Wind%20and%20Deserts

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Wind and Deserts
Dust blowing over the Atlantic ocean, originating
in the Sahara desert, northern Africa. Cape
Verde Islands are 600 km (370 miles) away from
the coast of Africa. Curious layers of red dust
were found in the Alps in Europe over 20 years
ago. We now know these originated in deserts of
China.
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Wind and Deserts

• What causes winds to blow?
• How is wind an influential process on the
  landscape? Why in some places, but not in
  others?
• What controls the locations of deserts?
• How does wind pick up and transport sediment?
• How does wind shape the landscape?

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What causes winds to blow?

• What is wind?
• Moving air - motion in the atmosphere.
• It is driven primarily by convection, which
  causes areas of high and low pressure.
• Denser air sinks and displaces less dense air,
  which moves upwards if it warms and picks up
  moisture.
• What controls air density?
• Temperature - heating causes air to expand (thus
  becoming less dense) and rise.
• Humidity - moist air is less dense than dry air
  at any given temperature - water molecules have
  lower mass than nitrogen and oxygen molecules of
  the same volume of air.
• So, warm moist air is bouyant and rises, cool dry
  air is dense and sinks.

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What causes winds to blow?
Atmospheric convection showing the basic
variables and components of motion.
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What causes winds to blow?

• Wind motion (convection) in the atmosphere.
• Convection is driven by density changes.
• Wind speeds are driven by pressure differences.
• The greater the difference between a
  high-pressure zone and a low-pressure one, the
  greater the wind speed.
• Globally, wind speeds average 1020 km/hr ( can
  be gt200 km/hr in hurricanes).
• Wind directions can vary because of the movement
  of high- and low-pressure zones around the Earth.
• Wind direction refers to the direction wind is
  coming from. For example, here it is usually
  westerly.

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What causes winds to blow?
If temperature were the only factor, then air
motion would
be simple convection between the warmer
equatorial zone to the cooler poles. Equatorial
regions receive more direct sunlight, and are
thus warmer. Does not match air circulation
patterns that we observe.
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What causes winds to blow?
When moist air rises, it expands and cools. This
causes
condensation of moisture (rain) and the cooled
and dryer air sinks. Warm air at equators
promotes evaporation. By 300 N or S air is cool,
dry and thus dense and sinks.
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What causes winds to blow?
The Earth is rotating - the Coriolis effect
results from this.
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How is wind an influential process on the
landscape?

• Winds are present everywhere on Earth, but the
  impact on landscapes is more apparent where
• Strong winds are common.
• There is little vegetation.
• Roots of plants act to hold soils in place.
• Fine grained soils with loose, dry particles are
  found.
• Small particles low mass, easier to move.
• Loose little compaction, particles are free to
  move.
• Dry presence of moisture increases cohesion.

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How is wind an influential process on the
landscape?
Air is 800x less dense than water, so has lower
capacity to move material at a given velocity.
Small particles (less than 0.5 mm) are generally
all that wind can move. So, for typical wind
velocities only sand and finer particles are
transported.
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How is wind an influential process on the
landscape?

• Where do we find sediment that wind can move?
• Fine-grained soils that are not covered by
  vegetation might be found at
• Stream floodplains and sand bars
• Beaches
• Deserts
• Strong winds, sparse or absent vegetation, and
  abundance of fine-grained, loose sediment that
  allows wind to be an effective transporting agent
  is most common in deserts. This is where we see
  the effects of wind erosion most clearly.

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What controls the locations of deserts?

• Deserts are the most common place where the three
  requirements for wind erosion are met. So, why
  are deserts where they are and what is a desert?
• Areas that receive less than 25 cm
  precipitation/yr (10 inches/yr) because of
• Dry air descending to the surface.
• The descending air has previously lost its
  moisture when it ascended and cooled.
• Mountains block moist air from oceans.
• Or, moist ocean air is blown away from land.

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Distribution of dry climate areas on Earth
Fig 12.9
Note that most are at 30o N and S or are located
downwind of major mountains.
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Distribution of dry climate areas on Earth
Fig 12.9
Three examples of major types of deserts.
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What controls the locations of deserts?
Subtropical deserts like the Sahara occur in the
high-pressure zones at 30o latitude where cool,
dry air descends constantly.
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What controls the locations of deserts?
Earths Largest Deserts Develop at 30o N and S.
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What controls the locations of deserts?
The Great Basin and Mojave deserts are
rain-shadow deserts. Moist air rides up along
mountains, rain forms due to expansion and
cooling, and descends as dry air down the other
side of the mountain.
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What controls the locations of deserts?
Formation of the rain-shadow deserts in the
western U.S.
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What controls the locations of deserts?
The Namib Desert is a coastal desert, formed
where prevailing winds come from the east and
blow toward the ocean. Thus, very little ocean
moisture is carried onto land.
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How does wind pick up and transport sediment?

• As in streams and glaciers, the force that wind
  exerts on particles causes them move.
• Recall that air has 800 times lower density than
  water or ice, so the amount of force is
  relatively low.
• Unlike water/ice which always flows downhill,
  winds move in different directions depending on
  location on Earth.

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How does wind pick up and transport sediment?

• Getting particles moving
• Fast winds needed to move any particle, large or
  small.
• Commonly only lt0.5 mm can be moved, sometimes
  larger.
• 0.1 mm is the size that requires only about 5
  km/hr wind velocity to move. When

these bounce, they dislodge 10 other particles on
average. Smaller ones may remain suspended in the
air. Larger ones fall back and dislodge more
particles.
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How does wind pick up and transport sediment?
Once they are moving, particles travel in a
similar manner to those in waterborne sediments.
Some roll, some bounce, and some remain
suspended in the air.
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How does wind pick up and transport sediment?
Volcanic eruptions eject very small particles
(ash) high into the atmosphere. Eruption columns
may be 10s of km high, e.g., Mt. St. Helens was
24 km, or about 80,000 ft. These fine particles
stay airborne for extended periods and travel
many thousands of kilometers, in fact they may
circulate around the Earth.
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How does wind pick up and transport sediment?
1991 Mt. Pinatubo eruption. Volcanic gas and dust
circulated around the Earth. Caused an 2 oC
drop in global temperature. Fine dust from
deserts can also be carried around the globe.
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Effects of wind-born dust can be seen across
oceans.
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20.4 How does wind pick up and transport sediment?
Dunes move in a manner very similar to cross-beds
in a stream. particles roll up the windward slope
and slide down the leeward face.
Dunes migrate in the direction of the prevailing
winds over time. Thus, cross-bedded sandstones
can yield ancient wind patterns.
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How does wind shape the landscape?
Wind shapes the landscape by both erosional and
depositional processes. Sand dunes are a well
known depositional process. Other processes are
primarily erosional and produce features peculiar
to deserts.
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How does wind shape the landscape?

• Deflation is the lowering of elevation in an area
  due to removal of surface material by winds.
• Pan - a small basin-shaped area caused by
  deflation.
• Abrasion is an erosional process driven by
  wind-blown particles such as sand.
• Ventifacts - rocks faceted by abrasion.
• Yardangs - an abraded rock remnant, larger on
  top, caused by wind-driven sand, which usually is
  near the surface as bouncing, rolling and sliding
  particles. Deflation may also contribute.

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How does wind shape the landscape?

• Examples of pans, yardangs and ventifacts.

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How does wind shape the landscape?
There are many types of sand dunes
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