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Deserts and Winds
Deserts
Deserts cover about 30% of the Earth’s 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
Deserts
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 India
The Low-latitude Deserts
What created this extensive desert environment?
The Low-latitude Deserts
Most desert regions
are created by the
pattern of the
Earth’s 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 1520 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
Earth
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
Africa’s Sahara Desert on September 13, 1922
This was the air
temperature 5 feet
above the ground
as measured at a
French colonial
meteorology
station
Australia’s Desert
In the Southern Hemisphere, dry air sinking down
between 20o-30o south latitudes creates the desert
in central Australia (and in Southwest Africa)
20o
30o
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,
Arizona
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 wellwatered 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 westfrom 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 remains
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
Loess
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
Loess
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 Earth
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)
Dunes
Many dunes have an asymmetrical profile, with
the windward slide having a gentle slope and
the leeward side being steep
wind
direction
Dunes
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 Sea”
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