A desert is an area that is characterized by having little and unpredictable rainfall; less than 250 mm/year. The result is often a scarcity of plant and animal life. The Koppen system of climate classification relates the definition of a desert to plant and animals by characterizing a desert as a place where more water would be lost through evaporation than gained from precipitation (MacKinnon 1999). Deserts are not static; they are constantly expanding and contracting with changes in the climate and landscape. Deserts are one of the most extensive environments on the planet, comprising up to 40% of the terrestrial surface (Allan and Warren 1994).

Lack of water is the fundamental factor in defining a desert. Perveril Meigs for UNESCO set up a system balancing rainfall with water lost via evaporation and transpiration. He described three zones: arid, semi-arid, and hyper-arid. Arid zones receive less than 200 mm/year while semi-arid zones receive less than 600 mm/year. Hyper-arid zones are by far the driest areas, receiving less than 25 mm/year (Allan and Warren 1994).

High temperatures are often correlated with deserts. These temperatures often experience fluctuations up to 20 C each day. These extreme temperature fluctuations along with unpredictable rainfall patterns (some areas, such as Death Valley in California [see figure to right], do not receive rain for up to 3 years!) make it very difficult for plants and animals to survive (Allan and Warren 1994).

Why are there deserts?

As stated above, low rainfall and high relative degree of water loss characterize deserts. These areas may be the result of various factors such as high pressure zones, continentality, cold ocean currents, and/or rain shadows.

High pressure zones in the subtropical regions are where many deserts are located, around 30 degrees north and 30 degrees south latitudes. These zones are created by Hadley Cells (see figure below), a convection pattern
resulting from solar energy. The process is such that:

1. Air is heated in greater amounts at the equator because the angle of light hitting the Earth is closer to perpendicular. That is there is less surface area exposed to higher amounts of radiation, and increased warming results.

2. This warm air expands and rises at the equator, creating low pressure zones.

3. These low pressure zones suck in moisture-bearing masses, or rain-clouds, while the equatorial air masses travel towards the poles. Also, these rising masses cannot retain as much water as they continue to rise because they are cooling, which results in high rainfall at the equator, i.e. equatorial rainforests.

4. As these equatorial air masses travel away from the equator, they cool in temperature, and descend closer to the surface of the globe.

5. High air pressure and increased dryness results as the air masses nears the surface.

6. Moisture is sucked from the surface at 30 degrees north and 30 degrees south because the air masses are increasing in temperature and are able to hold more water (Ricklefs 1993).

Rain shadows are also often involved in desert formation. Mountain ranges cause air to rise and cool, leading to precipitation. As a result, this cooled air drops and sucks up moisture from the area preceding the mountain range. Rain shadows have been one of the main factors in the formation of deserts in North America, especially the Rocky Mountains in the western United States.

As an area of land is located more toward the interior of a continent, less water from the ocean, via winds, tends to reach the area. This phenomenon is known as continentality and is thought to be one of the main factors in the formation of deserts in central Asia.

Cold ocean currents (blue lines on figure below) seem to play an important role as well. Currents coming from the poles to the tropics consisting of cold water create cold air masses which cannot hold much moisture. Because most of the moisture is dumped before the air mass reaches the land, this dry air sucks up moisture from the land, resulting in coastal deserts (Warren and Allan 1994).