Winds and Deserts

1. Winds and Deserts

2. Eolian • Aeolus: Greek god of winds. • Eolian: Geological processes driven by wind.

3. Wind patterns on Earth • Rules of thumb: • Warm air rises and cool air sinks. • Where winds converge, air rises. • At the Equator: • Sun’s rays strike the Earth’s surface nearly perpendicularly. • Air warms and rises, traveling toward the poles. • By about 30° S and N, air has cooled, so it sinks.

4. Wind patterns on Earth • Coriolis effect: • North-south motion of wind cells is affected by planet’s rotation. • In SOUTHERN Hemisphere, currents are deflected to the RIGHT. • In SOUTHERN Hemisphere, currents are deflected to the LEFT. NORTHERN SOUTHERN

5. Sun Sun Sun Fig. 15.1

6. Fig. 15.15

7. Wind velocity vs particle size Faster wind Larger particles carried

8. Reynolds Number • r = fluid density, U = fluid velocity, H = length, and m = viscosity. • If Re > 500, turbulent. • If Re < 500, laminar.

9. Turbulent or laminar? • Air has extremely low viscosity. • TURBULENT! • Turbulent air flow picks up particles from the surface. • Large particles are entrained for short distances. Cannot be held in air because of air’s low density and viscosity. • Small particles (<0.1 mm diameter, dust) carried to heights of many km and very long distances (across Atlantic).

10. Entrainment Fig. 15.2

11. Fig. 15.3

12. Fig. 15.4

13. Deflation • Surface particles (dust, silt, sand) become dry and loose. • Winds entrain surface particles. • This erosional process is called DEFLATION. • Vegetation inhibits deflation. • What’s left behind after delation is called DESERT PAVEMENT. Coarse, gravelly ground surface.

14. Deflation hollow (Co) Fig. 15.6

15. Alternative theory for pavement formation Fig. 15.7

16. Desert pavement Fig. 15.7

17. Fig. 15.9

18. Fig. 15.10

19. Fig. 15.10

20. Fig. 15.11

21. Barchan Blowout Transverse Linear Fig. 15.12

22. Fig. 15.17

23. Fig. 15.17

24. Playa lake Don’t drink me! Fig. 15.18