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Title: Deserts and Winds

Deserts and Winds
Deserts cover about 30 of the Earths land
surface (42 million square kilometers) No other
land climate type covers such a large area
Semiarid, Arid and Deserts
Semiarid lands, also called steppes, typically
receives 250 to 500 millimeters (10 to 20 inches)
of rain per year Arid lands receive less than
250 millimeters (10 inches) of rain per
year Deserts are formally defined as regions that
rarely receive precipitation, typically less than
100 millimeters (4 inches) per year
True deserts are characterized by having little
or no vegetation
Are All Deserts Sandy?
A common misconception is that deserts are only
composed of mile after mile of drifting sand
dunes For example, only 10 of the Sahara Desert
is covered with sand
The sandiest desert is the Arabian Desert, which
is still only 33 sand
Polar Deserts
Many polar regions are technically deserts
because they receive little precipitation Polar
deserts on Earth cover nearly 5 million square
kilometers and range from bedrock to gravel
plains to ice sheets
The Sahara Desert
The Sahara Desert in North Africa is considered
to be the largest desert on Earth In fact it is
just part of an even greater desert environment,
the low-latitude deserts
The Low-latitude Deserts
The low-latitude deserts are a virtually unbroken
desert environment stretching for more than 9300
kilometers (5800 miles) from the Atlantic coast
of North Africa across the Middle East and into
The Low-latitude Deserts
What created this extensive desert environment?
The Low-latitude Deserts
Most desert regions are created by the pattern of
the Earths air circulation and the topography of
the land
The Low-latitude Deserts
Along the equator, in the region referred to as
the equatorial low, heated air rises 15-20
kilometers up into the atmosphere and then
spreads out to the north and south reaching to
20o and 30o latitude
The Low-latitude Deserts
Air that rises through the atmosphere, expands
and cools, a process that creates clouds and rain
For this reason, the areas under the influenced
of the equatorial low is among the rainiest on
The Low-latitude Deserts
Just the opposite is true at 20o-30o north and
south latitudes, where high pressure
predominates Here in the zone known as the
subtropical high, the dry air from the equatorial
low sinks towards the ground
The Low-latitude Deserts
When air sinks, it is compressed and warmed This
is just the opposite of what is needed to create
clouds and rain So the air is very dry
The Sahara Desert
The Sahara Desert occurs where the very dry air
from the subtropical high sinks downward With no
clouds and no rain, the Sahara is dry
The Sahara Desert
The world record high temperature of 137o F
(57.8o C) was record in El Azizia, Libya, in
North Africas Sahara Desert on September 13, 1922
This was the air temperature 5 feet above the
ground as measured at a French colonial
meteorology station
Australias Desert
In the Southern Hemisphere, dry air sinking down
between 20o-30o south latitudes creates the
desert in central Australia (and in Southwest
Rainshadow Deserts
Most of the interior of the Western United States
is an arid to semiarid environment These dry
lands exist because they are in the interior of a
land mass, far removed from the ocean
Rainshadow Deserts
Many deserts in the middle latitudes, such as in
the Western U.S., are rainshadow deserts Moisture
laden clouds blowing in from the Pacific Ocean
encounter the Coast Range and the Sierra Nevada
Rainshadow Deserts
The mountains force the wind and clouds upward in
elevation into a region of lower atmospheric
pressure and cooler temperatures, where the
clouds cannot continue to contain their
moisture The clouds drop their rain onto the
windward side of the mountains
Rainshadow Deserts
Air descending on the leeward side of the
mountains is much drier The mountains have
effectively cut off the rain from the interior of
the U.S., creating a rainshadow desert region
Death Valley
Death Valley is a deep valley on the leeward side
of the Sierra Nevada Mountains in
California During the late Pleistocene, the
valley was inundated by prehistoric Lake Manly,
but the lake dried up thousands of years ago
Death Valley
Badwater Basin, located within Death Valley, is
the lowest point in North America with a depth of
-282 feet (87 meters) below sea
level Surprisingly, just 76 miles (123 km) west
of Death Valley, Mount Whitney rises to a height
of 14,505 feet (4,421 meters)
Death Valley
The highest temperature ever recorded in the
Western Hemisphere, 57oC (134oF), occurred on
July 10, 1913 in Death Valley
Atacama Desert
The Atacama Desert in South America is in the
rainshadow of the Andes Mountains and is the
driest desert on Earth (some areas may not have
had rain for 400 years)
Atacama Desert
The Atacama Desert is so dry and lifeless, that
it is used by researchers at UTK and Carnegie
Melon to test Mars rovers for NASA
Desert Water
In most dry environments, it does occasionally
rain, such as this thunderstorm over Tucson,
Ephemeral Streams
Most desert streambeds are dry most of the
time Deserts have ephemeral streams, which means
that they only carry water in response to
specific episodes of rainfall
Ephemeral Streams
A typical ephemeral stream might flow only for a
few days or perhaps just a few hours during the
year (or it may stay dry all year)
Ephemeral Streams
A basic characteristic of desert streams is that
they are small and die out before reaching the
sea (which is referred to as interior
drainage) Without a steady supply of water, the
combination of evaporation and infiltration
quickly depletes the stream
Desert Rivers
The few permanent rivers that cross arid regions,
such as the Colorado River in the western U.S.
and the Nile River in Africa, originate outside
the desert, often in well-watered mountains The
Colorado River starts in the Rocky Mountains
Desert Rivers
After the Nile River leaves the its headwaters in
the lakes and mountains of central Africa, it
traverses 3,000 kilometers (2,000 miles) through
the desert without a single tributary
Wind Blown Sediments
Wind, like moving water, is turbulent and able to
pick up sand and dust, and transport it to
another location However, wind is not confined to
a stream channel like water, therefore wind can
spread sediment over a larger area
Wind Blown Dust
Small particles of dust can be swept high into
the atmosphere by the wind and can be kept
suspended in the air for long periods of time
Dust storm North of Stanton, Texas June 13, 2002
Wind Blown Dust
The wind can blow fine dust (silt-size) particles
over great distances, even around the world
This dust storm blew off of the Sahara Desert in
North Africa on February 26, 2000 and reached
South America 7 days later
Dust Bowl
Now the wind grew strong and hard, it worked at
the rain crust in the corn fields. Little by
little the sky was darkened by the mixing dust,
and the wind fell over the earth, loosened the
dust and carried it away. - John
Steinbeck The Grapes of Wrath
Dust Bowl
The Dust Bowl era refers to a series of dust
storms in the central U.S. and Canada from 1931
to 1939, caused by decades of inappropriate
farming techniques
Dust Bowl
The fertile soil of the Great Plains was exposed
through removal of grass during plowing
Dust Bowl
Then a major, prolonged drought occurred The soil
dried out, became dust, and blew away eastwards,
mostly in large black clouds
Dust Bowl
A month of especially vicious dust storms
destroyed 5 million acres of wheat in March of
1935 An economic blow that was felt around the
entire nation
Dust Bowl
The Black Sunday dust storm of April 14, 1935
was so bad that day was turned into pitch black
night and most people thought they would die as
the storm raged on and on
Dust Bowl
During the Dust Bowl, the economy of the Great
Plains was nearly destroyed
Dust Bowl
"And then the dispossessed were drawn west- from
Kansas, Oklahoma, Texas, New Mexico from Nevada
and Arkansas, families, tribes, dusted out,
tractored out. Car-loads, caravans, homeless and
hungry twenty thousand and fifty thousand and a
hundred thousand and two hundred thousand. -
John Steinbeck The Grapes of Wrath
Dust Bowl
Economically ruined, perhaps as many as two
million people fled from the Great Plains
Desert Pavement
In portions of many deserts, the surface is a
closely packed layer of coarse pebbles and
cobbles too large to be moved by wind This stony
veneer is called desert pavement
Desert Pavement
Desert pavement is formed when deflation lowers
the surface by removing the silt and sand, until
only a continuous cover of coarse particles
Desert Pavement
Once desert pavement is establish, a process that
can take hundreds of years, the pavement prevents
any further wind deflation However, because the
pavement is only a couple of stones thick,
disruption by animals, people or vehicles and
dislodge the stones and expose fine-grained
material below to more deflation
Wind Deposits
As with the case for running water, wind drops
its load of sediment when its velocity falls and
the energy available for transport diminishes So
windblown silt and sand can accumulate, forming
distinctive geologic features
A deposit of windblown dust is called loess The
fine dust in loess is typically only 0.01 to 0.06
millimeter (0.0004 to 0.0024 inch) in
diameter There are two primary sources for loess
deposits Deserts Glacial outwash deposits
When breached by road cuts or streams, loess
deposits tend to maintain vertical cliffs and
lacks any visible layering
Desert Loess in China
The Loess Plateau (now called the Huangtu
Plateau) is an area of some 640,000 square
kilometers in China that contains the thickest
and most extensive deposits of loess known on
The source of this loess is dust blown in from
the extensive deserts of central Asia
Desert Loess in China
The Yellow River (Huang Ho) cuts through the
Loess Plateau and get its name from the yellow
brown sediment of the eroded loess
Desert Loess in China
Homes, called yaodongs, are created by digging
into the loess In the dry desert, these
structures can last for many centuries
Glacial Loess in the U.S.
The principle loess deposits in the U.S. are in
the mid-west, concentrated around the Mississippi
River drainage, in the Dust Bowl
Glacial Loess in the U.S.
The source of this loess is glacial rock flour
carried by the Mississippi River in glacial
meltwater from retreating ice caps
Windblown Sand
Where sand is transported and later deposited by
wind, the principle depositional feature is
called a dune (or sand dune)
Many dunes have an asymmetrical profile, with the
windward slide having a gentle slope and the
leeward side being steep
wind direction
Sand sliding down the steep slipface of a dune in
White Sand National Monument, New Mexico
Dune Migration
Continued sand accumulation, combined with
periodic slides down the slipface, result in the
slow migration of the dune in the direction of
the wind
Dune Migration
To keep Interstate 95 open neat Winnemucca,
Nevada, sand must be taken away about three times
a year Each time, between 1500 and 4000 cubic
meters of sand is removed
The Highest Dunes
The highest dunes in the world are located along
the southwest coast of Africa in the Namib Desert
in the country of Namibia For example, Dune 7
is 383 meters (1245 feet) high
Sand Sea
In Namibia, along the south Atlantic coastline,
great sand dunes offer a spectacular display
The Sand Sea
The sand is so extensive, it is called a Sand
The Sand Sea
At first glance the desert seems lifeless, but...
Sand Sea
The Atlantic Ocean and the occasional oasis
support a variety of life, including elephants
Sand Sea
The coastline is also called the Skelton Coast
because off shore sand bars have sunk many ships,
which are later claimed by the advancing dunes
Sand Sea
During diamond mining operations the wreak of a
500-year old Portuguese treasure gallon was
discovered in 2006
Sand Sea
Fortunately, most of this desert region is
protected in national parks
Glaciers, Ice Ages Oceans