Download Deserts Conditions Leading to Deserts Wind

Geol 104: Deserts
Deserts
• Any region that receives <25 cm (10 in) of
rain annually and can support little or no
vegetation (due to low soil moisture).
• Cover ~25% of Earth’s land surface outside
of polar regions.
• Deserification is increasing amount of
desert land annually.
Conditions Leading to Deserts
• Latitude and prevailing wind direction.
• Topography and prevailing wind direction.
• Orientation of coast and prevailing wind
direction.
• Distance from coast and prevailing wind
direction.
• Cold ocean currents and prevailing wind
direction.
Wind
• Wind is the flow of air from areas of higher
pressure to lower pressure.
• The ultimate driving force for wind is solar
energy.
• Wind is controlled by:
– Pressure gradient
– Coriolis effect
– Friction
1
Geol 104: Deserts
Pressure Gradient Effect
• The greater the difference in air pressures
the greater the wind speed.
• Isobars - lines on weather map connecting
areas of equal air pressure.
• Pressure gradient - amount of pressure
change over a given distance. The closer the
isobars the greater the pressure gradient and
the stronger resulting winds.
Coriolis Effect
• All free-moving fluids are deflected on the
globe due to the rotation of the Earth.
• Wind deflection due to the Coriolis effect:
– is always directed at right angles to the
direction of air flow,
– affects wind direction but not wind speed,
– is affected by the wind speed (ie, stronger the
wind, greater the deflection), and
– is strongest at the poles, weakening to zero at
the equator.
Friction Effect
• Friction slows surface winds which
weakens the Coriolis effect causing the
winds to cross isobars.
• Roughness of terrain dictates angle of air
flow across isobars (little effect over water
but large over rough terrain).
2
Geol 104: Deserts
Global Air Circulation
• Combination of Coriolis effect and the
differential heating of equator vs poles and
land vs water.
• General pattern consists of (fig. 13.3):
A. Equatorial Lows
• Near the equator, hot, moist air rises resulting in
abundant precipitation and tropical rain forests.
B. Subtropical Highs
• As upper-level flow from equatorial lows sinks back
down the associated heating produces hot arid
regions (subtropical deserts) centered at about 30°N
and S latitudes.
Global Air Circulation (cont)
C. Subpolar Lows
• Form at convergence of Subtropical High and Polar
High. Warm, moist mid-latitude air rises and meets
with cold, dry polar air in zone of atmospheric
instability at about 60°N and S latitudes.
D. Polar Highs
• Extremely cold, dry polar air subsides spreading
toward the equator due to greater density of cold air.
Prevailing Winds
A. Easterly Trade Winds
– Easterly flowing winds between the Subtropical
highs and Equatorial lows.
B. Prevailing Westerlies
– Westerly flowing winds between the Subpolar
lows and Subtropical highs.
C. Polar Easterlies
– Easterly flowing winds between the polar highs
and Subpolar lows.
3
Geol 104: Deserts
Global Precipitation Patterns
• Major precipitation patterns are influenced by:
1. Global Circulation Patterns (latitude)
• High precipitation at zones of convergence, low at zones
of divergence.
2. Coastline vs Prevailing Wind Direction
• Wind moving off water yields high precipitation.
3. Topography and Extent of Landmass
• Rainshadow effect - low precipitation on leeward side
of mountains (fig. 13.4).
• Precipitation greatly decreases with distance from shore
(source of moisture).
Global Distribution of Deserts
• Regions of low precipitation or high
evaporation.
• See fig. 13.3
• Note relationship between location of
deserts and global air circulation and
precipitation patterns.
Wind Action in Deserts
• Winds are very strong in desert areas
because:
1. Low humidity
2. Great temperature range
3. Little or no vegetation
4
Geol 104: Deserts
Wind Erosion
• Deflation - removal of loose materials by
wind; often results in lowering of land
surface.
• Desert Pavement - pebbles and cobbles left
behind as wind erodes smaller particles.
Wind Transportation
• Dust Storms
• Most sand transport is near the surface by
saltation.
• Ventifact - cobble of boulder with flattened.
Polished faces due to wind-blown sand.
Wind Deposition
• Dunes
– Mound or ridge of well sorted, rounded windblown sand.
– Types of Dunes
•
•
•
•
Barchan
Transverse
Parabolic
Longitudinal
• Loess
– Homogeneous, unlayered deposit of windblown silt and clay, usually of glacial origin.
5
Geol 104: Deserts
Desert Landforms
• Desert Streams - seasonal flow.
– Wash - stream bed that is dry most of the time.
• Desert Lakes - seasonal due to seasonal
stream flow.
– Playa - lake bed that is dry most of the year.
• Debris flows - desert soils can become
saturated and flow.
– Flash floods - slow infiltration of rain leads to
rapid increase in discharge and short lag time.
Desert Landscapes
• Plateau
– Large elevated area of fairly flat land, capped
by resistant rock.
• Mesa
– Broad flat-topped hill with steep cliff faces
capped by resistant rock, smaller than plateau.
• Butte
– Narrow flat-topped hill with very steep cliffs
capped by resistant rock, smaller than mesa.
• See fig. 13.7
Desertification
• Process whereby productive potential of
arid or semiarid land falls by 10% or more
(10-25% - moderate, 25-50% - severe, and
>50% - very severe).
• Estimates are:
– 3.1 million mi2 (area 12 times size of Texas)
have become desertified in past 50 years.
– Current trend could threaten livelihood of 1.2
billion people.Each year, 23,000 mi2 undergo
low to moderate, and 81,000 mi2 (area the size
of Kansas) undergo severe desertification.
6
Geol 104: Deserts
Human factors that leave topsoil
vulnerable to desertification:
• Overgrazing on fragile arid or semi-arid
range land.
• Deforestation without reforestation.
• Surface mining without reclamation.
• Irrigation in areas of high evaporation
leading to salt buildup.
• Soil compaction by machinery, livestock,
etc.
Global implications
• What are the global implications of
desertification?
• How is desertification (and remediation of
desertification) linked to social and
biophysical processes?
• What are the local implications of
desertification (how might it affect you)?
7