tigy CLIMATIC ZONES ORIGINATE The Controlling Climatic Factors.—The first step in understand ing climate is to know why it differs from place to place. These differences depend on four factors: (I) the earth's rotation, (II) the revolution of the earth and the inclination of its axis, (III) the dis tribution of land and water, and (IV) the relief of the lands. Let us take each of these four factors separately and consider its effect upon temperature, pressure, winds, and rainfall.
How Rotation Affects the Distribution of Temperature.—In order to understand the effect of rotation, let us for the present ignore the other three factors. Let us suppose that the earth's axis has no inclination, and that the relief and the distribution of land and sea have no effect. With such a simplified globe the sun's movement through the heavens would at all times follow the course it now follows at the equinoxes. Hence there would be no seasons. For the reasons pointed out in Chapter II the equator would always be the warmest part of the earth, and the temperature would decline steadily toward the poles.
The Distribution of Pressure.—The most prominent feature of the distribution of atmospheric pressure on the earth as a whole is the more or less regular rise of pressure from polar regions to approxi mately latitude 30°. Since the days of Ferrel this has been supposed to be due to a circumpolar whirl arising from the westerly winds which make an eddy with a depression in the center like a whirlpool in water. This explanation, however, is so doubtful that for the present we may think of a steady increase in pressure from equator to poles as the normal condition on a simple rotating planetwith a uniform surface.
This simple distribution of pressure, however, would be altered by the temperature conditions described above. The high temperature of equatorial regions causes the air there to expand. Hence, if the atmosphere had an upper surface, as we may say for convenience, the heat would raise this surface well beyond the level that it would occupy under the influence of rotation alone. Such bulging would not in it self cause a change in pressure, for the total amount of air would be the same after expansion as before. AS a matter of fact, however, a portion of the overbulging air flows away just as water flows from the center of a great fountain where the supply wells up from below.
The air that flows away from the equatorial bulge moves northward or southward, but not east or west, because there the same bulging occurs. So much air flows from the equator that the pressure at the earth's surface decreases. Hence an equatorial belt of low pressure is formed as shown in Fig. 68.
In the latitude of the tropics and a little farther poleward so much air is added by the flow from the equator that the pressure is increased, forming a subtropical belt of high pressure in each hemisphere. Farther poleward the air that has risen in equatorial latitudes gradually descends giving rise to westerly winds. These circle round the earth in a great whirl which reduces the pressure in high latitudes. Thus a simplified globe would have an equatorial belt of low pressure between two subtropical belts of high pressure, while in high lati tudes there would be two polar areas of low pressure.
How Winds Would Blow on a Simplified Globe.—Let us now see how the pressure belts would influence the winds on our simplified globe. Their distribution is shown in Fig. 68. We will begin with the subtropical belts of high pressure because they are the starting point of two of the earth's chief types of surface winds. The weight of the upper air in the regions of high pressure causes the air to settle slowly downward, but the movement is so gentle that it cannot be felt. Hence the subtropical high-pressure belts form regions of calms, the " horse latitudes," as they are called.
On the borders of the subtropical belt of high pressure the down ward movement in the central region forces the air outward and forms regular winds on the earth's surface. These blow toward the / equatorial belt of low pressure on one side and toward the polar area of low pressure on the other. In the warm equatorial belt of low pressure, as we have seen, the air is heated and expands so that part flows away at the top. This reduces the mass of the air so much that the cooler, heavier air from the neighboring high pressure areas is able to push in below and lift the warm, light air. This cooler air is in turn warmed and pushed up. Thus an ascending current of air is formed in the equatorial belt and there are no steady winds. In other words this ascending current is accompanied by equatorial calms just as the descending current is accompanied by subtropical calms.