World+Map+Desert

(A1) __**Introduction **__ Desert biomes all have one common factor: lack of rainfall. Deserts cover nearly one fifth of the Earth's surface and are classified as areas where rainfall is less than 50 cm/year (A2). Also known as badlands, wastelands, and arid regions, deserts have a surprising amount of variety in their forms. While many people picture deserts as endless dunes of sand and blazing hot temperatures, that's only one of four generally accepted types of deserts (A2).

Not all deserts are hot. These are the different types of deserts:
 * **Hot and dry **
 * Many major North American deserts are considered "hot and dry". These include the Chihuahuan, Sonoran, and Mojave deserts. Besides these locations, hot and dry deserts can also be found in the Southern Asian, Neotropical (South and Central America), Ethiopian and Australian regions.These deserts can get anywhere from 1.5-28 cm/yr of rainfall depending on their location.
 * **Semiarid **
 * The major deserts of this type are found in the Nearctic realm (North America, Newfoundland, Greenland, Russia, Europe and northern Asia).Average rainfall here is 2-4 cm/yr.
 * **Coastal **
 * Coastal deserts are located in the Neartic and Neotropical regions.They recieve an annual average of 8-13 cm/yr of rainfall, with maximum rainfall reaching 37 cm.
 * **Cold **
 * The majority of cold deserts occur in Antarctica, Greenland and the Neartic region. Most precipitation comes in the form of snow and these deserts tend to average about 15-26 cm/yr of snow and rain.

The United States Geological Survey (USGS) and NASA classify deserts into eight types instead. These include Trade wind deserts, Mid-latitude deserts, Rain shadow deserts, Coastal deserts, Monsoon deserts, Polar deserts, Paleodeserts, and Extraterrestrial deserts. These classifications refer to the conditions that formed them, in contrast to the above classifications, which refer to their current climate. For example, extraterrestrial deserts exist off planet, and paleodeserts are made of fossils (A3).

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