Chapter 19 Deserts and winds

1. Chapter 19 Deserts and winds

2. What is a desert? • Any region with an arid climate and a rainfall less than 25 cm/year.

4. How deserts form? • Descending warm and moist air. • Rain shadow • Distance from ocean • Coasts with cold ocean currents • Polar deserts

5. Distribution of deserts • Most of the deserts lie along a narrow belt 30° N and S of the Equator. • These two belts have warmer air and can hold a lot of moisture, leading to very low rainfalls.

6. Distribution of deserts • The two belts are characterized by clear skies, lots of sunshine, little rain and high evaporation.

9. Deserts due to Rain Shadow • Rain shadow is a dry region downwind from a mountain range. • Example: Sierra Nevada range in California causing desert formation in Nevada and N. Arizona.

10. Formation of Rain Shadow Deserts Moisture-depleted airmass sink and warm, reducing relative humidity… Moist airmass rise and cool, causing precipitation… …resulting in a rainy windward slope Moisture laden winds come onshore …forming a rain shadow on the leeward side of the mountains.

12. Distance from ocean: oceans are a great source of evaporated water. At greater distance from the oceans, deserts occur in the continental interior. • Example: Gobi desert in China.

13. Coasts with cold ocean currents: warm air which can hold more moisture and no rains on the coasts. • Example: Pacific coast of S. America.

14. Polar deserts: cold air with little moisture. No rains. • Examples: Antarctica, Greenland, Alaska.

15. Wind erosion and transportation • Wind can erode only finer particles such as clay silt and sand. • Deserts typically have strong winds which can erode loose dry sediments causing sand storms/dust storms.

17. Effects of wind action are strong only close to the ground (upto one meter from the ground surface).

18. Death valley, Ca

19. Death valley, Ca

20. Erosional features • Desert pavement: thin surface layer of closely-packed pebbles.

22. Desert Pavement in the Sonoran Desert, Arizona

23. Ventifacts: rock fragments with flat, wind-abraded surfaces.

26. Blowout: depression on the land surface caused by wind erosion.

29. Wind deposition • Loess • Sand dunes

30. Loess: deposit of wind-blown silt and clay consisting of quartz, feldspar and clay minerals. • Weakly cemented by calcite. • A desert or glacial outwash is needed as source material.

32. Sand dunes • Mounds of loose sand grains heaped up by the wind. • Composition of sand depends on; • Sand source • Chemical weathering • Quartz, feldspar and calcite are generally more abundant. • Well sorted and rounded.

34. How a dune forms? • A sand dune forms with a gentle upwind slope and a steeper slip face on the downwind side. • Sand eroded from the upwind side is deposited on the slip face, forming cross-beds.

39. Embryonic dunes forming on the lee-side of vegetation

40. Types of dunes • Factors controlling dune type: • Wind velocity and direction • Sand supply • Vegetation cover

41. 4 types of dunes: • Barchan • Transverse • Parabolic • Longitudinal

42. Barchan: crescent shaped dune convex in the upwind direction.

43. Transverse: relatively straight, elongate dune oriented perpendicular to the wind direction.

45. Parabolic: similar to a barchan dune except that it is convex in the downwind direction.

46. longitudinal: symmetrical ridge that forms parallel to the wind direction.

48. Deserts in US • Deserts in SW US are caused by; • Proximity to the 30° N latitude. • Rain shadow by Sierra Nevada. • Colorado plateau • Basin and Range Province, Nevada.