The common definition of a desert is a place that receives less than 10 inches of rain a year. The Köppen climate classification system, which is widely used to categorize different types of climate, recognises two types of desert. A "BWh" or hot desert or "BWk" temperate desert. Many people, when they think of a desert, imagine searing heat, cactus and sand dunes when, in actual fact, frozen Antarctica is a type of desert.
Although temperature alone does not make a desert, it's worth noting the range in temperatures from a hot desert to a temperate desert. Temperatures in the Sahara Desert can reach 136 degrees during the day and drop to below freezing at night. Contrast this with Antarctica which ranges from minus 4 degrees to minus 94 over the period of a year. Taking the maximum temperature from the Sahara region and the minimum from Antarctica, desert temperatures can range over 230 degrees depending on where the desert is located.
In order for an area to be classified as desert it must receive less than 10 inches of rainfall a year. Death Valley in the Mojave Desert receives under two inches of rain annually, making it a typical example of a hot desert. The American McMurdo Station in Antarctica records an average eight inches of rain annually, making its location on Ross Island a type of desert. The rainfall figures for Antarctica are based on the "water equivalent" of the snow that it receives.
Much of what makes a desert is the evaporation rate, which is in turn connected to temperature. An area which loses a significant amount of its rainfall to evaporation is classified as desert. Antarctica does not lose any of its rainfall to evaporation because of its consistently low temperatures, and so might be more accurately described as tundra.
Some deserts are formed due to their location near mountains. Clouds carried by the prevailing wind will deposit most of their moisture on one side of a mountain range, leaving the other barren and forming a desert. An example of this type of desert is the Mojave Desert in the "shadow" of the Sierra Nevada mountain range.
Desertification is the process by which otherwise fertile land is transformed into a desert-like region as a result of deforestation or other man-made abuse. The removal of trees or groundwater can lead to the top soil being blown away, preventing trees returning. These areas are rare and do not fall into the traditional definition of a desert because the climate of the area is unaffected.