Deserts of the World:

  • Desert cover 1/5 of the Earths land mass. The largest desert is the Sahara which is 3.5 million square miles (roughly the size of the U.S.

  • Sand covers approximately 10-20% of most deserts. The rest consists of gravel covered plain, rocky hills and mountains, dry lake beds. The soil is rich in salt, uranium and other minerals. Large deposits of oil and natural gas lie under some deserts.

  • Up to 95% of solar radiation reaches the ground as little vegetation is supported to reflective or absorb it - of this. 95% of this is reradiated back, and at night this translates into cool temperature..

As many factors can act to help develop deserts, a number of different types of desert exist, These include trade wind, rainshadow, coastal, midaltitude and human induced. Find out more about each of these in the sections below

[From:photos from: http://pubs.usgs.gov/gip/deserts/types/]; one of the best sites for info and great photos

1. Trade wind deserts The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds.See the notes on climate we recently covered for background on wind belts.

The world's largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert.

2. Rain shadow deserts

Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside "shadow" of the range.


3. Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama of South America, is the Earth's driest desert. In the Atacama, measurable rainfall--1 millimeter or more of rain--may occur as infrequently as once every 5-20 years.
Morning fog moistens the dunes of the Namib coastal desert (photograph by Georg Gerster).

4. Midlatitude deserts

Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical high pressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert.

5. Polar deserts are areas with annual precipitation less than 250 millimeters ( 1 in) and a mean temperature during the warmest month of less than 10° C. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This "freeze-thaw" alternation forms patterned textures on the ground, as much as 5 meters in diameter.-

6. Human impact causes desert expansion - a phenomenon called desertification appropriately Continual loss of fertile land on the outskirts of arid areas.

  • Overgrazing
  • loss of protective cover of plant life-wind and water causing erosion.

Steps taken to prevent-Plant tree, improve farming methods, limit livestock.

LEFT: Off-road vehicles significantly increase soil loss in the delicate desert environment of the western United States. In a few seconds, soils that took hundreds of years to develop can be destroyed (photograph by Terrence Moore).

RIGHT:Goat seeks food in the sparsely vegetated Sahel of Africa (photograph courtesy of the U S. Agency for International Development).

http://pubs.usgs.gov/gip/deserts/desertification/Still other desert types exist... see the excellent USGS site listed above from which much of this info and all the images were obtained if you want further detail

Soils

Soils that form in arid climates are predominantly mineral soils with low organic content. The repeated accumulation of water in some soils causes distinct salt layers to form as waters evaporating leave. their salts behind
Calcium carbonate precipitated from solution may cement sand and gravel into hard layers called "calcrete" that form layers up to 50 meters thick.

Caliche is a reddish-brown to white layer found in many desert soils. Caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction between water and carbon dioxide released by plant roots or by decaying organic material.

Next page: the NA deserts....