1. Atmospheric Forces and Winds
Chapter 6
Atmospheric Forces and Winds

2. Figure CO: Chapter 6, Atmospheric Forces and Wind
Courtesy of RMS, Inc

3. Figure UN01: Winds over France on Feb. 27-28, 2010
Data from Météo-France

4. Figure UN02: Flooding in La Faute, France
© Regis Duvignau/Reuters/Landov

5. Basics about Wind
Wind direction is the direction from which the wind is blowing
A north wind blows from the north to the south
It is reported according to compass directions
Prevailing wind direction is the most frequent direction
Wind speed
Reported on U.S. weather maps in knots
1 knot = 1.15 miles/hour = 0.5 meter/second
If wind > 15 knots and highly variable, the weather report will include the wind gust, the maximum speed

6. Figure 01: Wind directions in angles, compass headings.

7. Forces Have magnitude (or strength) and direction
Multiple forces can act on the same point
The resultant force is the net force
If two forces act in opposite directions, the net force will have the direction of the stronger force and a strength equal to the difference of the two forces
If two forces act at an angle to each other, the resultant force is along a diagonal and away from where the two forces are applied

8. Figure 02: Force diagram.

9. Figure 03: Graphical addition of force vectors.

10. Forces and Movement
A force applied to an object often results in movement
An object’s velocity is the magnitude and direction of its motion
The speed of the object, the distance traveled in a given amount of time, is the magnitude of the motion
Acceleration is a change in an object’s velocity—magnitude, direction or both

11. Forces cause the wind to blow
Forces that act on air create horizontal wind
A force acting through a distance does work
Work is equivalent to energy
Ultimately, the sun provides the energy that allows the winds to blow
Radiation causes temperature imbalances, which lead to pressure imbalances and a force

12. Newton’s second law of motion
Says that
the sum of the forces = mass x acceleration
Or that acceleration = sum of forces/mass
Helps scientist forecast changes in the wind direction and speed, or its acceleration
Requires that we specify which forces are acting and how strong they are
Is also called the Law of momentum
Momentum of an object is its mass x its velocity

13. Gravity, the strongest force
Does not act horizontally, so does not influence the horizontal winds.
Does influence vertical air motions
Is directed downward toward the center of Earth
Is a very strong force
Keeps our atmosphere from escaping
Equals the mass x 9.8 m/s2

14. The Pressure Gradient Force (PGF)
The force that results from pressure differences over distances in a fluid
A pressure gradient is a change in pressure over a distance
PGF always directed from high to low pressure
Is stronger when isobars more closely spaced
Is stronger when the difference in pressure is greater over a particular distance
Determines the way air moves only if no other forces are acting

15. Figure B01A: Fan blowing on paper

16. Figure B01B: Air over plane wing, with lift and drag

17. Figure 04: Pressure gradient force in highs and lows.

18. The horizontal pressure gradient force
Is always directed from high to low pressure
Is stronger where the density is less—higher in the troposphere
When stronger, causes stronger winds
Is always important in horizontal winds
Is not generally in the same direction the wind blows, because other forces can act

19. Figure 05: Surface weather map
From Plymouth State University Weather Center, [

20. Isobaric Charts Plot the altitude of a given pressure surface
Units of altitude are called geopotential meters
Also called a constant-pressure chart
Common levels are 850, 700, 500, 250, and 200 mb
Are useful for portraying horizontal pressure gradients above the ground
The spacing between the lines of constant height is proportional to the PGF
The winds in general blow parallel to the height contours, at right angles to the PGF

21. Figure 06: 500-mb isobaric chart
From Plymouth State University Weather Center, [

22. Figure 07A: Isolines of constant height are proportional to the PGF

23. Figure 07B: Isolines of constant height are proportional to the PGF

24. Figure 07C: Isolines of constant height are proportional to the PGF

25. Centrifugal Force/Centripetal Acceleration
Centripetal acceleration is a change in direction even if the speed does not change
From the point of view of an observer experiencing the centripetal acceleration, there is an apparent force called the centrifugal force
The faster the speed and the tighter the curve, the larger is the centripetal acceleration
The sign of the centripetal acceleration is positive for cyclones, negative for anticyclones, and always directed inward to the center

26. Figure 08: Centrifugal force schematic

27. The Coriolis Force Deflects the wind to the right in the NH
Deflects the wind to the left in the SH
Is strongest at the poles
Is zero at the equator
Is stronger for stronger winds
Is weaker for weaker winds
Is zero for calm. It cannot start a wind

28. Figure 09A: Curving path of ocean buoy
Adapted from Joseph et al., Current Science, 92 (2007).

29. Figure 6.10: The centrifugal (CENTF) and Coriolis forces acting on an air parcel moving with respect to the rotating Earth
Modified from A. Persson, Bull. Amer. Meteor. Soc., 79 [1998]: 1378.).

30. Figure 11A: Coriolis force at different latitudes.

31. Figure 11B: variation of Coriolis force with latitude and wind speed

32. Figure B02B: Carl-Gustaf Rossby
Courtesy of University of Chicago News Office

33. The Friction Force
Acts in the direction opposite to the direction the wind is blowing
Acts to slow down the wind
Is most important at Earth’s surface
Gets stronger when the winds are stronger
Is not important above the boundary layer (the lowest 1 km in the atmosphere)
The rougher the surface and the stronger the wind the greater is the friction force

34. Figure 12: Frictional force diagram

35. Why force-balances are important
Force-balances simplify Newton’s second law of motion by limiting the number of forces
Force-balances describe winds that come close to describing the observed winds
Even though the forces are balanced, the wind need not be calm
The PGF is important in every force balance
Only the PGF can set calm air into motion

36. Figure T01: Some Atmospheric Force-Balances

37. Hydrostatic Balance
Gravity (downward) balances the Vertical Pressure Gradient Force (upward)
Does not apply inside cumulus clouds, because buoyancy is important there
Does apply generally in the atmosphere
Limits vertical motions to be much weaker than horizontal winds

38. Figure 6.13: Air parcel in hydrostatic balance
Reproduced from Lester, P., Aviation Weather, Second edition. With permission of Jeppsen Sanderson, Inc. Not for Navigation Use. Copyright © 2010 Jeppesen
Sanderson, Inc.

39. More on Hydrostatic Balance
The pressure gradient force is stronger when the air is less dense
The density of air is less when the air Temperature is higher
Pressure decreases upward less rapidly when the air has a higher temperature
Hydrostatic balance helps explain the sea breeze and other thermal circulations

40. Pressure levels on weather maps
The atmosphere is very close to hydrostatic balance
This means that the height of a particular pressure level is roughly equivalent to the pressure at a related height level
An altimeter is a barometer with a height scale
Upper-level weather maps are labeled in m
Winds on a weather map are strong when the height contours are close together, weak where they are farther apart

41. Geostrophic Balance
Is a balance between the horizontal pressure gradient force and the Coriolis force
Ignores the friction force
Has isobars that are straight lines
Does not mean that the wind is calm
Has a wind called the geostrophic wind
Winds on weather maps above the surface are close to the geostrophic wind
Blows with lower pressure (height) on the left (NH)

42. Figure 14: Geostrophic balance

43. The Geostrophic Wind Is a wind in geostrophic balance
Is parallel to the isobars
In the NH has low pressure on the left
In the SH has low pressure on the right
In the NH the wind blows clockwise around high pressure centers and counterclockwise around low pressure centers
In the SH CW flow around lows and CCW flow around highs

44. Figure 15: Geostrophic wind in highs and lows

45. Gradient Balance and the Gradient Wind
Gradient balance is between the PGF, the Coriolis force and the centrifugal force
Gradient balance allows curving wind patterns called the gradient wind
The centrifugal force is always outward
Around a low the centrifugal force opposes the PGF and the resulting flow is subgeostrophic
Around a high the centrifugal force adds to the PGF and the resulting flow is supergeostrophic

46. Figure 16: As in Figure 6-15, except now we also include the centrifugal force, leading to gradient balance.

47. Adjustment to Geostrophic Balance
Initially there is an imbalance of forces
Air parcels move toward lower pressure (PGF)
As soon as there is a wind, the Coriolis force acts
Parcels oscillate towards a balance between the PGF and the Coriolis force
Adjustment takes minutes to hours
Adjustment is temporary and incomplete

48. Figure 17: Wavy path of parcel adjusting to balance

49. Guldberg-Mohn Balance
Is a balance between the PGF, the Coriolis force, and friction
Friction slows the wind and the Coriolis force weakens
The winds blow across the isobars at an angle toward low pressure (away from high pressure)
Between 15° and 30° over water
Between 25° and 50° over land
Friction damps oscillations during adjustment to balance

50. Figure 18: Guldberg-Mohn balance

51. Figure 19: A numerical simulation of how varying amounts of friction affect the adjustment to Guldberg–Mohn balance.
Modified from Knox, J., and Borenstein, S., J. Geoscience Ed., 46 [1998]: 190–192.

52. Figure B03A: Chart of wind speeds and max wave heights

53. Figure 21: The isobars at the surface drawn over a satellite image of a cyclone
Image created by Prof. Joshua Durkee, Western Kentucky University, using GREarth software.

54. The Thermal Wind
The thermal wind relates temperature and winds to each other
The winds are more westerly as you go up wherever it’s colder toward the poles

55. Putting horizontal and vertical winds together
At the surface, the wind blows across the isobars into low-pressure areas
At the center of the low-pressure area the air must rise
Low-pressure areas are usually cloudy and wet
At the surface, the wind blows across the isobars out of high-pressure areas
At the center of the high-pressure area the air must sink
High-pressure areas are usually clear and dry
These patterns are the result of Guldberg-Mohn balance

56. Figure 22: Schematic of pressure levels when air is heated

57. Figure 23: Cross-section of winds at various pressure levels

58. Figure 24A: How surface wind patterns induce vertical wind motions
Figure 24B: How surface wind patterns induce vertical wind motions

59. Figure 24A: How surface wind patterns induce vertical wind motions

60. Figure 24B: How surface wind patterns induce vertical wind motions

61. The thermal circulation
The sea breeze is a thermal circulation
A thermal circulation has both horizontal and vertical air motions
The horizontal pressure gradient force is most important in a thermal circulation
Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column

62. The sea breeze Is a daytime circulation
Depends on differential heating at the surface between land and water
Has the warmer, rising air column over the land, which absorbs more incoming solar radiation
Has the cooler, sinking air column over the water, which absorbs less radiation
Air flows from warmer to cooler column aloft
Air flows from cooler to warmer column at the surface

63. Figure 25: Sea breeze schematic

64. Figure 26A: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

65. Figure 26B: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

66. Figure 26C: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

67. Figure 26D: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

68. The sea breeze and the land breeze
As solar heating diminishes in the late afternoon, the sea breeze weakens
At night, differential cooling occurs
The cooler, sinking air column is over land, where radiational cooling is more rapid than over the water
The warmer, rising air column is over the water
The land breeze develops at night
Air flows towards the land aloft
Air flows towards the water at the surface

69. Figure 27: Schematic of land breeze

70. Scales of motion in the atmosphere
Describe the size and lifetime of wind patterns in the atmosphere
Determine which forces are most important to forming the wind patterns
Are largest when the lifetimes are longest
Are smaller when the lifetime is shorter
Have a variety of names and definitions

71. More on scales of motion
Microscale: <1 km in diameter
PGF, centrifugal, friction forces are important
Mesoscale: Between 1 and 1000 km in size
PGF, centrifugal, friction, and Coriolis Force for largest sizes
Synoptic scale: At least 1000 km in size
Balance between PGF and Coriolis Force dominates
Planetary scale: Roughly 10,000 km in size