Welcome to You Ask Andy

Helene MCBride, age 11, of Ottawa, On , for her question:

 What causes a desert?

A desert region is parched for lack of rainfall. For some reason the winds that blow over it drink up more moisture than they shed. They are dry, thirsty winds. Rainfall occurs because air can no longer carry its load of vapor. Water vapor is a gas, like the other gases of the air. The warmer the air, the more vapor it can hold. When warm, vapor‑loaded air is cooled it must give up its extra. vapor as rain, snow or some other downpour.

The prevailing winds that blow over a desert have little surplus moisture to give. Sometimes the rain has already been stolen by mountains in the path of the prevailing winds. The dry desert of Utah lies in the path of the westerlies. Their rain has already been stolen by mountains to the west.

Many of the worlds big deserts stretch along the Tropics of Capricorn and Cancer. These are regions where cool, dry air is descending to the warmer earth. As this air is warmed by the ground it becomes thirsty. It drinks up more moisture than it gives. The Sahara Desert is caused mainly by this situation.

Not all deserts are scorching hot. There are cold deserts along the shores and islands of Canada which lie within the Arctic Circle. Here the winds spilling down from the polar regions are too cold to drink up any vapor. Therefore they have little or none to shed as rain or snow. Your gift encyclopedia has a map of the world’s deserts. When you know the prevailing winds and the geography of highlands and lowlands you can figure out for yourself why each desert region in the world is parched for rainfall.

Not all deserts are wastelands, unable to support life. Naked stretches of rolling sands, like parts of the Sahara, are rare. What’s more, no desert is entirely without rainfall though desert regions average less than five inches of rainfall a year and showers are undependable. There may be no rain for years, then a deluge may come pelting down lasting minutes days or many days.

Desert showers are usually sudden and violent. Soil, sand and pebbles are washed from the high ground4 Gullies, gorges and sometimes canyons are cut into the dry ground by the racing runoff. For desert showers do not have time to sink into the thirsty soil.

Rare running water and constant blowing sand carves a pattern on the face of the desert. Each desert has its own beauty. The vast sands of the Sahara are brilliant with sunshine. The deserts of the southwestern United States are vivid with color and fascinating rock formations.

These American deserts are far from lifeless. They stretch as flat plateaus studded with mountain ridges: The flat ground is spotted with cactus and desert scrub. Rodents and other small mammals scurry about. Snakes glide and squirm, lizards bask in the sun. Their flat lands stretch to where the purple hills meet the blue sky. The golden puma lives in those hills and the hungry coyote comes down to howl at the starry night.

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