1. Fundamentals of Physical Geography 1e
Chapter 4: Atmospheric Pressure, Winds, and Circulation
Petersen
Sack
Gabler

2. Representations of Earth: San Francisco Bay Region and Airport
Chapter cover photo (p. 75)
Representations of Earth: San Francisco Bay Region and Airport
The swirling circulation patterns in Earth’s atmosphere are created by changes in pressure
and winds.

3. Pressure Atmospheric Pressure
Variations in pressure create atmospheric circulation (including wind).
Chapter cover photo (p. 75)

4. Variations in Atmospheric Pressure
Standard Sea level pressure is:
millibars (mb)
29.92 inches of mercury
When air pressure increases, what happens to the mercury in the tube?
Figure 4.1

5. Variations in Atmospheric Pressure
Air Pressure and Altitude
Air Pressure decreases with increasing elevation (altitude).
Mount Everest (29,028 feet) has only 1/3 of the pressure at sea level.
By approximately how much does density drop between 0 and 100 km?
Figure 4.2

6. Variations in Atmospheric Pressure
Cells of High and Low Pressure
Low (Cyclone) = L
Air is ascending (rising)
Low pressure
Convergence at surface
High (anticyclone) = H
Air is descending (subsidence)
High pressure
Divergence at surface
Figure 4.3

7. Low pressure (converging air) High pressure (diverging air)
CYCLONE
ANTICYCLONE
Low pressure
(converging air)
High pressure
(diverging air)
Stepped Art
Fig. 4-3, p. 78

8. Variations in Atmospheric Pressure
Horizontal Pressure Variations
Determined by thermal (temp) or dynamic (motion of atmosphere) conditions.
Thermal
Warm/hot air is less dense and wants to rise. This creates low pressure near the equator.
Cold air is more dense and wants to sink, creating high pressure, near the poles.
Dynamic:
High pressure in the subtropics.
Low pressure in the subpolar regions (e.g o N and S)

9. Variations in Atmospheric Pressure
Mapping Pressure Distribution
Adjust to sea level pressure
Isobars: lines of equal pressure
Pressure Gradient
Strong pressure gradient (isobars close together causes stronger winds
Weak pressure gradient (isobars farther apart) causes weaker winds

10. Wind Pressure Gradients and Wind Wind
Horizontal movement of air due to pressure differences.
High to Low
Corrects radiational imbalances between N and S pole
Where on this figure would winds be the strongest?
Figure 4.4

11. Wind Wind Terminology Windward Leeward
Winds are named for where they come from
Wind from NE is called NE wind
Prevailing winds
How might vegetation differ on the windward and leeward sides of an island?
Figure 4.5

12. Leeward
Windward
Stepped Art
Fig. 4-5, p. 79

13. Wind The Coriolis Effect and Wind Coriolis Effect
Apparent deflection of the wind
N. hem: wind is deflected to the right
S. hem: wind is deflected to the left.
If no Coriolis effect exists at the equator, where would the maximum Coriolis effect be located?
Figure 4.6

14. Wind The Coriolis Effect and Wind Surface Wind Geostrophic Wind
Pressure Gradient Force (PGF)
Friction force
Coriolis effect
Wind crosses isobars
Geostrophic Wind
PGF
Coriolis force
Wind parallel to isobars
Figure 4.7

15. Wind Cyclones, Anticyclones, and Wind Direction
Anticyclone (H) – wind moves away from center in a clockwise spiral in N. hem.
Wind goes form high to low pressure
Cyclone (L) – wind moves towards center in a counterclockwise spiral in N. hem
Figure 4.8

16. Wind
What do you think might happen to the diverging air of an anticyclone if there is a cyclone nearby?
Figure 4.8

17. Global Pressure and Wind Systems
A Model of Global Pressure
Equator low (trough)
Subtropical High -30oN and S
Subpolar low (L)
Polar high (H)
This idealized pressure pattern is affected by landmasses and topography
Figure 4.9

18. Global Pressure and Wind Systems
A Model of Global Pressure
Figure 4.9

19. Global Pressure and Wind Systems
Seasonal Variations in the Pressure pattern
January
Shift southward in due to location of sun’s direct rays.
Icelandic Low
Aleutian low
Figure 4.10a

20. Global Pressure and Wind Systems
Seasonal Variations in the Pressure Pattern
July
Shift northward due to location of sun’s direct rays.
Bermuda/Azores High
Pacific High
Figure 4.10b

21. Global Pressure and Wind Systems
What is the difference between the Jan. and July average sea-level pressure at your location?
Why do they vary?
Figure 4.10

22. Global Pressure and Wind Systems
A Model of Atmospheric Circulation
Convergence and Divergence
Surface Winds (H  L)
Differential heating, Earth’s rotation, Coriolis force, and atmospheric dynamics
Figure 4.11

23. Global Pressure and Wind Systems
Polar Easterlies
Westerlies
Trade Winds (5o-25o)
Northeast trades
Southeast trades
Figure 4.11

24. Global Pressure and Wind Systems
Trade Winds (5o-25o)
Northeast trades
Southeast trades
Tropical easterlies
Figure 4.11

25. Global Pressure and Wind Systems
The Intertropical Convergence Zone (ITCZ)
Equatorial low
Strong convergence, rising air, heavy rain, and calm winds
Doldrums
Figure 4.11

26. Global Pressure and Wind Systems
Subtropical Highs
Westerlies
Polar Winds
Polar Front
Relatively warm air (westerlies) meets cold air (polar)
Subpolar low
Figure 4.11

27. Global Pressure and Wind Systems
Latitudinal Migration with the Seasons
5o-15o
ITCZ and subtropical high
30o-40o
Subtropical high in summer
Wetter westerlies in winter along polar front
Figure 4.12

28. Global Pressure and Wind Systems
Longitudinal Variation in Pressure and Wind
Subtropical Highs
West coasts
Subsidence and divergence
Stable
Relatively dry
East coasts
Unstable and moist
Figure 4.13

29. Upper Air Winds and Jet Streams
Polar front
Very strong, narrow band of winds embedded within the upper air westerlies
Subtropical
Which jet stream is most likely to affect your home state?
Figure 4.14

30. Upper Air Winds and Jet Streams
Rossby Waves
How are Rossby waves closely associated with the changeable weather of the central and eastern United States?
Figure 4.15

31. Regional and Local Wind Systems
Monsoon Winds
Monsoon
Seasonal shift of the winds
Low pressure (summer) – wet
High pressure (winter) – dry
Cherapunji, India
Figure 4.16

32. Regional and Local Wind Systems
Chinook (Foehn)
The term Chinook means “snow eater.” Can you offer an explanation for how this name came about?
Santa Ana
Katabatic (drainage) winds
Figure 4.17

33. Regional and Local Wind Systems
Land-Sea Breeze
Diurnal (daily reversal of wind)
Differential heating between land and water
What is the impact on daytime coastal temperatures of the land and sea breeze?
Figure 4.18

34. Regional and Local Wind Systems
Mountain breeze-valley breeze
How might a green, shady valley floor and a bare, rocky mountain slope contribute to these changes?
Figure 4.19

35. Ocean-Atmosphere Interactions
Ocean Currents
Gyres: major surface currents
What influences the direction of these gyres?
Figure 4.20

36. Ocean-Atmosphere Interactions
Ocean Currents
Warm Currents
Gulf Stream
Kuroshio Current
Cold Currents
California Current
Labrador Current
Upwelling
Humboldt (Peru) Current
Figure 4.21

37. Ocean-Atmosphere Interactions
Ocean Currents
How does this map of ocean currents help explain the mild winters in London, England?
Figure 4.21

38. Ocean-Atmosphere Interactions
El Niño
Weak warm countercurrent that replaces cold coastal waters off the coast of Peru (equatorial Pacific)
Figure 4.22

39. Ocean-Atmosphere Interactions
El Niño Southern Oscillation (ENSO)
Easterly surface winds weaken and retreat to the eastern Pacific, allowing central Pacific to warm and the rain area migrates eastward.
La Niña
opposite of ENSO
Figure 4.23

40. Ocean-Atmosphere Interactions
El Niño and Global Weather
Past few decades on average every 2.2 years
Figure 4.23b

41. Ocean-Atmosphere Interactions
North Atlantic Oscillation (NAO)
Relationship between Azores High and Icelandic Low
Positive NAO
Larger pressure difference between Azores and Icelandic
Eastern US may be mild and wet during winter
Figure 4.24a

42. Ocean-Atmosphere Interactions
North Atlantic Oscillation (NAO)
Negative NAO
Weak Azores High and a weak Icelandic Low
Eastern US may be colder and snowier
Figure 4.24b

43. Fundamentals of Physical Geography 1e
End of Chapter 4: Atmospheric Pressure, Winds, and Circulation
Petersen
Sack
Gabler