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Upon hearing the word "desert" most people conjure up images of lots of sand and heat. It's a common misconception. Some deserts are blazing hot and others happen to be the coldest, least sandy places on Earth, like Antarctica, for instance, or the frigid Gobi Desert of China.
About the only thing all deserts have in common is their lack of abundant and reliable water. They are dry. That, as the word implies, makes them some of the most open, treeless, deserted lands in the world. And because they are already living on the edge, the habitats of desert animals and plants also tend to be among the most easily disrupted, according to desert researchers.
Desert Recipe
In the simplest terms, a desert can be defined as anyplace where the rain- or snowfall is less than the rate that things dry up. As to why they exist at all, one need look no further than a satellite image of Earth, or a world map with all the major deserts highlighted, to see that there's a method to the planet's deserts: The majority of desert lands fall inside two bands north and south of the tropics, at midlatitudes.
The reason for this is atmospheric. It starts in the tropics where there the sun beats down ferociously and evaporates a lot of water, causing the thunderheads to pile up. As that warmed air reaches higher altitudes, it cools and rains out its moisture, then is pushed toward the poles by global air currents. This dried-out air tends to cycle back down over the midlatitudes where it creates high-pressure systems. High-pressure systems result in fair, dry weather, as any meteorologist can attest, whereas low-pressure systems spawn storms.
Another thing that grows deserts is the rain shadow effect, which is created by mountains blocking wet weather from inland areas. When a storm has to climb a mountain, its air cools with the higher altitude and produces more rain and snow — dumping the moisture on the mountains. By the time the air makes it to the desert, it's squeezed dry.
Some desert areas have multiple mountain ranges blocking moisture, like Death Valley, in California, which is cut off from Pacific Ocean storms by no less than three mountain ranges. Other deserts are behind only one range, but they are real doozies: the Atacama of Peru, which is behind the Andes, and the Tibetan Plateau and Gobi, which are behind the gigantic Himalayas.
A third ingredient for a desert is cool water offshore — if an ocean is nearby, that is. Cold currents, like those off the western United States, western and southern Australia, or southwestern Africa, don't tend to spawn summer rainstorms, and they keep coastal and interior areas dry. It's the opposite case along the Atlantic Coast of the U.S., which has very warm waters from the Gulf Stream. The warm, moist air makes for muggy summers and wetter winters as well.
