Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit (i.e. they lose more moisture than they receive). Measurement of rainfall alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. Other regions have cold deserts, including high altitude areas.
Desert terrain exists in warm, temperate, and cold climates, but all deserts share one common trait: little rain. The three categories of desert terrain are tundra (cold desert), rocky deserts (often temperate), and sandy deserts (often warm).
Tundra differs from the other desert categories in two important ways. Because snow and ice cover much of the landscape, it's easy to find water. During the height of summer, the permafrost thaws to a depth of a foot or so, turning the landscape into a vast field of mud. The muddy tundra affects movement and skill use as the shallow bogs described in Marsh Terrain, although there's little standing water.