Why do deserts occur

Even in such a dry climate, most of the landforms are carved by the rare periods of heavy rainfall that result in flash floods, erosion, and sediment deposition. Hot air rises at the equator, where the land receives the greatest amount of the sun's radiation. Most of the world's deserts are located near 30 degrees north latitude and 30 degrees south latitude, where the heated equatorial air begins to descend.

The descending air is dense and begins to warm again, evaporating large amounts of water from the land surface. The resulting climate is very dry. Some deserts are found on the western edges of continents. They are caused by cold ocean currents, which run along the coast.

They cool the air and make it harder for the air to hold moisture. Most moisture falls as rain before it reaches the land, eg the Namib Desert in Africa.

The most spectacular results of the action of the winds are eroded and polished rocks in the most bizarre shapes. Dunes are also generated by the wind that builds and "moves" these sandy mountains around; the draughts lift the grains and settle them down again when friction increases.

The shape of the dunes mostly depends on the direction and variability of the wind; there may be parabolic, dome-shaped, boat-shaped, crosswise , straight, opposing or star-shaped. To inform younger students about Energy and Environment, Science, Chemistry, English culture and English language, with accompanying images, interviews and videos. CLIL will no longer be a secret with"clil in action"!

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And for the winners, production of animated short films! They travelled over large areas with their herds, moving to new pastures as seasonal and erratic rainfall encouraged new plant growth. They took with them their tents made of cloth or skins draped over poles and their diet included milk, blood and sometimes meat. The desert nomads were also traders. The Sahara is a very large expanse of land stretching from the Atlantic rim to Egypt.

Trade routes were developed linking the Sahel in the south with the fertile Mediterranean region to the north and large numbers of camels were used to carry valuable goods across the desert interior.

The Tuareg were traders and the goods transported traditionally included slaves, ivory and gold going northwards and salt going southwards. Berbers with knowledge of the region were employed to guide the caravans between the various oases and wells. Round the rims of deserts, where more precipitation occurred and conditions were more suitable, some groups took to cultivating crops. This may have happened when drought caused the death of herd animals, forcing herdsmen to turn to cultivation.

With few inputs, they were at the mercy of the weather and may have lived at bare subsistence level. The land they cultivated reduced the area available to nomadic herders, causing disputes over land.

The semi-arid fringes of the desert have fragile soils which are at risk of erosion when exposed, as happened in the American Dust Bowl in the s. The grasses that held the soil in place were ploughed under, and a series of dry years caused crop failures, while enormous dust storms blew the topsoil away.

Half a million Americans were forced to leave their land in this catastrophe. Similar damage is being done today to the semi-arid areas that rim deserts and about twelve million hectares of land are being turned to desert each year.

Vegetation plays a major role in determining the composition of the soil. In many environments, the rate of erosion and run off increases dramatically with reduced vegetation cover. Although overgrazing has historically been considered to be a cause of desertification, there is some evidence that wild and domesticated animals actually improve fertility and vegetation cover, and that their removal encourages erosive processes.

Deserts contain substantial mineral resources, sometimes over their entire surface, giving them their characteristic colors. For example, the red of many sand deserts comes from laterite minerals. Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form. Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates.

Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world. Oil and gas form on the bottom of shallow seas when micro-organisms decompose under anoxic conditions and later become covered with sediment.

Many deserts were at one time the sites of shallow seas and others have had underlying hydrocarbon deposits transported to them by the movement of tectonic plates.

Traditional desert farming systems have long been established in North Africa, irrigation being the key to success in an area where water stress is a limiting factor to growth. Techniques that can be used include drip irrigation, the use of organic residues or animal manures as fertilisers and other traditional agricultural management practises. Once fertility has been built up, further crop production preserves the soil from destruction by wind and other forms of erosion.

A study of these microbes found that desert farming hampers desertification by establishing islands of fertility allowing farmers to achieve increased yields despite the adverse environmental conditions. They used terracing techniques and grew gardens beside seeps, in moist areas at the foot of dunes, near streams providing flood irrigation and in areas irrigated by extensive specially built canals.

The Hohokam tribe constructed over miles km of large canals and maintained them for centuries, an impressive feat of engineering. They grew maize, beans, squash and peppers. A modern example of desert farming is the Imperial Valley in California, which has high temperatures and average rainfall of just 3 in 76 mm per year. Other water from the river is piped to urban communities but all this has been at the expense of the river, which below the extraction sites no longer has any above-ground flow during most of the year.

Another problem of growing crops in this way is the build-up of salinity in the soil caused by evaporation of river water.

This prospect has proved false as it disregarded the environmental damage caused elsewhere by the diversion of water for desert project irrigation. Desertec proposes using the Saharan and Arabian deserts to produce solar energy to power Europe and the Middle East. Deserts are increasingly seen as sources for solar energy, partly due to low amounts of cloud cover. Many successful solar power plants have been built in the Mojave Desert. These plants have a combined capacity of megawatts MW making them the largest solar power installation in the world.

The potential for generating solar energy from the Sahara desert is immense. European interest in the Sahara Desert stems from its two aspects: the almost continual daytime sunshine and plenty of unused land. The Sahara receives more sunshine per acre than any part of Europe.

The Sahara Desert also has the empty space totalling hundreds of square miles required to house fields of mirrors for solar plants. The Negev Desert, Israel, and the surrounding area, including the Arava Valley, receive plenty of sunshine and are generally not arable. This has resulted in the construction of many solar plants. Both world wars saw fighting in the desert. The Turks were defeated by the British, who had the backing of irregular Arab forces that were seeking to revolt against the Turks in the Hejaz, made famous in T.

Warfare in the desert offered great scope for tacticians to use the large open spaces without the distractions of casualties among civilian populations. Tanks and armoured vehicles were able to travel large distances unimpeded and land mines were laid in large numbers. However the size and harshness of the terrain meant that all supplies needed to be brought in from great distances.

The victors in a battle would advance and their supply chain would necessarily become longer, while the defeated army could retreat, regroup and resupply. For these reasons, the front line moved back and forth through hundreds of kilometers as each side lost and regained momentum.

Marco Polo arriving in a desert land with camels. The desert is generally thought of as a barren and empty landscape. It has been portrayed by writers, film-makers, philosophers, artists and critics as a place of extremes, a metaphor for anything from death, war or religion to the primitive past or the desolate future. There is an extensive literature on the subject of deserts.

View of the Martian desert seen by the probe Spirit in Mars is the only planet in the Solar System on which deserts have been identified. The Martian deserts principally consist of dunes in the form of half-moons in flat areas near the permanent polar ice caps in the north of the planet. The smaller dune fields occupy the bottom of many of the craters situated in the Martian polar regions.

Skip to main content. Search for:. Cold desert: snow surface at Dome C Station, Antarctica. Exfoliation of weathering rocks in Texas. One square centimeter 0. Dust storm about to engulf a military camp in Iraq, Aerial view of Makhtesh Ramon, an erosion cirque of a type unique to the Negev. Diagram showing barchan dune formation, with the wind blowing from the left.

Windswept desert pavement of small, smooth, closely packed stones in the Mojave desert. Flash flood in the Gobi. Tadpole shrimp survive dry periods as eggs, which rapidly hatch and develop after rain.

Shepherd near Marrakech leading his flock to new pasture. Salt caravan travelling between Agadez and the Bilma salt mines. A mining plant near Jodhpur, India. Mosaic of fields in Imperial Valley. Deployment of forces on the eve of the Second Battle of El Alamein in Online Etymology Dictionary.

Retrieved The Free Dictionary. Yale University Press. ISBN Essentials of Geology, 3rd ed. United States Geological Survey. Cold Deserts of India. Indus Publishing. The biology of polar regions. Oxford University Press. Berry; Howard-Williams, C. Nordic Hydrology 29 4—5 : — ISSN Oliver 1 January The Encyclopedia of World Climatology. Deserts: Geology and Resources. Oracle ThinkQuest Education Foundation.

USA Today. Encyclopedia Britannica online. Physical Geography. Encyclopedia of Deserts: Deserts, Montane. University of Oklahoma Press. Arctic, Antarctic, and Alpine Research 34 3 : — A Dictionary of Ecology. Physical Geography: Process and System. Journal of Sedimentary Petrology 38 3 : — United Nations Convention to Combat Desertification. Geophysical Research Letters 30 Bibcode: GeoRL..