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: When air is cooled:
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:
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 5oN - 5oS Intertropical Convergence Zone (ITCZ)
thermal Low
high sun angles, long days, available energy
ascending air
heavy precipitation
cloud cover

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

15. Subpolar Lows 55o - 70o 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 90o 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. the ball’s
direction
Ball appears to be deflected to the right, but it has been going in the same direction all along.

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?
360°
YOU!

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

42. Pressure gradient
Resulting wind
direction
Coriolis
friction

43. Northern Hemisphere
High

44. Northern Hemisphere
High
OUT and CLOCKWISE

45. Southern Hemisphere
High

46. Southern Hemisphere
High
OUT and COUNTERCLOCKWISE

47. Northern Hemisphere
Low

48. Northern Hemisphere
Low
IN and COUNTERCLOCKWISE

49. Southern Hemisphere
Low

50. Southern Hemisphere
Low
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
High

56. Northern Hemisphere
High
CLOCKWISE

57. Southern Hemisphere
High
COUNTERCLOCKWISE

58. Northern Hemisphere
Low
COUNTERCLOCKWISE

59. Southern Hemisphere
Low
CLOCKWISE

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

61. Global Wind Systems (Surface Winds)

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

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

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

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

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

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

68. Hadley Cells

69. Winds Aloft Upper Level Westerlies (25o - 90o) Polar Low
Tropical High Pressure Belt (15o - 20o 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!