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
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. Mounds of loose sand grains heaped up by the wind.
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. Factors controlling dune type:
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; Colorado plateau
Proximity to the 30° N latitude.
Rain shadow by Sierra Nevada.
Colorado plateau
Basin and Range Province, Nevada.

51. Associated features
Plateaus: broad, flat-topped areas elevated above the surrounding land and partly bounded by cliffs.
Mesa: broad flat-topped hill bounded by cliffs from all sides.
Butte: narrow hill of resistant rock with a flat top and very steep sides.

54. The Basin and Range province is characterized by rugged mountain ranges separated by flat valley floors.

58. Runoff water in the valley may collect at the floor forming a Playa lake.
Mud cracks develop when a playa lake dries.

60. When individual alluvial fans join with on another, they form a Bajada.

62. Desert Weathering is Characterized By:
a dominance of physical weathering
minimal chemical weathering, consisting mainly of oxidation of mafic minerals

63. Desert Weathering is Characterized By:
formation of desert varnish: a weathering veneer formed of a mixture of clays and smaller amounts of iron and manganese oxides.

64. Oxidation of mafic
minerals results in
the formation of iron
oxides and the typical
reds and browns of
a desert landscape

66. Petroglyphs scratched in desert varnish