People often imagine a desert as a hot, scorching place with blowing sand and no vegetation. In fact, a desert landscape can be covered by gravel, clay, coarse soil, or even ice. But to be termed a desert, a region must either receive 10 inches of rain or less in a given year or lose more moisture through evaporation than it receives through rainfall. The impact of strong winds reacting with other weather patterns—called Aeolian processes—also play a big part in shaping the desert landscape.
Scientists group deserts into three main categories: Arid, semi-arid, and extremely arid. Arid deserts receive less than 10 inches of rain annually. Semi-arid deserts receive between 10 and 20 inches of rain per year. And extremely arid deserts many not see any rain for more than a year.
Deserts are also characterized by the type of precipitation they receive. Those that receive their moisture from snow or fog are considered cold deserts. Asia’s Gobi desert is covered in snow during the winter months. Deserts that receive their precipitation from rain are considered hot deserts. The world’s largest desert—the Sahara, covering 3,500,000 square miles in Northern Africa—is hot all year round.
Because all deserts are dry, they have significant daily temperature variations. Temperatures are high during the day because there is very little moisture in the air to block the sun's rays from reaching earth. Once the sun sets, the heat absorbed during the day quickly escapes. These extreme temperature fluctuations make survival in the desert difficult.
Deserts are located throughout the world and are created by the interaction between a landscape’s topography and the region’s dominant weather patterns. The three most common types of deserts are “rain shadow” deserts, “trade wind” deserts, and “polar” deserts.
Rain shadow deserts are expanses of land that lie in the shadow of mountain ranges that block moist ocean air from reaching them. As a result, only dry air crosses these landscapes, causing soil erosion and leaving behind sandy and rocky surfaces with little vegetation. The great North American Desert, formed in the rain shadow of the Sierra Nevada, was created by this process. Like all deserts formed in rain-starved mountain valleys, they are often made up of sand and rocky surfaces. Soil development is poor and vegetation is generally sparse.
Trade wind deserts are formed when dry winds dissipate a region’s cloud cover, allowing more sunlight to heat the land. High atmospheric pressure causes dry, cold air from the upper altitudes to compress and come down to earth, effectively sweeping away a region’s moisture. The Sahara in North Africa is an example of a trade wind desert.
The third type of desert, called a polar desert, is formed in areas where the cold climate prevents air from holding even a small amount of moisture. Rain or snowfall freezes so quickly that the surface water that would otherwise support plants and animals is locked up in frozen landscapes of ice. Antarctica is the world’s largest polar desert.
Deserts are expanding at an alarming rate, encroaching upon productive grass and farming land and adversely affecting millions of people on almost every continent.
The causes are complex, but human population growth is at the crux. The more people, the higher the demand for habitable land, for livestock grazing, for firewood, and for water. The more people, the greater the chance of poor agricultural practices that result in nutrient-deficient soil unsuitable for plant growth.
The U.S. experienced the phenomenon of encroaching deserts first-hand during the 1930's when southern states such as Oklahoma saw their farmland turn into a "Dust Bowl" as a result of severe seasonal drought and poor farming practices.
Researchers now use satellites to monitor desertification globally so that long-term changes are better documented. Scientists are working to reclaim damaged landscapes surrounding deserts and learning how to minimize future human effects on these fragile landscapes. For example, the Chinese government is planting a 2,800-mile “green wall” of trees around the Gobi Desert in hopes of stopping the encroaching sands. By 2010, these green belts are expected to extend from outer Beijing through Inner Mongolia—creating a forested fence longer than the Great Wall of China.