It is common belief that all deserts are hot and sandy places. While this is generally not true, a common factor of deserts is aridity, the temporal and/or spatial scarceness of water. True deserts can be delineated from other biomes based on their aridity. Of the following groups, only the first two are considered as true deserts here. Aridity can be divided into four groups:
• extreme arid: less than 60-100 mm mean annual precipitation;
• semiarid: from 150-250 to 250-500mm; and
Since evaporation depends largely on temperature, bio-climatic aridity cannot be defined solely by the amount of precipitation. Therefore, the higher limits given above refer to areas with high evaporativity in the growing season (e.g., in subtropical areas with rainfall in warm seasons). This is taken into consideration in UNESCO's 'World Map of Arid Regions' that defines bioclimatic aridity by P/ET ratios (annual precipitation/mean annual evapotranspiration). P/ET ratios smaller than 0.03 qualify for hyperarid zones (roughly corresponding to the extreme arid zone above) and a ratio of 0.03-0.20 as arid zone (thereby corresponding to the arid zone mentioned above).
Another common way of delineating deserts is based on their vegetation pattern and optional land use. Extreme arid zones typically show contracted vegetation restricted to favorable sites or lack vegetation altogether. Arid zones are characterized by diffuse vegetation. Semiarid zones mostly are characterized by continuous vegetation cover (if edaphic conditions allow for it) and only very locally dry-land farming (without irrigation) is possible. Farming without irrigation becomes a reliable option at larger scale in nonarid zones only.
Based on geographic location and a combination of temperature and geographical causes of aridity, deserts can be separated into five classes:
• Subtropical deserts. They are found in the hot dry latitudes between 20° and 30°, both north and south. These deserts lie within the subtropical high pressure belt where the descending part of the Hadley's cell air circulation causes general aridity.
• Rain shadow deserts. They are found on the landward side of coastal mountain ranges.
• Coastal deserts. Found along coasts bordering very cold ocean currents that typically wring moisture as precipitation from the air before it reaches the land, these deserts are often characterized by fog.
• Continental interior deserts. They are found deep within continents and far from major water sources.
• Polar deserts. They are found both in the northern and southern cold dry polar regions.
This article focuses on extreme/hyperarid and arid zones and on the first four ofgeographic desert classes listed above.
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