Deserts

A Desert is a type of Habitat or Biome characterised mainly by extreme temperatures and extremely low amount of precipitation. Scientifically speaking a Desert is a region which receives rainfall less than 10 inches or 25cm or less in a year. Deserts are one of the Earth’s major types of ecosystems, supporting a community of distinctive plants and animals specially adapted to the harsh environment. About one-third of the Earth's land surface is Desert. Deserts are not only made of sand as commonly believed, they also consist of rocks and mountains. Desert sands are often rock which has weathered down to sand over the centuries. There are many Deserts in the World. The Largest Desert is the Sahara Desert found in North Africa. The largest continuous stretch of sand is called the 'Empty Quarter' which is part of the Arabian Desert that stretches 250,000 sq. miles. Deserts are inhabited by very few Plants and Animals. They are often able to survive by avoiding the driest times and only growing after the rains. 

Classification / Types of Deserts

There are almost as many definitions of Deserts and classification systems as there are Deserts in the World. Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. 

1.  Basic or General Desert Classification

• Cold Deserts - The Deserts found in temperate regions at higher altitudes and near the Arctic are called Cold Deserts. Some of these deserts are found in Greenland, Antarctic and the Nearctic realm. These Deserts are characterized by extremely cold and dry winters, high amount of snow fall and low overall temperatures throughout the winter and sometimes even in the summers too. The cold deserts found in the temperate regions may have mean winter temperatures averaging between –2 to 4°C and the mean summer temperature can average between 21-26°C. The mean annual precipitation in cold deserts ranges from 15-26 cm. The soil in these areas is salty, silty and heavy. The vegetation in these areas is scarce and widely scattered. The plants found in the most extreme regions of these deserts are short. While in temperate regions of cold deserts the plant length varies between 15 cm to 22 cm in height. The plants here are deciduous, most of them having spiny leaves.

• Hot and Dry Deserts - Most Hot and Dry Deserts are located near the Tropic of Cancer or the Tropic of Capricorn. Hot and Dry Deserts' temperatures range from 20 to 25° C.  Summer temperatures in these Deserts often reach 100 degrees Fahrenheit or higher. At night the temperature may drop to 45 degrees Fahrenheit or lower.They usually have very little rainfall and/or they get a lot of rainfall in a very short time, and then there are long periods of time (especially the winter) when there is no rain. The average rainfall is usually less than 15 cm a year. The soils in the hot and dry Deserts are coarse-textured, shallow, rocky and gravely with good drainage and have no subsurface water, the reason of the coarse texture is the chemical weathering. Some of the plants in this area open their stomata only at night when evaporation rates are lowest.

• Semiarid Deserts - Semiarid Deserts are located in North America, Europe, and northern Asia. In these Deserts the summers are moderately long and dry, and like Hot Deserts, the winters normally bring little rainfall. Summer temperatures usually average between 21-27° C. It normally does not go above 38° C . Also, like the Hot Desert, rainfall is often very low and/or concentrated. The average rainfall ranges from 2-4 cm yearly. However, the evening temperatures in these Deserts are cool, around 10° C , and because the nights are cool, condensation occurs. Dew is formed, and the amount of dew formed may equal or exceed the rainfall received by some Deserts. The soil in the semiarid Deserts range from sandy and fine-textured to loose rock fragments, gravel or sand. Near the mountain slopes the soil is shallow, rocky, gravely, well drained and with good drainage whereas in the bottom land the soil is sandy and fine-textured. Many of the plants in a semiarid Desert have spines and silvery or glossy leaves that let them reflect more radiant energy.

• Coastal Deserts - Coastal Deserts are found in moderately cool to warm areas such as the Nearctic and Neotropical realm which means areas of southern California, Baja California, south-west Africa, and Chile. Atacama of Chile is a well known Coastal Desert. Coastal Deserts have cool winters and moderately long, warm summers. The average summer temperature ranges from 13-24° C. Winter temperatures are 5° C or below. The average rainfall measures 8-13 cm in many areas. The soil in the coastal Desert is fine-textured with moderate soil content, and is fairly porous with good drainage.

2.  Peveril Meigs Classification

In 1953, Peveril Meigs divided Desert regions on Earth into 3 Categories according to the amount of precipitation they received. 

According to this system

• 'Extremely Arid Lands' have at least 12 consecutive months without rainfall.

• 'Arid Lands' have less than 250 millimetres of annual rainfall.

• 'Semiarid Lands' have a mean annual precipitation of between 250 and 500 millimetres.

'Arid' and 'Extremely Arid Land' are Deserts, and 'Semi-arid' Grasslands generally are referred to as 'Steppes'.

3.  Köppen Climate Classification System

• BWh - Hot Desert temperature ranges from 20 to 25° C. The extreme maximum temperature for Hot Desert ranges from 43.5 to 49° C. Hot Deserts usually have very little rainfall and/or concentrated rainfall in short periods between long rainless periods. This may average out to under 15 cm a year. Plants are almost all ground-hugging shrubs and short woody trees. All of the leaves are replete (packed with nutrients). Some examples of these kinds of plant are Turpentine Bush, Prickly Pears, and Brittle Bush.

• BWk - Temperate Deserts have strong temperature contrasts between summer and winter and low rainfall. Temperate Deserts temperature in winter ranges from -2 to 4° C and in the summer 21 to 26° C a year. Most precipitation falls in winter, despite a peak in May. Temperate Deserts usually have lots of snow. They also have rain around spring. This may average out to 15-26 cm a year. Aridity increases markedly in the rain shadow of the Pacific mountain ranges. Even at intermediate elevations, winters are long and cold, with temperatures falling below 32°F (0°C). All plants here are either deciduous and more or less contain spiny leaves. Sagebrush is a commonly found Plant in this area.

4.  Classification according to Geographical Location & Weather Pattern

• Trade Wind Deserts - These type of Deserts occur where the Trade Winds (surface winds found in the tropics) blow over the land. The Trade Winds blowing in two belts on the equatorial sides of the Horse Latitudes (subtropic latitudes between 30 and 35 degrees both north and south) 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. The Sahara Desert is a Trade Wind Desert.

• Mid Latitude Deserts - Mid Latitude 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. Dominant high pressure and lack of moisture combine to inhibit the production of clouds. The Sonoran Desert of south-western North America is a typical Mid Latitude Desert.

• Rain Shadow Deserts - Rains Shadow Deserts are those areas that lie in the shadow of mountain ranges and receive little precipitation. These 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. Death Valley, USA is a Rain Shadow Desert.

• Coastal Deserts - Coastal Deserts are formed when cold waters move from the Arctic and Antarctic regions toward the equator and come into contact with the edges of continents. The cold waters are augmented by upwellings (rising up) of cold water from ocean  depths. As the air currents cool as they move across cold water, they carry fog  and mist, but little rain. Coastal Deserts are 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. These type of Deserts are found southern California, Baja California, south-west Africa, and Chile.

• Monsoon Deserts - Monsoon Deserts are those Deserts which receive rainfall owing to monsoons developing in response to temperature variations between continents and oceans. The south east trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon Desert region west of the range.

• Polar Deserts - Polar Deserts are high-latitude Deserts where the existing moisture is frozen in ice sheets and is thus unavailable for plant growth. Polar Deserts are areas with annual precipitation less than 250 millimetres and a mean temperature during the warmest month of less than 10° C. Polar Deserts on the Earth cover nearly 5 million square kilometres and are mostly bedrock or gravel plains. It is are also known as Arctic Desert. 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.

Distribution

Deserts are found all over the World. There are Deserts like the Sahara of North Africa and the Deserts of south western U.S-Mexico and Australia that occur at low altitudes and then there are the Cold Deserts that occur in the basin and range area of Utah and Nevada and in parts of western Asia. 

Major Deserts of the World

Antarctic Desert (Antarctica) -  Area = 13,829,430 km²  or  5,339,573 mi².

Arctic Desert - Area = 13,700,000+  or    5,300,000+ mi².

Sahara Desert (Africa) - Area =    9,100,000+ km²  or  3,320,000+ mi².

Arabian Desert (Middle East) - Area = 2,330,000 km²  or  900,000 mi².

Gobi Desert (Asia) - Area = 1,300,000 km²  or  500,000 mi².

Kalahari Desert (Africa) - Area = 900,000 km²  or  360,000 mi².

Patagonian Desert (South America) - Area = 670,000 km²  or  260,000 mi².

Great Victoria Desert (Australia) - Area = 647,000 km²  or  250,000 mi².

Syrian Desert (Middle East) - Area = 520,000 km²  or  200,000 mi².

Great Basin Desert (North America) - Area = 492,000 km²  or  190,000 mi².

Formation of Deserts

Deserts form under certain geographical conditions.

Primarily there are 3 geographical conditions which can result in a Desert.

1.  When an area is not located near a large body of water it may result in Arid or dry region. High atmospheric pressure in the region brings cold, dry air from higher altitudes closer to land. The sun heats the air causing low humidity and high ground temperatures.

2.  When an area lies in the interior of a land mass, it may lack moisture leading to a dry conditions. This may happen because of the lack of rain because the moisture in the air has already fallen by the time it reaches the inland of a land causing them to dry up.

3.  When mountains prevent rain from reaching an area, making that area dry and eventually into a Desert. If there are two mountain ranges, one to the east and one to the west, they can prevent ocean air from reaching the land. The region between the mountains becomes very dry. Cold deserts such as those in Antarctica are products of the extreme cold climate. Surface water remains frozen and the air is too cold to contain more than minute amounts of moisture. 

General Characteristics of Deserts 

Extreme Temperatures - Deserts are generally known as 'Areas of Extremes'. The reason being that temperatures in Deserts are either too cold or too hot. Temperatures are high during the day because there is very little moisture in the air to block the Sun's rays from reaching Earth. The arid conditions of the major Desert areas result from their position in subtropical regions to either side of the moist equatorial belt. The high temperature is caused by the intense radiation it receives. The sky is not sheltered by clouds, causing more than 70% of sunlight to penetrate into the ground. Once the Sun goes down, the heat absorbed during the day quickly escapes back into space. Temperatures in excess of 100 degrees fahrenheit during day are not uncommon. But at night, the same Desert experiences a temperatures fall into the 40s or 50s. 

Imbalance between Precipitation and Evapotranspiration - A desert is an area where more water is lost through evaporation than is gained from precipitation. Due to the high temperature, the rate of evaporation is fifteen times the amount of rain. Whatever little amount of water it does receive is quickly lost through evaporation. Average annual Precipitation in the world's deserts ranges from about 0.4 to 1 inch (10 to 25 millimetres) in the driest areas to 10 inches (250 millimetres) in Semi arid regions. The little precipitation and rain that falls in deserts is usually erratic and varies from year to year. But when it rains, it comes in high quantities that disappear as quickly as they came. This rain is called 'Intermittent' or 'Episodic Rain'. Such powerful bursts of water can erode the landscape with great force.

Location near the Tropics - Most of the World's Desert ecosystems are located in two belts near the tropics (also called the torrid zone) at 30 degrees north and 30 degrees south of the equator. These areas receive little rainfall because of the downward flow of dry air currents that originate at the equator. As this equatorial air moves north and south, it cools and loses whatever moisture it contains. Once this cool, dry air moves back toward Earth's surface, it is rewarmed, making it even drier. Over the Desert areas, the dry air currents draw moisture away from the land on their journey back toward the equator. 

Free Flowing Winds - Winds flow freely unhindered in Deserts because Deserts have few Plants, especially Trees, to stop the air flow and bind the soil to prevent it from erosion. That is why sands and soils are easily blown away in Deserts.

Erosion - In Deserts the wind erodes rocks. This process is known as 'Abrasion' in which sand is carried up and hits against the rocks having a sand-papering effect. Over a period of time, landforms like pillars, rock pedestal and natural arches are formed. 

Unique Geographical Features - Deserts have unique physical features which are not found elsewhere.

• Dunes - Prominent among the Hot Deserts is the presence of Dunes. A Dune is a hill of sand built by winds. Dunes also form under the action of water flow (alluvial processes), on sand or gravel beds of rivers, estuaries  and the sea-bed. The form of the sand is also shaped by the wind in the form of Barchans (crest shaped dune). Blown about sand usually collect in basins or piles up against fixed objects or rocks which may create different physical features. Nearly all Desert surfaces are plains where removal of fine-grained material by the wind has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles.

• Mesas and Buttes - Where rock lie in horizontal layers, some harder than others, water erosion with wind and weathering over thousands of years can produce Mesas and Buttes. A Mesa is a large, flat-topped island of rock that is left standing when the surrounding areas have been eroded away. When these erode or weather further, columns of rock called 'Buttes' remain jutting out of the landscape.

• Canyons - Canyons are formed where river flows. As the water slices down into the rock, it leaves behind almost vertical sides.

• Bajadas - Bajadas are shallow slopes that lie at the base of rocky hills, where materials accumulate from the weathering of the rocks. Fast-flowing streams caused by sudden rainstorms have great power and scour out huge amounts of pebbles and rock, which they carry downhill. When they deposit their load at plains, fan-shaped deposits of stone and rocks called Alluvial Fans are formed. Bajadas are formed when several Alluvial Fans merge together.

• Hoodoo - It is a bizarre-shaped column or pillar caused by differential erosion on rocks of different hardness.

Unique Plants - Most Desert Plants are drought or salt-tolerant. Some store water in their leaves, roots, and stems. Other Desert Plants have long tap roots that penetrate the water table, anchor the soil, and control erosion. The stems and leaves of some Plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Cactus is one of the most common Desert Plant which is adapted to Desert life. Most of the Cacti store water in their leaves, stems and roots. They have very few or no leaves and in most Species, the leaves are modified to needles. Apart from Cacti, Deserts also feature Plants from the Pea Family and Sunflower Family. Cold Deserts have Grasses and Shrubs as dominant vegetation. 

Some of the Desert Plants

1. Date Palm.
2. Thyme.
3. African Peyote Cactus.
4. Olive Trees
5. Acacia.
6. Needle Grass.
7. Reeds.
8. Black Sage.
9. Buckwheat Bush.
10. Rice Grass.
11. Salt Bush.
12. Arctic Azaleas.
13. Arctic Lupine.
14. Arctic Poppies.
15. Cinquefoil.
16. Campanulas.
17. Buttercups.
18. Lichens.
19. Wild Crocus.
20. Yarrow.

Desert Adapted Animals - Animals that live in the hot Desert have many adaptations. Some animals never drink, but get their water from seeds (some can contain up to 50% water) and plants. Many Animals are nocturnal, sleeping during the hot day and only coming out at night to eat and hunt. Some animals rarely spend any time above ground. Spadefoot toads spend nine months of every year underground. Deserts are home to many Reptiles, Insects, Birds, and small Mammals. The kangaroo mice of North America and the Bilby and Red Kangaroo of Australia are Desert Specialists. The most universal behavioural adaptation used by small Mammals, Reptiles, and Insects to deal with high temperatures is staying in the shadow (shade) of plants or rocks, thus avoiding the direct rays of the Sun. These animals also seek shelter by burrowing into the ground.  a burrow, even a few feet underground, can decrease the temperature by several degrees. Another behavioural adaptation used by Desert animals is to remain inactive during the hot daylight hours. They hunt at night when temperatures are cool and when there is less risk of losing precious body water. Animals that use this adaptation are referred to as Nocturnal. Some animals get all of the water they need from the insects, bulbs, and seeds they eat. They will not drink water even when it is available. Some animals have developed salt glands, a physical adaptation that allows the secretion of salt without the loss of water. The absence of sweat glands, and the concentration of urine are other physical adaptations made by Desert animals. Because fat intensifies heat, a unique physical adaptation of some Desert animals is the storage of fat in humps or tails, rather than throughout the entire body. Camel is the best example of such a adaptation. Animals found in Cold Deserts have adopted to the cold environment by having thick fur and fatty layered bodies which protect them from cold and save them during harsh times. Some are even coloured as snow to camouflage themselves from predators and to catch prey as well. Arctic Fox and Polar Bear are examples of such colour adaptations.

Some of the Desert Animals

1. Camel.
2. Desert or Kangaroo Rat.
3. Armadillo Lizard.
4. Desert Tortoise.
5. Bilby.
6. Red Kangaroo .
7. Ground Squirrel.
8. Jack Rabbit.
9. Arctic Fox.
10. Rock Ptarmigan.
11. Arctic Wolf.
12. Polar Bear.
13. Musk Ox.

Origin of Deserts

The present day Deserts are of relatively recent origin as per geological studies. They represent the most extreme result of the progressive cooling and consequent aridification (drying up) of global climates during the Cenozoic Era (65.5 million years ago). It has been suggested that many typical modern desert plant families, particularly those with an Asian centre of diversity such as the chenopod and tamarisk families, first appeared in the Miocene (23 to 5.3 million years ago), evolving in the salty, drying environment of the disappearing Tethys Sea along what is now the Mediterranean–Central Asian axis.

Importance of Deserts

Scientific Study - Deserts are scattered with fossils, human artifacts, ancient rocks and minerals. Because Deserts are dry, they are ideal places for human artifacts and fossils to be preserved. Researchers and scientist can and do study these in understanding the ununderstood. They are open environments wherein the wind and sometimes water interact with arid surface, these phenomena are the basis of geological studies with far reaching implications. 

Solar Energy - As Sun is nearest the equator it's heat occurs as well near these. This light can and is being used to produce solar energy and meet energy requirements of a country and save the environment as well. The Negev Desert and the surrounding area, including the Arava Valley, are the sunniest parts of Israel and little of this land is arable, which is why it has become the centre of the Israeli solar industry.

Minerals - Valuable minerals located in arid lands include copper in the United States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia; and gold, silver, and uranium  deposits in Australia and the United States. Non-metallic mineral resources and rocks such as beryllium, mica, lithium, clays, pumice, and scoria also occur in arid regions. Sodium carbonate, sulphate, borate, nitrate, lithium, bromine, iodine, calcium, and strontium  compounds come from sediments and near-surface brines formed by evaporation of inland bodies of water.

Indicators of Land Degradation - There are certain Deserts which have been recently formed and some have been the result of deforestation and improper land usage. Deserts are constant reminders to humanity that if proper care of environment is not taken, it can lead to Desertification. They also highlight the importance of Afforestation.

Source of Inspiration - Deserts have been sources of inspiration for various writers and artists. Deserts have been portrayed as fascinating environments of adventure and exploration in the form of Narratives like 'Lawrence of Arabia' and it's form in movie as well, along with movie like Dune.