Definition

 Definition

Potential supplements the measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P − PE ± S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example receives about 300 millimeters (12 in) of rain per year, however about 2500 millimeters (100 in) of water could evaporate over the course of a year.^[ In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack ofheat to aid in the evaporation process.

Measurement of alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on For example, 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. The of Alaska's also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the and other high altitude areas in other parts of the world. Polar deserts cover much of the ice free areas of the arctic and Antarctic.