Sherry Dowling, age 13, of Darlington, South Carolina, for her question:
How come it never rains in the desert?
Actually it does rain in the deserts once in a while. After long, weary droughts, rain may come deluging down in cloudbursts. Some¬times a fierce downpour pelts down a few hailstones. Most but not all, deserts are blistering hot, at least during the day. But some have cool or even cold winters. Here there are desert frosts and occasional snows, especially on high plains and mountains.
In the scorching center of the sandy Sahara, not a drop of rain may fall for a decade. But sooner or later, the long drought ends. Maybe it ends with a drenching torrent that dumps five inches of rainfall during a single night. The next shower may come next week, next month or next year or wait another decade. If all these uneven downpours are added together, perhaps through half a century, the annual rainfall might be about five inches.
Practically no desert region has an annual rainfall of less than five inches. But any arid region can qualify as a desert if its annual rainfall does not exceed ten. Even the dampest desert is a very dry place. Its plants and animals, if any, must be specially adapted to survive in such arid conditions.
These parched places occupy 14 per cent of the earth's land, which is about eight million square miles. They are caused, naturally, by minimum rainfall and the mysterious reasons for this seem to be behind the scenes. If you mark the major deserts on the world map, you notice that most of them straddle the Tropics of Cancer and Capricorn, somewhat north and south of the equatorial region.
These locations are related to the prevailing global weather systems, which certainly play a major role in desert making. The equator is a zone of low pressure, where light rising air forms rainy clouds. The global desert regions tend to breed high pressure weather systems. There the dry descending air heats the ground and absorbs up to 99 per cent of its scanty moisture.
Mountains and prevailing winds also conspire to rob the deserts of their fair share of rain. Prevailing winds gather moisture from the oceans and tend to shed rain when they clash with cooler air over the land. If there are coastal mountains, the rain is shed on their seaward slopes. The winds that continue on over the top are dry, with little or no moisture to shed in the inland desert region.
The major deserts have been there for thousands and perhaps millions of years. They are regions where geography and global weather systems create dry air and skimpy cloud covers. Dense clouds provide more than rain. They shield the ground from the scorching rays of the sun and at night they prevent the day's heat from escaping. In the deserts the sun beats down from a cloudless sky. At night, the temperature plummets, maybe 50 degrees or more, as the day's heat escapes to the upper atmosphere. And the dry desert air becomes drier.