CAUSE OF LOCAL WINDS AND OF GENERAL CIRCULATION
General Circulation. Differences in temperature, changing the specific gravity of the air, are the cause of the general circulation of the atmosphere about the earth, modified by the rotation of the earth; likewise the local circulation between land and water is caused by the different quantities of heat radiated by the two widely differing forms of matter, each attaining to a different temperature under the influence of the same solar radiation; and the inflow of winds to the cyclone and the outflow from the anti-cyclone are due to the same forces that cause the general and the local circulations.
If there were no difference in temperature between the equator and the poles the atmosphere would soon adjust itself in accordance with the laws of gravity, modified by the centrifugal force developed from the rotation of the earth, and the atmosphere forever would be at rest relative to the earth, moving with it as if it were a part of the solid sphere throughout its diurnal rotation on its axis and its annual movement about the sun. But there is a decided difference in temperature between the equator and the poles and between land and water surfaces; hence a general circulation, modified and distorted by numerous local movements, which, in turn, may be modified by the height of hills and mountains and the direction of their trend.
Fig. 12.—Trade wind circulation and reason for belts of high pressure at latitudes 30° N. and S. that extend around globe as shown by [Figure 13].
Let us trace a current of air through its course as shown in [Figure 12] and the reason for the blowing of the trade winds will be apparent, as will the reason for the location of a belt of high pressure at latitudes 30° north and south encircling the globe. At the equator there is a belt of calms. Here the air gently ascends under the intense heat of vertical sunshine. It is humid, for there is much water surface in the region of the equator, and the air carries vast quantities of water vapor aloft, later to be precipitated as torrential rains in the Tropical Zone, as the air cools by expansion in its ascent. This air expands or bulges upward and overflows aloft northward and southward, causing low air pressure at the equator, because of the quantity of air moved to other latitudes, which more than compensates for the amount banked up over the equator by the centrifugal force of the earth’s rotation.
Chart 1.—High and Low Centers of Action and Prevailing Winds of the Globe for July (Buchan).
Since air, passing away from the equator, must pass successively over parallels of latitude having less easterly velocity than that with which it started its journey, it runs ahead of the earth, and, relative to the surface of the earth, has a direction from the southwest north of the equator, and from the northwest south of the equator. Our current was divided at an altitude probably of six miles above the equator, one half following the northern and the other half the southern circuit. It was cooled by elevation and by radiation outward to space and as a result gained in weight and gradually descended, reaching the earth at about latitudes 30° north and south, and causing an accumulation of air at those latitudes and the belt of high pressure that irregularly surrounds the earth. In descending in the belt the air breaks up into a number of anti-cyclonic systems, sub-permanent highs or Centers of Action, which have so much to do with initiating the migratory Highs and Lows that create the weather of the earth, as will be fully explained in the [Chapter on Weather Forecasting]. The intensity of these centers of action is modified and their geographic positions shifted with change of season. (See [Charts 1] and [2.])
