THE TROPIC GRASSLANDS —.. --THE SUDAN 743. Appearance.—A traveler going south ward from the Sahara at the end of the rainy season,will find that the sand dunes and thorn bushes become less and less, and that the grass becomes more and more abundant the farther south he goes. (Sec. 568.) Very grad ually he has entered a country where there are bunches of grass and scattered mimosa trees. (Fig. 552.) As he rides on, better grass and more trees will appear, and then, after days of journeying by camel and mule, clumps of forest will be seen. Still farther on, he will finally see the solid mass of the equatorial forest. In making the journey, the traveler has crossed the great region of hot grassland that is called the Sudan (Arabic word for black people). It stretches from the Atlantic Ocean to the Nile and the plateau of Abyssinia. It is as broad as the United States, and two-thirds as large. Most of the Sudan is a low plain (less than 1500 feet), but there are some highlands in it.
744. Belts of climate.—Why is it that belts of forest and of grass stretch east and west across Africa? (Fig. 445, Bl, B3.) It is because of the unevenness of rainfall, due to the way the winds blow in that part of the world. They move very much as the air moves in a room which has a stove or a lamp in the middle of it. (Fig. 546.) The heat of the stove or the lamp expands the air near to it. The expanded air is lighter than the rest of the air; it rises as heavier air pushes in from the sides of the room to take its place.
The hot land near the equator acts like a stove. The air above it is heated and then slowly rises. The zone at the equator, at which the air is thus rising, has no wind. For this reason it is called the Zone of Calms, or Doldrums. To take the place of the rising air, air comes•in from the north and south. (Fig. 545.) This air, blowing toward the equator, makes what is called a trade wind. (Sec. 364.) The one north of the equator is called the Northeast Trade Wind, and the one south of the equator is called the South east Trade Wind.
After the air brought in by the trade winds rises, it goes back northeastward (or south eastward in the southern hemisphere) as a high, upper current (Fig. 547). The only way to study the high currents is by going up in balloons, or by visiting the tops of high mountains, such as the Peak of Teneriffe in the Canary Islands.
745. The Doldrum climate and the forest belt.—When the moist, heated air along the equator rises and is cooled its moisture forms clouds and falls as rain. Every after noon, for weeks at a time, rain falls. This Zone of Calms, or Doldrums, is one of the meanest places in the world.
It is breeze 1 e s s, hot, sultry, and very damp. These cli matic con ditions make that part of Cen tral and West Africa which is near the equator a great, damp, hot forest. For the same reasons we find the same kind of a forest near the equator in South America, and in the East Indian Islands.
746. The tropic grassland climate. ' —Let us again con sider the stove in the room. Now suppose you move it a few feet from the center of the room. What effect will this have on the direction of the air movements in the room? Draw two diagrams on the board or on a piece of paper and see for yourself. The air will be circulating as it did before the stove was moved; but since the stove has moved, the center of circulation or the zone of rising air will have moved with it, and the air will now be blowing straight across the place where the stove first stood.
Something very much like that happens in Africa. The Zone of Calms, or the Dol drums, follows the sun, just as the calm place in the room followed the stove. (Fig. 545.) In the time of our summer, the sun moves north of the equator and shines straight down, so the center of rising air, or Zone of Doldrums, moves north. Land that before had the trade winds now has the doldrums. Since the doldrum belt is the rain belt the grasslands of the Sudan receive a good soaking at this time.