A desert is defined as a region that has an arid climate that is unable to support fauna or flora to any great extent. They're normally thought of as hot and sunny, but deserts can be cold. The central plateau of Antarctica and regions where the topography restricts the rainfall, like the Atacama in southern America, are examples of this.

Many of the hot sandy deserts are formed by the global circulation of winds. The sun heats the equatorial regions of Earth far more than the poles. As the heated air rises above the equator it starts to flow north and south at high altitudes around 30000 feet to 50000 feet. As it moves away from the equator it slows down, and at about 25 to 30° north and south, starts to descend to the ground. This downward movement of the air warms it up and often evaporates any clouds.

This is also the mechanism for forming anticyclones, or areas of high pressure, which gives us their characteristic sunny weather. Since this wind circulation is fairly constant throughout the year, the weather doesn't change very much.

If you look at an atlas of the world you will see that at about 30° north is where most of the hot deserts occur. It happens in the south as well to a lesser extent because of the lack of landmasses.

One of the big problems of climate change, as I see it, is that as we increase the heat at the equator, the air at high levels could move away at a faster rate. It would then take longer to slow down before descending to, for example, the Mediterranean area at 35° north. In this way, the Sahara desert could migrate northwards into the Mediterranean itself.

Many cold deserts form in the same way because the air above the north and south poles is also descending and drying out the precipitation bearing clouds. One of the largest cold deserts is in the Antarctic. Despite the fact there can be quite a lot of snowfall here during certain times of the year, it is of no use to any life form since it never melts.

The third major desert is really a rain shadow desert, where hills and mountains take all the precipitation out of the clouds on the windward side before they descend and decay on the lee side. The most famous of these is the Chilean Atacama desert where, in some places, it hasn't rained for several decades.

As always in geography the three types of desert here can be split into many sub-types. But the common factor they all share is that conditions make it difficult, if not impossible, to live there.

Related Links:

- Desert Climate Zone (hot)
- Polar Climate Zone
- Weatherwise: Hot and Cold Deserts