OASES 568. Bounds and appearance.—South of the Mediterranean region is the Sahara, or Great Desert, as the Arabs call it. The desert of Arabia, east of the Sahara, is just another part of the same great desert region and separated from it by a narrow sea.
What is the northern boundary of the desert? (Fig. 445.) The southern boundary is a climate boundary, where the rainfall gradually increases and the desert gradually changes into a land with bunch grass and bushes. Farther south the increasing rain makes a belt of grasslands, reaching across Africa from sea to sea. (Sec. 743, Fig. 552.) This desert of North Africa and South west Asia is one of the largest natural regions in all the world. The distance from the Atlantic Ocean to the Red Sea is greater than that from New York to San Francisco. From the Red Sea to the Persian Highlands beyond the Euphrates is as far as from New York to Chicago. A traveler crossing the desert on his camel might ride for days across bare rocks, from which the hot wind had blown every grain of sand. In other parts of the region he might travel fordays across heaps of sand which are always drift ing with the wind. The desert is not every where the same. In some places the earth is bare; in other places where there is a little moisture, coarse tufts of grass and clumps of , thorny bushes grow. Later we shall read about the oases (Secs. 578-582).
569. Climate.—Why is it that Europe is green with crops and forests, while just to the south of it, in Africa, millions of square miles are brown and bare? The winds make the difference. We have seen the effects of the trade wind (Sect 379) as it blew across the Caribbean, and carried moist winds against the West Indian mountain sides. In Africa this northeast trade wind blows across lowland, and becomes hotter and hotter as it goes toward the equator. This hot wind dries up the moisture from the land instead of giving it moisture. Thus the trade wind makes the Sahara a desert.
Trade winds do this in other continents too. Geographers often speak of "Trade Wind Deserts." (Secs. 776, 857).
570. Summer heat.—The sun heats the dry ground very hot. The air over it becomes
so hot that it quivers like the air over a hot stove. The desert Arab wraps thick layers of wool around his head to keep out the heat. He wears a long, flowing robe, or burnoose, made of pure wool. At night he needs this woolen garment to keep him warm.
The temperature is often 130° or 140° F.
during the day, but there may be frost at night, because the sand cools off so quickly.
In our country moisture and clouds at night keep the earth warm just as our clothes keep us warm. By watching the thermometer for a while in winter, you can notice that cloud less nights are colder than cloudy ones.
Traveling in the desert is often done at night, especially if the journey is short, or the traveler does not need to rush on with all speed to reach some distant spring.
571. Rain storms and dust storms.—In the winter the cyclones crossing the Mediter ranean bring a few inches of rain to northern Arabia, Egypt, and the northern edge of the desert. No part of the Sahara is entirely rainless, but insome places years passbetween showers. Thunder clouds sometimes form (Sec. 71), and rain may fall violently. But often the rain actually dries up before it reaches the earth.
There are wind storms which gather clouds of sand and dust and drop them in burning hot, stifling showers. Sometimes people and animals perish in sandstorms.
572. Desert watercourses and lakes.—It seems strange to think of watercourses being in a desert, but many such are there. On the southern slope of the Atlas Mountains, in some of the highlands in central Sahara, and in central Arabia, streams flow at the time of the winter rains, but soon sink into the desert sands. Some of these streams run every year; some of them run but rarely. The watercourse that passes through the oasis of M'zab (Algeria) has flowed only twelve times in one hundred and fifty years. These old water-courses are often the best of roads r.nd may be used in safety for years and years. Then suddenly a cloudburst may descend, turning the road into a raging torrent, which rises like a wall and rushes onward like the wind, overwhelming and sometimes drowning people.