Friday, September 4, 2009

Why Does the Sahara Constantly Change Shape?

The Sahara, the world's greatest dry hot desert, stretches right across the north of Africa where there is almost no rainfall and consequently, little or no vegetation to anchor the soil. The sand is blown constantly by the wind, much of it into a landscape of great shifting dunes which constantly change shape, while the edges of the desert eternally encroach upon the land around.

The Sahara extends over three and a half million square miles of sand where the average rainfall is generally much less than 10 inches a year. The prevailing winds come from the heart of Asia and carry little moisture.

The temperature during the day exceeds 100°F in the summer and even in the winter averages 60°- 70°F. The surface of the sand is sometimes as hot as 170°F. The sun beats down from a clear sky all day, but at night the same cloudless sky allows the land to cool quickly, and there is often frost at dawn in winter.

The wind acts as a great sand-blasting machine, constantly wearing down rocks and carrying sand and small pebbles along. The few desert plants survive because they have long roots or thick fleshy leaves and stems that reduce water loss and may even store moisture.

A desert oasis is simply a place where there is water. The greatest oasis of all is Egypt, where for thousands of years life has depended on the careful use of the waters of the River Nile.

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