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.

Wind and

Deserts

Dust blowing over the Atlanticocean, originating in the Saharadesert, northern Africa.

Cape Verde Islands are ~600 km(~370 miles) away from the coastof Africa.

Curious layers of red dust werefound in the Alps in Europe over20 years ago. We now know theseoriginated in deserts of China.

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?

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.

Atmospheric convection showing the basic variables and components of motion.

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 10–20 km/hr ( can be >200 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.


If temperature were the only factor, then air motion wouldbe 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.


When moist air rises, it expands and cools. This causescondensation 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.


The Earth is rotating - the Coriolis effect results from this.

Direction of Earth’s rotation


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.

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.


• 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.


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.

Fig 12.9

Distribution of dry climate areas on Earth

Note that most are at ~30o N and S or arelocated downwind of major mountains.

Fig 12.9

Distribution of dry climate areas on Earth

Three examples of major types of deserts.

Subtropical deserts like the Sahara occur in the high-pressure zones at ~30o latitude where cool, dry air descends constantly.


Earth’s Largest Deserts Develop at ~30o N and S.


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.


Formation of the rain-shadow deserts in the western U.S.


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.


• 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.

How does wind pick up and transport sediment?

• Getting particles moving:• Fast winds needed to move

any particle, large or small.• Commonly only <0.5 mm can

be moved, sometimes larger.• ~0.1 mm is the size that

requires only about 5 km/hr wind velocity to move. Whenthese bounce, they dislodge 10 other particles on average. Smaller ones may remain suspended in the air. Larger ones fall back and dislodge more particles.


Once they are moving, particles travel in a similar manner to those in waterborne sediments. Some roll, some bounce, and some remainsuspended in the air.


Volcanic eruptions eject very small particles (ash) high into the atmosphere.

Eruption columns may be 10’s 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.


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.


Effects of wind-born dust can be seen across oceans.

20.4 How does wind pick up and transport 20.4 How does wind pick up and transport sediment?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.

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.


• 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.

Examples of pans, yardangs and ventifacts.


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

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.

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dense air

air motion

direction wind

warm air

wind directions

volume of air

wind motion convection

cool dry air