Atmospheric Currents.—All storms owe their origin to the heat of the sun, which produces differences of temperature in different portions of the earth, and thereby causes all the movements and currents which take place in the atmosphere around the globe. As the air at the equator is more highly heated by the sun than that of any other region, it expands, becomes lighter and rises, causing a partial vacuum or deficiency there at the surface of the earth. The air north and south of it at once moves forward from opposite directions to supply this deficiency at the equator, and this in turn becomes heated and ascends. Other air again moves forward from north and south to replace it, and thus an upward current at the equator, and a north and south polar current at the surface toward the equator, are established. These north and south polar currents cause a deficiency of air at the poles, and the heated air which has risen at the equator into the upper region of the atmosphere divides and moves forward toward the opposite poles to supply the deficiency caused there. Thus, upper currents in opposite directions from the equator to the poles are also established in order to restore the equilibrium disturbed by the surface polar currents flowing toward the equator.
But by the time the air of the upper currents has reached the region of the tropics, it has become cooler and heavier, and descends to the surface of the earth. Here it divides into two currents—one flowing back to the equator, forming the trade-winds; and the other, becoming warmer again at the surface, flows toward the poles, meeting the polar current somewhere north of the tropic in the northern hemisphere, and south of it in the southern. This meeting of the equatorial or tropical and polar currents in the temperate zone, and the various phenomena attending and resulting from it, are the most significant and important facts which constitute the basis of Prof. Blasius's theory of storms, in distinction from the centripetal theory of Espy, and the rotary theory of Colonel Clapper, as developed by Piddington, Thorn, Dove, and others, and better known in this country as the cyclone theory of Redfield.
The following diagram (Fig. l) will serve to indicate the movements and courses of the general atmospheric currents of the earth, as above described, the arrows showing the directions in which they move.
The two currents above referred to—the polar and the equatorial or tropical—are of different temperatures, and move horizontally in opposite directions toward each other. When they meet they overlap each other somewhat like two wedges with their sharp ends forward. The warmer current, being lighter, glides obliquely over the cooler current, and moves northward; and the cooler current, being heavier, moves beneath it on the surface of the earth southward, just as two currents, warm and cold, flow over each other in opposite directions through an open window or door of a heated room.
