are mainly found around the tropic of Cancer and tropic of Capricorn in
Northern and Southern Hemispheres respectively. The common definition of desert is a
region that receives less than 25cm of rain a year on average.
When people think of deserts, the following image comes to their minds : hot and arid land, vast expanses of sand, soil of reddish brown color, a sky of brilliant blue, no or very few plants, cacti, spiny leaves and camel is the only animal that they can think of.
The truth is not like that. It is not very common to find sand in a desert, but small rocks, pebbles and loose gravel on the surface layer instead. Only 15% of the world' s desert surface is pure sand. (Parts of the Sahara and Arabia desert)
Desert occupies about one fifth to one third of the earth' s surface. The rainfall pattern is not a seasonal one. Instead, rain usually falls in the form of sudden, violent thunderstorms. There may be several storms in a year or none for several years. The " average rainfall each year" is not calculated based on one year' s rainfall, but on the total rainfall in a long period of time.
Cacti dominate the Sonoran Desert vegetation near Tucson,Arizona (photo. by Peter Kresan) From USGS
How about the polar regions with sheets of floating ice?" You may ask. Large amount
of water is locked up in the icecap. The average rainfall in the cold deserts located in
polar regions is within 25cm.
The rate of evaporation ( known as " Moisture Index" ) can be used to classify deserts as there are extremely high temperatures, low humidities and very little cloud cover.
How extreme are the temperatures ? The daytime air temperature can reach 58oC (136oF) in the Sahara. Soil temperature may reach 80oC (176oF). For half a year, the average temperature of the Mongolian desert is below the freezing point. In Antarctica, the winter mean temperature is -30oC (41oF).
Ripples on a dune in Mexico. (photo. by Peter Kresan) From USGS
|The subjective definition of desert:
An empty, arid region with only a few highly specialized plants can survive. The thin soils, being attacked by wind, lack humus and so are generally infertile. The moisture lost through evaporation is greater than that gained in rainfall. The rainfall is irregular. The drought period is usually longer than one year except for the Antarctica. The extremely dry and hot condition made crop growth quite impossible.If enough vegetation grows by following the rain to provide grazing for flocks and herds, seasonal or nomadic pastoralism may be carried out.
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