1. CHAPTER 21: DESERTS AND WIND

2. What is the annual precipitation in your region?
A desert is a region that receives less than 25 cm of precipitation per year.
What is the annual precipitation in your region?

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

4. 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.

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

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

7. 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?

8. Atmospheric moisture circulation determines the location of most deserts
Polar Cells
Ferrel Cells
Hadley Cells

9. 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.

10. Other Factors Determining Desert Formation
Not all deserts lie around 30° latitude
Several other factors contribute to desert locations:
• orographic effect

11. Other Factors Determining Desert Formation
dry, cold air descending over polar regions
the distance atmospheric moisture is transported

12. Other Factors Determining Desert Formation
cold ocean current adjacent to a tropical coast

13. Other Factors Determining Desert Formation
poor management of farmland

14. Other Factors Determining Desert Formation
deforestation

15. Each desert has unique characteristics
Trade Wind Deserts

16. Each desert has unique characteristics
Mid-latitude Deserts

17. Each desert has unique characteristics
Rain Shadow Deserts

18. Coastal deserts

19. Ripple and dune Terminology
Wind is an important geological agent

20. Ripple and dune Terminology
Suspension: Silt carried in suspension produces well-sorted deposits of progressively smaller sediments with distance.

21. Ripple and dune Terminology
Loess: Eolian deposits of silt are left by continental glaciations
Mineralogically how does loess differ from typical wind-blown sand?

22. Ripple and dune Terminology
Desert Pavement (lag deposit of coarse sediment left after fines blow away) Yardang (rock outcrop sculpted by sand abrasion)
Suggested tips to give students: Pebbles, cobbles, and boulders left behind in Desert Pavements are called Ventifacts (can be seen upper left).

23. Sand Dunes reflect sediment availability and dominant wind direction
Abundant loose sediment (usually sand)
Energy to move sediment (usually wind)
An obstacle to trap sand (often a bush)
A dry climate
Suggested tips to give students: In this illustration, parent and daughter isotopes can migrate in and out of a sample causing "uncertainty" in the sample's specific age. The time of metamorphism, however, can be determined.

24. 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

25. 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

26. 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

27. Sand Dunes
What are the differences in shape and wind characteristics of Barchan and Parabolic dunes?

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

30. Evolution of Desert Landscapes
Suggested tips to give students: The Basin and Range region of the southwestern United States (recall Chapters 11 and 15) includes many particularly well-developed examples of desert landscapes and their evolution.

31. What is the evidence that this area was recently wet?
Playa Lakes
Playa Lakes are products of rainfall and evaporation
What is the evidence that this area was recently wet?

32. 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?

33. Environmental refugees

34. Desertification
Desertification threatens all six inhabited continents

35. COPYRIGHT
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