Deserts and Winds

DesertsDeserts 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 DesertsSemiarid 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 DesertsMany 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 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 Sahara Desert

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

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 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 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 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

The Sahara Desert

This was the air temperature 5 feet above the ground as measured at a French colonial meteorology station

Australia’s DesertIn the Southern Hemisphere, dry air sinking down between 20o-30o south latitudes creates the desert in central Australia (and in Southwest Africa)

20o

30o

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

Rainshadow DesertsMany 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 DesertsThe 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 DesertsAir 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 ValleyDeath 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 ValleyThe highest temperature ever recorded in the Western Hemisphere, 57oC (134oF), occurred on July 10, 1913 in Death Valley

Atacama DesertThe 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 DesertThe 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 WaterIn most dry environments, it does occasionally rain, such as this thunderstorm over Tucson, Arizona

Ephemeral StreamsMost 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 StreamsA 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)

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

Ephemeral Streams

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, 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 Sediments

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

Wind Blown Dust

Dust storm North of Stanton, Texas June 13, 2002

The wind can blow fine dust (silt-size) particles over great distances, even around the world

Wind Blown Dust

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 BowlThe fertile soil of the Great Plains was exposed through removal of grass during plowing

Dust BowlThen a major, prolonged drought occurred

The soil dried out, became dust, and blew away eastwards, mostly in large black clouds

Dust BowlA 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 BowlThe “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 BowlDuring 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 PavementIn 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 PavementDesert pavement is formed when deflation lowers the surface by removing the silt and sand, until only a continuous cover of coarse particles remains

Desert PavementOnce 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 DepositsAs 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

LoessWhen breached by road cuts or streams, loess deposits tend to maintain vertical cliffs and lacks any visible layering

Desert Loess in ChinaThe 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 ChinaThe 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 ChinaHomes, 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 SandWhere sand is transported and later deposited by wind, the principle depositional feature is called a dune (or sand dune)

DunesMany dunes have an asymmetrical profile, with the windward slide having a gentle slope and the leeward side being steep

wind direction

DunesSand sliding down the steep slipface of a dune in White Sand National Monument, New Mexico

Dune MigrationContinued sand accumulation, combined with periodic slides down the slipface, result in the slow migration of the dune in the direction of the wind

Dune MigrationTo 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 DunesThe 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 SeaIn Namibia, along the south Atlantic coastline, great sand dunes offer a spectacular display

The Sand SeaThe sand is so extensive, it is called a “Sand Sea”

The Sand SeaAt first glance the desert seems lifeless, but...

Sand SeaThe Atlantic Ocean and the occasional oasis support a variety of life, including elephants

Sand SeaThe 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 SeaDuring 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