How the Earth's Revolution and the Inclination of its Axis Affect the Climatic Belts.—If the earth's axis were not inclined to the plane of the orbit in which the earth moves around the sun, the climatic belts would always remain in the location shown in Fig. 68. Hence ?there would be no seasons. Since the axis is inclined, however, and the vertical rays of the sun migrate back and forth from latitude 231° S. to 231° N, the heat equator and with it all the climatic belts migrate similarly and cause seasons. These, as we have seen, produce a profound effect upon man's life. If the sun stood always at its most southerly position the climate of the northern United States would permanently become almost like that of the Poles, and only people like the Eskimos could live there. If the sun stood always at its most northerly position conditions would not be quite so bad, but the heat of July would prevail all the year, and people would become as lazy as those of the tropics.
The effect of the migration of the climatic belts upon rainfall is especially interesting. It causes the rainy season to come in summer in some places and in winter elsewhere, and thus determines which regions are the best for farming. Suppose you had a choice between a farm in northern Texas and one in northern California, each farm being in the center of a plain where the soil is excellent, but where irrigation is expensive. Suppose also that you knew that both places had the same rainfall, an average of 20 to 25 inches a year, and the same temperature, an average of about F. for the year as a whole. In Texas and California there are places of just this kind. What kind of farming would you plan in each case and how would you expect to live? If you were wise you would ask the Weather Bureau at Washington for monthly rainfall records of each sec tion. You would find that Texas lies far enough south to receive abundant summer rains while this part of California is in the sub tropical belt of winter rains. Hence during the six months of the growing period from April to September the Texas farm would get 16 inches of rain and the California farm only 4. Unless you could spend a large sum to bring water for irrigation this particular Cali fornia farm would be of value chiefly as a cattle range, while on the Texas farm you could raise excellent crops of corn as well as other farm products.
Why Equatorial Regions have Two Wet and Two Dry Seasons.— Fig. 70 illustrates the effect of the seasonal migration of the earth's climatic belts upon rainfall. The shaded areas indicate the rainfall
month by month throughout the year in various latitudes. To begin with the equatorial belt in the center, notice that in January there is almost no rain. The sun is then so far south that the equator is under the influence of the northeast trades with their drying effect. As the sun moves northward the abundant equatorial rains come with it. Hence the rainfall increases. It reaches a maximum in April or May, a month or more after the sun has passed the equinox, for the seasons usually lag a little behind the sun. Then as the sun goes northward to the Tropic of Cancer, the rainfall once more diminishes. The belt of southeast trades swings over the equator and in July there is almost no rain.
As the sun and the accompanying rain belt move southward once more, the rainfall at the equator increases until after the September equinox, only to diminish as the equatorial belt passes southward and the northeast trades again prevail at the end of the year. Thus at the equator, although there is no summer or winter, there are four seasons: (1) the dry season when the sun is in the south, (2) the wet season when the sun crosses the equator northward, (3) the dry season when the sun is in the north, and (4) the wet season when the sun crosses the equator on its way south. This type of rainfall with two wet and two dry seasons prevails almost unmodified in the equatorial regions of South America, Africa, and the East Indies.
Why Sub-equatorial Regions have One Wet and One Dry Season. —In the two diagrams (2N and 2S in Fig. 70) illustrating the con ditions of rainfall in the trade wind latitudes 15 to from the equa tor quite a different seasonal distribution is seen. These latitudes are near the margin of the equatorial belt, and hence are called sub equatorial. When the sun is far south in January it carries the equa torial rain belt with it, so that the southern sub-equatorial regions re ceive a heavy rainfall as appears in Fig. 70. On the other side of the equator, however, the southward migration of the climatic belts causes the drying trades to blow over the sub-equatorial regions and gives them a dry season in January, as is shown in the figure. Six months later the conditions are reversed. Since the sub-equatorial belts lie in such low latitudes that they are always warm, they really have two seasons, wet and dry. Southern Mexico, northern Australia, and a strip of northern Africa just south of the Sahara have this type of rainfall.