An area having an annual rainfall of 10 in. (25 cm) or less is considered to be a desert. Some deserts have no rain for intervals of several years. Deserts and semideserts exist in some regions having up to about 20 in. (50 cm) of rainfall where evaporation is very high and loss by runoff is great. The largest desert regions of the world lie between 20° and 30° north and south of the equator, either where mountains intercept the paths of the trade winds or where atmospheric high-pressure areas cause descending air currents and a lack of precipitation. Other factors contributing to the formation of deserts include the amount of sunshine, rate of evaporation of water, and range of temperature. Temperature ranges in deserts are often extreme.
Plants of the desert have leaves and stems adapted to lessen their loss of water, and individual plants are more widely spaced than those in more humid regions; their roots form a spreading network sometimes penetrating to 50 ft (15 m) underground. Among the animals living in deserts of North America are species of squirrels, mice, bats, foxes, rabbits, and deer; reptiles, e.g., the Arizona coral snake, species of rattlesnakes, the desert tortoise, and the horned toad, gila monster, and many other lizards; a number of birds, e.g., the cactus wren, the road runner, species of owls, sparrows, and hawks; and spiders, scorpions, termites, and beetles. See dune; oasis.