A desert is a deserted and lonely stretch of land. Only a few sturdy plants can live there and in some deserts no plants can live at all. Only a few hardy animal can survive. And the world's wide deserts are deserted even by the rain. In fact, deserts are deserts because there is not enough rainfall to turn their land into fertile soil.
Some deserts are worse than others and. the worst of them are called the arid, or dry, deserts. These lonely stretches of dry land get less than ten inches of rain a year. Sometimes it does not rain for five or ten years and then the region is doused with a downpour. But the pouring torrent does little or no good to the parched land.
In a dry desert region, the rainfall is soon lost. Some of the water sinks quickly into the gritty sand. Some gushes along in shallow streams which soon disappear for their water evaporates into the air. In regions of fertile farmlands, the air gives rain and dewy moisture to the thirsty soil. But the air above a desert takes away more moisture than it gives.
The air which is descending from high above the ground is usually thirsty air, the kind which makes deserts of the land below. There are certain places in the world where currents of air tend to descend most of the time. The trade winds blow in the tropics in a wide belt Fund the globe. Where they stop, north and south of the equator, they meet the westerly winds and here we have regions of descending air.
These regions of thirsty, descending air are just where we have the world’s dry deserts.
North of the equator, the sandy Sahara straddles the Tropic of Cancer. South of the equator, there are deserts across the Tropic of Capricorn in Australia and South America. In North America, the westerly winds blow up and over the mountain ranges and descend on the other side. This thirsty air makes six small deserts on the eastern side of our far western mountains.
Altogether, the arid deserts cover about one quarter of the world’s dry land, A little of this wasteland has been reclaimed, but the job takes lots and lots of water. For the thirsty air above a desert region is always eager to take more moisture than it needs
Some 85 million people live in the arid desert regions. If they were spread out Evenly, there would be only eight of them on each square mile. But most of them crowd around the small oasis where there is ground water to fill their wells and springs.