Transcript How Does Earth Work?

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.
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.
What causes winds to blow?
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.
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.
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.
What causes winds to blow?
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.
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.
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.
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.
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.
Distribution of dry climate areas on Earth
Fig 12.9
Three examples of major types of deserts.
What controls the locations of deserts?
Subtropical deserts like the
Sahara occur in the highpressure zones at ~30o latitude
where cool, dry air descends
constantly.
What controls the locations of deserts?
Earth’s Largest Deserts Develop at ~30o N and S.
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.
What controls the locations of deserts?
Formation of the rain-shadow deserts in the western U.S.
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.
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.
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. 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.
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.
How does wind pick up and transport sediment?
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.
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.
Effects of wind-born dust can be seen across oceans.
Dunes
in adoes
mannerwind
very pick up
20.4move
How
similar to cross-beds in a stream.
sediment?
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.
and transport
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.
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.
How does wind shape the landscape?
Examples of pans, yardangs and ventifacts.
How does wind shape the landscape?
There are many types of sand dunes: