Desert

A desert is a region of land that is very dry because it receives low amounts of precipitation (usually in the form of rain but may be snow, mist or fog), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside the area.

Classification of Deserts
Deserts generally receive less than 250 mm of precipitation each year. Semi -arid deserts are regions which receive between 250 and 500 mm and when clad in grass are known as steppes. About one third of Earth’s land surface is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.

Sand deserts such at Earth’s Rub' al Khali,  are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which consequently break in pieces. Rain falling on hot rocks can cause them to shatter and the resulting fragments and rubble strewn over the desert floor is further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes.

Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. (Pre terraformed Mars, Sol System).

Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur.

Desert Life
Plants and animals living in the desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and often spines to deter herbivores. Animals need to keep cool and find enough food and water to survive. Many are nocturnal and stay in the shade or underground during the heat of the day.

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Pastoral nomads have moved their flocks and herds to wherever grazing is available. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert farming is possible with the aid of irrigation. Many trade routes have been forged across deserts, and traditionally were used by caravans of camels carrying salt, gold, ivory, slaves and other goods. Some mineral extraction also takes place in deserts and uninterrupted sunlight gives potential for the capture of large quantities of solar energy.

