of these disadvantages, cyclonic storms are one of the best things that a country can have, for they bring rain at all seasons. In the same way anti-cyclones are both a trouble and a blessing. They trouble the peach grower in New Jersey, for example, by bringing frosts when the trees are in blossom. They do great good, however, because in combination with cyclones they cause constant changes of weather and these are one of the best aids to health and a great stimulus to work.
Rainfall of Polar Regions.—The two polar belts might be expected to have much precipitation in the form of either rain or snow, because the pressure is low. As a matter of fact, however, they have little because the air is so constantly cold that it can absorb little moisture. Hence when the air rises in storms it yields only slight precipitation.