A desert is an area with very little rainfall, usually receiving no more than 250mm (10 inches) a year. They can be either extremely hot or extremely cold. Water shortage in the desert is due to low rainfall, humidity, and lack of cloud cover. Without the cloud cover, the surface is exposed to direct sunlight, causing rapid evaporation of water. The rates of evaporation are then in turn too high for any rainfall to be able to replenish. Interestingly, evaporation rates in deserts can be 20 times that of the annual precipitation. This is because most rain evaporates before it ever hits the ground. Some deserts won't receive any rainfall for years. Infact, in the Atacama desert in Chile, no rain fell for over 40 years!

Deserts occur in 6 of the world's 7 continents, with Europe being the only continent without a desert. Antarctica is entirely a desert and Australia is almost completely a desert, having some fertile lands along the east coast. Deserts cover almost one third of the Earth's land surface, and 13% of the world's population live in these dry areas. Some of them choose to live near Oases, and some are nomads that wander throughout the desert.

What causes deserts to form?

  • Descending air
    A lot of deserts tend to occur in two belts that circle the planet. These belts, one in each hemisphere, are situated between 15 and 35 degrees latitude. This happens because most air in these areas are descending. As this air descends, it becomes warmer and more able to hold water vapour, therefore absorbing it rather than releasing it.

  • The rain shadow effect
    When moist air is pushed over land and hits a mountain, it is forced up the range where it will then cool and condense, releasing rain. By the time the air is pushed down the other side of the mountain, it has lost most of its moisture. All the deserts in North America are influenced by this 'rain shadow effect'.

  • Distance from the sea
    This reason is simple to explain. Air masses will lose all of their moisture before reaching some areas far from the sea. Sometimes deserts formed this way are called 'remote interior basins'. The Gobi desert is one example of a remote interior basin.

  • Ocean currents
    Some deserts occur along coastline. This is usually because the seas next to the deserts have cold currents. Water in cold oceans are less likely to evaporate than they would in warmer oceans. If cool air from the sea moves over the hot land, it warms, and thus there is little chance of any vapour condensing and producing rain. These deserts tend to occur on the Western edge of continents near the Tropic of Cancer or Tropic of Capricorn.

  • Desertification
    Overcultivation, Overgrazing, deforestation, salinisation, drought] and rise in temperature can lead to desertification, a process that turns previously fertile lands barren.

  • Cold temperatures
    In places like Antarctica, it is so cold that any potential rainfall is frozen when it is released. It may snow in places like Antarctica, but it doesn't rain.

    Why are some deserts so cold at night?
    Usually deserts have cloudless skies. This allows the sun's rays to heat up the land during the day, making the temperature extremely hot. But as the sun sets, if there is no cloud cover to keep the heat in, it radiates into the atmosphere. Of course, this is only applicable to hot deserts. The difference between the hottest and coldest parts of the day can be quite distinct. Some people call nighttime 'the Winter of the desert'.

    Why are the physical characteristics of a desert?
    These vary greatly. Deserts come in many forms, from claypan deserts which are dry and cracked, to sandy deserts which are full of dunes. Other types of deserts include mountain deserts (any water runs down the steep sides of a mountain, eroding and creating deep gorges), stony deserts (covered with small pebbles and rock) and shield deserts (a mixture of sand and gravel, often with an underlying granite base).

    There are many interesting landforms to be found in deserts. They include:

  • Gibber Plains - These form when small stones and rocks are left behind after loose material has been blown away.
  • Wadis - Steep sided valleys, or dry riverbeds.
  • Alluvial Fans - Heavy rainfall forms streams that carry loads of sand and rock fragments as they cut through gorges and gullies. When the streams hit flat land they 'spray' their loads out in a fan like way.
  • Mesas - Flat topped hills with hard rock caps
  • Buttes - Smaller versions of mesas
  • Mushroom Rocks - Also known as rock pedestals, they have been eroded, but harder rock left intact on top leaves a mushroom shape.
  • Playa Lakes - When land surface is flat and low, rainfall collects in holes to form playa lakes.
  • Oases - Springs/Waterholes
  • Arches - Rock has been eroded underneath these, but the harder rock on top is left untouched. Thus, they are arch shaped.
  • Inselbergs - A hard rocky outcrop that resists erosion. Think Uluru (aka Ayers Rock).
  • Plateaus - Areas of higher land
  • Badlands - When hilly or mountainous country is severely chopped up by heavy storms and flash floods, badlands are formed.
  • Sand Dunes - Hilly bumps of sand that are created by sand particles being transported by the wind

    How does a hot desert's flora and fauna survive on so little water?
    Over time animals and plants have learned to adapt to the desert's harsh conditions. There are different ways that they do this, and they can be categorised into the following for groups.

  • Evaders - will avoid the hot, dry conditions by laying dormant or going to sleep until cooler weather and/or rains arrive. Some evaders include toads, rodents that burrow and plant seeds.

  • Escapers - animals move out of the desert when the temperature gets too high. They return during cooler and wetter times. They include birds and the gazelle. Plants escape drought by working hard to find a source of moisture. They send out long roots and tunnel their way to any underground water.

  • Resisters - Have developed ways of resisting with the harsh climate of deserts and make the best possible use out of any water available. Some examples of resisters are nocturnal animals, plants that have extensive root systems that reach and store water and animals that get the moisture they need from the leaves of plants. A lot of resister plants limit the amount of water they lose from evaporation through having no leaves at all, curling up their leaves from the sun and insulating their trunks and stems with cork cells.

  • Conservers - Stores whatever water it can. Plants tend to be much better at storing water, yet the camel is a good example of an animal conserver. Plants have large spreading roots that swell up when full of water. To prevent loss of water through evaporation they usually have thorns instead of leaves. An example of a plant conserver is the cactus.

    The desert tends to lack large mammals because most of them are incapable of withstanding the high temperatures and storing enough water. The Sahara desert in Africa is the world's largest desert and covers 8% of the world's land. It is estimated that approximately 1200 plant species can be found in the Sahara, proving that with the right characteristics, a plant can easily survive in the harsh desert climate.

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