Atmospheric Forces and Winds

1. Atmospheric Forces and Winds • Atmospheric pressure • Measuring air pressure • Surface and upper-air charts • Why the wind blows • Surface winds

2. Atmospheric Pressure

3. Atmospheric Pressure • air pressure - definition • air pressure and temperature • pressure gradient force • Air pressure is, quite literally, the weight of the atmosphereabove us.

4. Stepped Art Fig. 6-2, p. 143

5. Measuring air Pressure

6. Barometers • mercury barometer • aneroid barometer • altimeter • barograph

7. Pressure Readings • station pressure and sea-level pressure • isobars

8. Surface and Upper Air Charts

9. Surface and Upper Air Charts • isobaric maps • contour lines • ridges • troughs • Color-filled contour maps are the same as ordinarycontour maps, except that the area between adjacentlines is filled in with color.

10. Filled Contour Maps

11. Why the Wind Blows

12. Newton’s Laws of Motion • Newton’s first law: “An object at rest will remain at rest and an obect in motion will move in a straight line at constant speed unless acted on by an unbalanced force.” • Newton’s second law: • Newton’s third law: “Every action has an equal and opposite reaction.”

13. Forces that Influence the Wind • net force and fluid movement • Wind is the result of a balance of several forces.

14. Pressure Gradient Force • pressure gradient • pressure gradient force • strength and direction of the pressure gradient force • The horizontal (rather than the vertical) pressure gradient force is responsible for causing air to move horizontally.

15. Fig. 6-11, p. 151

16. Coriolis Force • real and apparent forces • Coriolis force • strength and direction of the Coriolis force • factors that affect the Coriolis force • It is sometimes claimed that “water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres”. This is not true.

17. Fig. 6-14, p. 153

18. Straight-line Flow Aloft • combination of the pressure gradient and Coriolis forces • geostrophic wind • Geostrophic winds can be observed by watching the movement of clouds.

19. Curved Winds Around Lows and Highs Aloft • cyclonic and anticyclonic flow • centripetal force • gradient wind

20. Winds on Upper-level Charts • gradients in contour lines • meridional and zonal winds • Height contours on upper-level charts are interpreted in the same way as isobars on surface charts.

21. Stepped Art Fig. 6-19, p. 158

22. Surface Winds

23. Surface Winds • planetary boundary layer • friction • frictional effects on the wind • Most people rarely venture out of the planetary boundary layer.

24. Winds and Vertical Motions

25. Winds and Vertical Motions • divergence and convergence • hydrostatic equilibrium

26. Summary of Atmospheric Forces • “True” forces: • Gravity • Pressure Gradient • Friction • “Ficticious” forces: • Coriolis force • Centrifugal force

27. Summary of Atmospheric Force Balances • Vertical: • Hydrostatic Balance • Horizontal: • Geostrophic Balanice • Gradient Balance • Ekman Balance • (see Table 6-1 in Ackerman and Knox)

28. Atmospheric Circulations • Scales of atmospheric motions • Eddies - big and small • Local wind systems • Global winds • Global wind patterns and the oceans

29. Scales of Atmospheric Motions

30. Scales of Atmospheric Motions • scales of motion • microscale • synoptic scale • planetary scale • Lots of important weather events occur on microscales, like evaporation of liquid water molecules from the earth’s surface.

31. Eddies - Big and Small

32. Eddies - Big and Small • eddy • rotor • wind shear • turbulence • Wind shear can sometimes be observed by watching themovement of clouds at different altitudes.

33. Local Wind Systems

34. Thermal Circulations • isobars and density differences • thermal circulations

35. Stepped Art Fig. 7-4, p. 172

36. Sea and Land Breezes • sea breeze • land breeze • Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes.

37. Stepped Art Fig. 7-5, p. 174

38. Seasonally Changing Winds - the Monsoon • Monsoon wind system • Asian monsoon • other monsoons

39. Mountain and Valley Breezes • valley breeze • mountain breeze • The nighttime mountain breeze is sometimes calledgravity winds or drainage winds, because gravitycauses the cold air to ‘drain’ downhill.

40. Katabatic Winds • drainage winds • bora • Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds.

41. Chinook Winds • Chinook winds • compressional heating • chinook wall cloud • In Boulder, Colorado, along the eastern flank of the Rocky Mountains, chinook winds are so common thatmany houses have sliding wooden shutters to protecttheir windows from windblown debris.

42. Fig. 7-14, p. 180

43. Santa Ana Winds • Santa Ana wind • compressional heating • wildfires • Many Southern California residents regularly hose downtheir roofs to prevent fires during Santa Ana wind season.

44. Desert Winds • dust storms • dust devils

45. Global Winds

46. General Circulation of the Atmosphere • cause: unequal heating of the earth’s surface • effect: atmospheric heat transport • Ocean currents also transport heat from the equator tothe poles and back.

47. Single-cell Model • basic assumptions • Hadley cell • why the single-cell model is wrong • One of the world’s premier atmospheric science research facilities,the Hadley Centre for Climate Research, is named after George Hadley.

48. Three-cell Model • model for a rotating earth • Hadley cell • doldrums • subtropical highs • trade winds • intertropical convergence zone • westerlies • polar front • polar easterlies • Many global circulation terms,including ‘trade winds’ and ‘doldrums’, were named by mariners who were well acquaintedwith wind patterns.

49. Fig. 7-21, p. 185

50. Average Surface Winds and Pressure: The Real World • semipermanent highs and lows • Bermuda high & Pacific high • Icelandic low & Aleutian low • Siberian high • The Bermuda High frequently brings hot, muggy weather to the eastern US.