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**Desert Characteristics and Classification:**
– Deserts are regions with low precipitation, little plant coverage, and dry streams.
– Most deserts receive less than 250mm (10in) of precipitation annually.
– Semi-deserts receive between 250 and 500mm (10-20in) of precipitation.
– Deserts like the Sahara occur at low altitudes.
– Actual evapotranspiration in deserts can be close to zero.
– Deserts are classified based on total precipitation, rainy days, temperature, and humidity.
– Cold deserts exist in high-altitude regions like the Himalayas.

**Water Sources in Deserts:**
– Deserts may have underground water sources like aquifers, springs, or underground rivers.
– Notable examples include the Nubian Sandstone Aquifer System under the Sahara and the Great Man-Made River project in Libya.
– Oases like Kharga Oasis in Egypt form where porous sandstone allows for water extraction.
– Seepages in canyon walls and shaded pools can provide water sources for desert life.

**Desert Climate and Temperature:**
– Hot deserts have high summer temperatures, low humidity, and minimal precipitation.
– Daily temperature variations can be significant in deserts.
– Cold deserts, found at higher latitudes, are arid due to dry air and lack of moisture.
Polar deserts, like those in the Arctic and Antarctic, receive minimal precipitation.
– Hyperarid deserts get less than 25mm of rainfall annually, while arid deserts receive between 25-200mm.

**Types of Deserts and Their Features:**
– Trade wind deserts, mid-latitude deserts, monsoon deserts, rain shadow deserts, and montane deserts.
– Deserts vary in composition, with around 20% being sand globally.
– Different types of dunes include barchan, seif, transverse, star, and dome dunes.
– Features like gypsum dune fields, desert pavement, and various landforms are common in deserts.

**Specific Desert Examples and Unique Characteristics:**
– The Atacama Desert is one of the driest places on Earth with minimal precipitation.
Antarctica is the world’s largest cold desert, with 98% thick ice sheet and 2% barren rock.
– The Great Man-Made River project in Libya taps into aquifers to supply water to coastal cities.
– White Sands National Park in New Mexico showcases vast gypsum dune fields.
– The Tassili Plateau in Algeria features eroded sandstone outcrops and unique formations.

Desert (Wikipedia)

A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the land surface of the Earth is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts or "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 and dunes of the Libyan Desert
view of high desert through a mountain crevasse
Valle de la Luna ("Moon Valley") in the Atacama Desert of Chile, the world's driest non-polar desert

Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks, which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter, and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind. This picks up particles of sand and dust, which can remain airborne for extended periods – sometimes causing the formation of sand storms 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, often forming desert pavements, and little further erosion takes place. 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.

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 herbivory. Some annual plants germinate, bloom and die in the course of a few weeks after rainfall, while other long-lived plants survive for years and have deep root systems able to tap underground moisture. 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. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy.

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available, and oases have provided opportunities for a more settled way of life. 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, and the Imperial Valley in California provides an example of how previously barren land can be made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy.

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