1. Atmospheric Pressure and Wind

2. Atmospheric pressure: –force exerted by a column of air per unit area –Normal atmospheric pressure at sea level = 1013 millibars

3. Air pressure patterns controlled by: 1. Temperature changes 2. Rotation of earth

4. 1.Temperature changes: When air is heated: –air expands and PRESSURE DROPS When air is cooled: –air compresses and PRESSURE INCREASES

5. Result: WARM surfaces develop thermal LOWS COLD surfaces develop thermal HIGHS

6. THERMAL HIGHS THERMAL LOW

7. 2. Rotation of earth: Earth’s rotation causes air to accumulate in certain latitudes and to be deflected away from certain latitudes accumulation : HIGH pressure deflection: LOW pressure

8. Highs and Lows in cross-section: HIGHS: –clear skies rising barometer means good weather LOWS: –cloudy skies falling barometer means bad weather

9. Global Patterns of High and Low Pressure

10. Equatorial Low 5 o N - 5 o S Intertropical Convergence Zone (ITCZ) thermal Low –high sun angles, long days, available energy –ascending air –heavy precipitation –cloud cover

14. Subtropical Highs 25 o - 40 o N & S rotation-induced Highs air deflected to subtropics descending air clear skies hot dry air great deserts here

15. Subpolar Lows 55 o - 70 o N & S rotation-induced Lows warm air from low latitudes is lifted as it meets cold polar air ascending air storm centers here

17. Polar Highs 90 o N & S thermal Highs cold polar temps at high latitudes descending air Note: all pressure belts shift seasonally

19. What causes wind? Wind is air moving from High to Low pressure. Wind is named after direction it comes FROM. ( a “west wind” comes out of the west; flows eastward)

21. Two components of wind 1.Speed 2. Direction

22. 1.Wind Speed is determined by: a. Steepness of pressure gradient Steep gradient: closely spaced isobars Gradual gradient: widely spaced isobars b. Friction Friction from surface lowers wind speed

24. 2. Wind Direction is determined by: a. Direction of pressure gradient b. Coriolis force c. Friction

25. a. Direction of pressure gradient from High to Low makes wind would blow perpendicular to isobars

28. 2. Wind Direction is determined by: a. Direction of pressure gradient b. Coriolis force c. Friction

29. b. Coriolis force apparent deflection of moving things (like the wind) on a rotating surface (like the earth) Imagine tossing a ball across a rotating room…

30. Ball appears to be deflected to the right, but it has been going in the same direction all along. the ball’s direction

31. Airplanes, rockets, migrating birds, ocean currents, air are deflected from their paths of motion because the earth is rotating. in Northern Hemisphere, deflection to RIGHT of movement in Southern Hemisphere, deflection to LEFT of movement

32. Watch this animation…

33. Deflection increases with latitude: no Coriolis at equator; greatest deflection at poles Imagine sitting on a chair on a platform at varying latitudes….

34. If you are sitting on the north pole, how many degrees will the room rotate/spin in one day? YOU! 360°

35. If you are on the equator, how many degrees will the room rotate/spin in one day? 0 !

36. If you are between the poles and the equator, how many degrees will the room rotate/spin in one day? Between 0 and 360, depending on latitude

39. If Coriolis effect were only influence on wind direction, wind would blow parallel to isobars

40. 2. Wind Direction is determined by: a. Direction of pressure gradient b. Coriolis force c. Friction

41. the “drag” produced by earth’s surface –applied opposite direction of motion –reduce angle of Coriolis deflection

42. Pressure gradient Coriolis friction Resulting wind direction

43. Northern Hemisphere

44. OUT and CLOCKWISE

45. Southern Hemisphere

46. OUT and COUNTERCLOCKWISE

47. Northern Hemisphere

48. IN and COUNTERCLOCKWISE

49. Southern Hemisphere

50. IN and CLOCKWISE

54. Winds in Upper Atmosphere no friction only the pressure gradient and Coriolis effect –wind is parallel to isobars: GEOSTROPHIC WIND

55. Northern Hemisphere

56. CLOCKWISE

57. Southern Hemisphere COUNTERCLOCKWISE

58. Northern Hemisphere COUNTERCLOCKWISE

59. Southern Hemisphere CLOCKWISE

60. Trade Winds 5 o - 25 o N & S –NE, SE –steady, persistent

61. Global Wind Systems (Surface Winds)

62. Westerlies 35 o - 60 o N & S –not steady or persistent

63. Polar Easterlies 65 o - 80 o N & S –more prevalent in Southern, variability in Northern

64. Equatorial Belt of Variable Winds and Calm 5 o N - 5 o S ITCZ “Doldrums”

65. Subtropical Belt of Variable Winds and Calm 30 o - 35 o N & S “Horse Latitudes”

66. Polar Front Zone 60 o - 65 o N & S zone of conflict between differing air masses

67. Polar Zone of Variable Winds and Calm 80 o - 90 o N & S

68. Hadley Cells

69. Winds Aloft Upper Level Westerlies (25 o - 90 o ) Polar Low Tropical High Pressure Belt (15 o - 20 o N & S) Equatorial Easterlies

70. Jet Streams Narrow zones of extremely high wind speeds occur where there are strong temp contrasts Polar Jet (westerly) Subtropical Jet (westerly)

71. Summary!