Deserts

Deserts are characterized by extreme environmental conditions. They get at most

50 centimeters (cm), or 20 inches, a year of precipitation; more commonly,
they're lucky to get half of that. Most of them are found at low latitudes, that is,
closer to the equator than to the poles. The massive Sahara, probably the most
famous desert on Earth and its third largest, lies just north of the equator in
Africa. While they are far less densely populated than other biomes owing to how
dry they are and being poorly hospitable overall, most deserts do feature a range
of vegetation as well as both vertebrate and invertebrate animal life.

Large mammals are uncommon in deserts because most of them cannot store
sufficient water and tolerate the heat (camels are a notable exception). While
smaller animals might be able to find patches of shade sufficient to cover their
bodies, deserts typically offer little protection from the sun for larger animals.
The dominant animals of the warm deserts are non-mammalian vertebrates,
mainly reptiles. Whatever mammals have managed to thrive in these biomes tend
to be small, such as the kangaroo mice that inhabit some deserts in North
America.

Types

Hot-and-dry deserts probably best fit the average person's idea of what a desert
should look and feel like. The Sahara is one such desert. Others appear in
Australia, South Asia, and Central and South America. The U.S. features the
Chihuahuan, Sonoran, Mojave and Great Basin deserts.

The seasons are warm to quite hot year-round, and owing to the low humidity of
these environments, the temperature swing from the hottest time of day to the
coldest time of day may be extreme – over 45 C (about 80 F) in some regions.
This is mainly because the surface receives twice as much solar radiation during
the day as would the surface in comparable but more humid environment, and
loses twice as much heat at night.

Rainfall is usually very sparse in hot-and-dry deserts, and evaporation rates

routinely outpace rainfall rates. Falling rain has even been noted to evaporate
before reaching the ground. What little rain these deserts get occurs in short, brief
and sometimes intense bursts, though monsoons and remnants of tropical systems
that drift into some deserts can provide abundant moisture at times. The Atacama
Desert in Chile on the western coast of South America, known as the world's
driest place, receives an average of 1.5 cm per year of rain – barely a half-inch.

Plants in hot-and-dry deserts are mostly low shrubs and short, woody trees.
Animals include small nocturnal carnivores, with comparatively high populations
of burrowers and kangaroo rats. Insects, arachnids, reptiles and birds are also
common. The animals hide from the sun and then come out to forage at dusk or at
night, when the desert is coolest.

Cold winter deserts, also called semiarid deserts, are characterized by

moderately long, dry summers and winters that include brief intervals of rain.
This pattern is like that of hot-and-dry deserts, but the overall temperatures are
somewhat cooler. U.S. examples include the sagebrush zones of Utah, Montana
and the Great Basin. They also include northern, but subarctic, portions of North
America, Newfoundland, Greenland, Russia, Europe and northern Asia.

Summer temperatures in these deserts usually average between 21-27 C (70-80

F). It normally does not rise above 38 C (100 F), and evening temperatures are
cool, at around 10 C (50 F). Annual rainfall can be as low as only 2 to 4 cm
(about 0.8 to 1.5 inches).

Soil can range from sandy and fine-textured to loose rock fragments, gravel or
sand. There is no subsurface water in these environments. As for vegetation, cacti
(the plural of "cactus") are found here. The spines of cacti and other plants in cold
winter deserts provide protection in a difficult natural setting. The multitude of
spines offer enough shade for the surface of these plants to reduce water losses
through transpiration. Many plants have glossy leaves, which lets them reflect
more light energy. Semiarid desert plants include Creosote bush, bur sage, white
thorn, cat claw, mesquite, brittle bushes, lyciums and jujube.

As for animals, insects and jack rabbits are seen during the day, staying in
shadow as much as possible. Many animals seek protection in burrows
underground, where they are insulated from the hot, dry air. These include
kangaroo rats, rabbits, skunks, some insects, birds and reptiles.

Coastal deserts are found in regions that are generally cool to moderately warm.
Portions of the aforementioned Atacama Desert in Chile represent the coastal
desert biome. Here, cool winters alternate with relatively long and warm
summers. Temperatures are moderate compared to the two desert biomes already
discussed. Average summer temperatures range from 13-24 C (55-75 F); winter
temperatures are 5 C (41 F) or colder. The maximum annual temperature is close
to 35 C (95 F), and the minimum is about -4 C (25 F).

Rainfall, while sparse, exceeds that of hot-and-dry and cold-winter deserts,

averaging around 8 to 13 cm (3 to 5 inches) per year. The soil in these deserts is
high in salt and other nutrients. Some of the plants have extensive root systems,
unlike the flora in the aforementioned desert types. These plants are almost
botanical analogs of camels in that they can store very large amounts of water for
future use when it is available. These plants include the salt bush, buckwheat
bush, black bush, rice grass, little leaf horsebrush, black sage and chrysothamnus.

Coastal-desert animals boast special adaptations for dealing with heat and lack of
water. For example, some toad species seal themselves in burrows with sticky,
gel-like secretions and remain inactive for eight or nine months until a heavy rain
washes them out. Amphibians that include larval stages of development feature
accelerated life cycles, bettering their chances of reaching maturity before the
rainwater evaporates. Some insects lay eggs that are able to stay dormant in
adverse conditions, maturing only when their environment is more suitable for
hatching; fairy shrimps do the same. Coastal-desert mammals include coyotes and
badgers; birds include the famed great horned owl, golden eagle and the bald
eagle. Lizards and snakes are the chief reptilian representatives.

Polar deserts or cold deserts are curiosities, like almost everything about the
Earth's poles. Compared to other desert biomes, they receive a veritable flood of
precipitation, especially in the winter months. Mean annual precipitation is about
15 to 26 cm (6 to 10 in). Winter in the Arctic Polar Desert – which spans 5.4
million square miles in parts of Alaska, Canada, Greenland, Iceland, Norway,
Sweden, Finland and Russia – occurs between mid-December and mid-March,
while that in the 5.5-million-square-mile Antarctic Desert that spans the continent
after which is was named falls between mid-June and mid-September.

Polar desert plants are widely scattered over the vast lands in which they grow.
Plant heights can reach 122 cm (about 4 feet) in some areas. The main plants are
deciduous, meaning that they have leaves they shed seasonally with most of these
having spiny leaves. Fungi and dwarf shrubs are also common.

Deserts are classified by their geographical location and dominant weather

pattern (U.S Geological survey) as trade wind, midlatitude, rain shadow, coastal,
monsoon, or polar deserts. Former desert areas presently in non-arid
environments are paleo-deserts, and extraterrestrial deserts exist on other planets.

Trade wind desert

The trade winds in two belts on the equatorial sides of the Horse Latitudes 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 world's largest desert, the Sahara of North Africa,
which has experienced temperatures as high as 57° C, is a trade wind desert.

Midlatitude deserts

Midlatitude 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. The Sonoran Desert of
southwestern North America is a typical midlatitude desert.

Rain shadow deserts

Rain shadow deserts 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

Coastal deserts

Coastal deserts 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. A coastal desert, the Atacama of South
America, is the Earth's driest desert. In the Atacama, measurable rainfall--1
millimeter or more of rain--may occur as infrequently as once every 5-20 years.

Monsoon desert

"Monsoon," derived from an Arabic word for "season," refers to a wind system
with pronounced seasonal reversal. Monsoons develop in response to temperature
variations between continents and oceans. The southeast 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 areas with annual precipitation less than 250 millimeters and a
mean temperature during the warmest month of less than 10° C. Polar deserts on
the Earth cover nearly 5 million square kilometers and are mostly bedrock or
gravel plains. 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

Palaeo deserts
Data on ancient sand seas (vast regions of sand dunes), changing lake basins,
archaeology, and vegetation analyses indicate that climatic conditions have
changed considerably over vast areas of the Earth in the recent geologic past.
During the last 12,500 years, for example, parts of the deserts were more arid than
they are today. About 10 percent of the land between 30? N. and 30? S. is covered
now by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied
almost 50 percent of this land area. As is the case today, tropical rain forests and
savannahs were between the two belts.

Fossil desert sediments that are as much as 500 million years old have been found
in many parts of the world. Sand dune-like patterns have been recognized in
presently nonarid environments. Many such relict dunes now receive from 80 to
150 millimeters of rain each year. Some ancient dunes are in areas now occupied
by tropical rain forests.

The Nebraska Sand Hills is an inactive 57,000square kilometer dune field in

central Nebraska. The largest sand sea in the Western Hemisphere, it is now
stabilized by vegetation and receives about 500 millimeters of rain each year.
Dunes in the Sand Hills are up to 120 meters high.

Extraterrestrial deserts

Mars is the only other planet on which we have identified wind-shaped (eolian)
features. Although its surface atmospheric pressure is only about one-hundredth
that of Earth, global circulation patterns on Mars have formed a circumpolar sand
sea of more than five million square kilometers, an area greater than the Empty
Quarter of Saudi Arabia, the largest sand sea on our planet. Martian sand seas
consist predominantly of crescent-shaped dunes on plains near the perennial ice
cap of the north polar area. Smaller dune fields occupy the floors of many large
craters in the Polar Regions.

What is a Sand Dune?

Sand dunes are some of nature’s most scintillating creations. These eye-catching
land masses are found around the globe in different climatic conditions. They are
not only limited to deserts, but can form in any landscape on the earth’s surface
provided the conditions are right. Every sand dune is formed as a result of the
interaction between thewind and soil in the form of sand grains. There are many
ways thrill seekers can explore sand dunes, for example, sliding down the dunes,
Bird viewing in wetlands, skiing, sand boarding or just sledging on the sand
slopes. Sky divers or those on airplanes experience the true beauty of sand dunes.

A Sand Dune is a small ridge of hill of sand found in a desert or on top of a

beach. When they form on a beach, they are typically above the normal maximum
reach of the waves.They form from millions of finely divided sand particles that
are blown by the wind and get deposited against some obstacle such as a piece of
drift wood, bush or rock.

How Are Sand Dunes Formed?

For a sand dune to form, the following conditions must be met:

 Deposition of sand must take place quicker than the obstruction material
is eroded to enable build up of sand.

 There must be abundant supply of sand transported to the beach from

longshore drift.

 There must be a huge range between high and low tides so that when the
tide subsides, a large area of land is exposed and can dry out to enable the
wind to pick up the sediments.

 Energy for movement, which is strong winds to transport sand

particles. However, dunes can also form underwater in rivers.

 An obstructer or obstacle, for example, a piece of wood or a rock to allow

accumulation of sand. Areas with sparse vegetation tend to form many
dunes.

 Dry climate: Moisture usually causes grains of sand to bond

together. Large grains present challenges during wind
 transportation. Additionally, moisture helps plant growth. Plant growth
stabilizes loose sediment and stops its movement.

Sand dunes are formed due to the earth’s erosional and depositional forces. The
sand that results in the formation of sand dunes is eroded from rocks by physical
and chemical processes such as wind and water. The erosion leads to accumulation
of sand. Strong winds pick up the sand and transport it (typically slightly above the
ground) to another location.

Types of Sand Dunes

1. Barchan dune

It’s sometimes called barkhan dune or crescentic dune. As the name suggests, it’s
crescent-shaped and produced as a result of the action of wind majorly from one
direction. It’s the most common kind of sand dune that occurs in sandy deserts
across the globe. Barchans, on numerous occasions, occur in groups. However, on
few occasions, they occur separately.

2. Traverse dune

These kinds form long lines of ridged dunes that are at right angles (perpendicular)
to the direction of the wind. They feature steep slip faces at their back. They
commonly occur in areas with bountiful supply of sand and less plant life.

Dunes that form on beach areas are traverse dunes and are as a result of strong
winds emanating from the ocean. If you move farther inland, you tend to stumble
across more plant life.

3. Blowout dunes

Their shapes are exactly opposite of barchans dunes. In blowout dune, the slip face
and crescent-shaped curve faces away from the wind. The obstacle that enables the
formation of blowout dunes is vegetation, but due to sand accumulation over the
years, the vegetation forms a mold leading to a stable dune.

4. Linear dunes

Linear dunes are commonly parallel to the wind direction and result in long,
straight ridges. Linear dunes can grow or accumulate as tall as 100 meters and
extend for many kilometers. Geologists have found out that these kinds of dunes
are triggered by winds that travel from one direction (northwest) in one season of
the year and later shift and travel from a separate direction (southwest) during
another season of the year. Linear sand dunes generally move easterly forming a
long and thin shape.

5. Composite dunes

This is a combination of two or more kind of dunes. Composite dunes culminate to

very hilly, large and tall types of dunes called draas. They are commonly a blend of
linear and traverse dunes that accumulate up to 400 meters high. These gigantic
sand dunes travel a lot less slowly (about z-2 meter each year) than their smaller
counterparts. They culminate into wide dune fields known as ergs. Ergs can spread
up to 500,000 km2.