CHAPTER 21:DESERTS AND WIND

A desert is a region that receives less than 25 cm of precipitation per year.

What is the annual

precipitation in your region?

Life Support

• Vegetation is so sparse, and water so rare, that a desert is unable to support significant populations of animals.

Temperature

• Desert regions can have extreme temperatures ranges because the atmosphere contains little humidity to block the Sun's rays or retain heat at night.

Natural Climate Change

• Climates can change naturally. The Sahara region was humid 5,000 to 10,000 years ago

Deserts may be hot or cold, but low precipitation is a common trait

Arctic circle

Some would argue that the “Arctic Desert” isn’t really a desert. Why do most cool dry areas have more vegetation than hot dry areas?

Atmospheric moisture circulation determinesthe location of most deserts

Polar Cells

Ferrel Cells

Hadley Cells

Formation

• Air warms and contracts as it sinks closer to Earth’s surface.

• Evaporation exceeds condensation.

• Deserts form, clustered around 30°N and 30°S latitudes.

Other Factors Determining Desert Formation

• Not all deserts lie around 30° latitude• Several other factors contribute to desert locations:

– • orographic effect

Other Factors Determining Desert Formation

– dry, cold air descending over polar regions

– the distance atmospheric moisture is transported

Other Factors Determining Desert Formation

– cold ocean current adjacent to a tropical coast

Other Factors Determining Desert Formation

– poor management of farmland

Other Factors Determining Desert Formation

– deforestation

Each desert has unique characteristics

• Trade Wind Deserts

Each desert has unique characteristics

• Mid-latitude Deserts

Each desert has unique characteristics

• Rain Shadow Deserts

Coastal deserts

Ripple and dune Terminology

• Wind is an important geological agent

Ripple and dune Terminology

• Suspension: Silt carried in suspension produces well-sorted deposits of progressively smaller sediments with distance.

Ripple and dune Terminology

• Loess: Eolian deposits of silt are left by continental glaciations

Mineralogically how does loess differ from typical wind-blown sand?

Ripple and dune Terminology

Desert Pavement (lag deposit of coarse sediment left after fines blow away)

Yardang (rock outcrop sculpted by sand abrasion)

Sand Dunes reflect sediment availability anddominant wind direction

• Abundant loose sediment (usually sand) • Energy to move sediment (usually wind)• An obstacle to trap sand (often a bush)• A dry climate

Ripple and dune Terminology

• Barchan (Crescentic Dune)• Strong wind in one direction, small amounts of sand, limited

vegetation

• Transverse (Crescentic Dune)• Weak wind in one direction, large amounts of sand deposited

perpendicular to wind, limited vegetation

Ripple and dune Terminology

• Parabolic Dune• Arms stabilized upwind by vegetation, often start as semi-circular blowouts

that elongate

• Longitudinal Dune• Winds flowing in opposing directions, two slip faces, aligned with wind

Ripple and dune Terminology

• Star dune• Multidirectional winds, largest

dunes, grow tall instead of moving

• Sand seas • Large regions (>125 km2) of

windblown sand numerous, very large dunes, sand covers >20% of the ground surface

Sand Dunes

What are the differences in shape and wind characteristics of Barchan and Parabolic dunes?

Arid Landforms

• Arid landforms can be shaped by water• Flash floods are common• Streams tend to be ephemeral, flowing only after heavy rain.

Evolution of Desert Landscapes

Playa Lakes

• Playa Lakes are products of rainfall and evaporation

What is the evidence that this area was recently

wet?

Desertification

• Desertification is the process by which land loses its vegetation and turns into a desert

Both natural and anthropogenic factors lead to desertification. What are some of the factors?

Environmental refugees

Desertification

• Desertification threatens all six inhabited continents

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