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 atmosphere above us.

4. Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) Because it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the addition of air into column 1 causes its surface pressure to rise. (The difference in height between the two columns is greatly exaggerated.)
Watch this Active Figure on ThomsonNow website at
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 ordinary contour maps, except that the area between adjacent lines 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
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. Figure 6.11: The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure gradient force.
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. Figure 6.14: On nonrotating platform A, the thrown ball moves in a straight line. On platform B, which rotates counterclockwise, the ball continues to move in a straight line. However, platform B is rotating while the ball is in flight; thus, to anyone on platform B, the ball appears to deflect to the right of its intended path.
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. Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above sea level. Dashed red lines are isotherms in °C.
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
Fig 7.1
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 the movement of clouds at different altitudes.

33. Local Wind Systems

34. Thermal Circulations isobars and density differences

35. Figure 7.4: A thermal circulation produced by the heating and cooling of the atmosphere near the ground. The H’s and L’s refer to atmospheric pressure. The lines represent surfaces of constant pressure (isobaric surfaces).
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. Figure 7. 5: Development of a sea breeze and a land breeze
Figure 7.5: Development of a sea breeze and a land breeze. (a) At the surface, a sea breeze blows from the water onto the land, whereas (b) the land breeze blows from the land out over the water. Notice that the pressure at the surface changes more rapidly with the sea breeze. This situation indicates a stronger pressure gradient force and higher winds with a sea breeze.
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
mountain breeze
The nighttime mountain breeze is sometimes called gravity winds or drainage winds, because gravity causes 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 that many houses have sliding wooden shutters to protect their windows from windblown debris.

42. Figure 7.14: A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind side.
Fig. 7-14, p. 180

43. Santa Ana Winds Santa Ana wind compressional heating wildfires
Many Southern California residents regularly hose down their 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 to the 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 acquainted with wind patterns.

49. Figure 7.21: The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth.
Watch this Active Figure on ThomsonNow website at
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.

51. Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ.
Fig. 7-22a, p. 188

52. Fig. 7-22b, p. 189

53. The General Circulation and Precipitation Patterns
major controls
ITCZ, midlatitude storms, polar front
Most of the world’s thunderstorms are found along the ITCZ.

54. Westerly Winds and the Jet Stream
jet streams
subtropical jet stream
polar front jet stream

55. Global Wind Patterns and the Oceans

56. Winds and Upwelling upwelling wind flow parallel to the coastline
Upwelling frequently occurs along the coast of California.

57. El Niño and the Southern Oscillation
El Niño events
Southern Oscillation
La Niña
teleconnections
ENSO is an example of a global-scale weather phenomenon.

58. Figure 7.32: In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Niña. During El Niño conditions—diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Niño to El Niño.
Fig. 7-32, p. 196

59. Other Atmosphere-Ocean Interactions
North Atlantic Oscillation
Arctic Oscillation
Pacific Decadal Oscillation
Other atmosphere-ocean interactions may very well be discovered in the coming years.