1. Atmospheric Circulation and
Weather Systems

2. Take-Away Points
1. Weather is driven by unequal solar heating and
cooling
2. Air motions are affected by the Coriolis Effect and
“centrifugal” force
3. High and Low Pressure Systems
4. Air flows parallel to pressure contours (Geostrophic
winds)
5. Air masses meet along sharp boundaries or fronts
6. Weather is inherently chaotic and that limits our
ability to forecast it

3. The Seasons

4. Heat Lag
1. Weather is driven by unequal solar heating and cooling

5. Heat Lag

6. Atmospheric Circulation
1. Weather is driven by unequal solar heating and cooling

7. Atmospheric Circulation
1. Weather is driven by unequal solar heating and cooling

8. Asymmetric Earth
1. Weather is driven by unequal solar heating and cooling

9. Asymmetric Earth
1. Weather is driven by unequal solar heating and cooling

10. Atmospheric Circulation
1. Weather is driven by unequal solar heating and cooling

11. Zonal and Meridional Flow
1. Weather is driven by unequal solar heating and cooling

12. Semi-Permanent Features, January
1. Weather is driven by unequal solar heating and cooling

13. Semi-Permanent Features, July
1. Weather is driven by unequal solar heating and cooling

14. Rotation Effects
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

15. The Coriolis Effect
• Due to moving on a
rotating earth
• Things on equator are
moving faster than
points near poles
• Affects:
– Winds
– Ocean Currents
– Tides
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

16. The Coriolis Effect
• Things moving toward
the equator are
deflected west
• Things moving
poleward are
deflected east
• Deflected to Right in
Northern Hemisphere
• Deflected to Left in
Southern Hemisphere
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

17. The
Coriolis
Effect
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

18. The
Coriolis
Effect
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

19. The
Coriolis
Effect
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

20. Inertial Circles
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

21. “Centrifugal” Force ?
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

22. Or No Force?
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

23. “Centrifugal” Force Does Not Exist
• When anything turns, the only forces that
act are in the direction of the turn
• These forces are called centripetal (centerseeking) force
• “Centrifugal” force is an illusion
• “Centrifugal” force is due to inertia and
centripetal force opposing each other
2. Air motions are affected by the Coriolis Effect and “centrifugal” force

24. High Pressure Systems
3. High and Low Pressure Systems

25. High Pressure Systems
•
•
•
•
•
•
Air flows out from center
Spin clockwise in Northern Hemisphere
No air mixing
Stable, fair weather
Sinking Air, few clouds
Long duration can result in inversions,
pollution
• Winter: often extreme cold
– Cold Air is Dense
– Clear Skies and Radiational Cooling
3. High and Low Pressure Systems

26. Low Pressure Systems
3. High and Low Pressure Systems

27. Why Counterclockwise?
3. High and Low Pressure Systems

28. Low Pressure Systems
• Air flows in toward center
• Spin counter-clockwise in Northern
Hemisphere
• Mixes air of different properties
• Associated with fronts
• Stormy, sometimes violent weather
• Passage often results in sharp change in
weather conditions
3. High and Low Pressure Systems

29. Geostrophic Winds
• As air flows in or out of pressure cells, Coriolis
Effect deflects it
• At surface, friction limits the deflection.
– Winds blow about 45 degree angles to isobars
• Aloft, friction not a factor
– Deflection continues until limited by pressure
gradient (winds can’t go against pressure)
– Winds blow parallel to contours
– This is called geostrophic flow
4. Air flows parallel to pressure contours (Geostrophic winds)

30. Geostrophic Flow

31. Geostrophic Flow

32. Geostrophic Flow
4. Air flows parallel to pressure contours (Geostrophic winds)

33. 1905 Weather Map of US

34. First U.S. Weather Map With
Fronts

35. Fronts and Low Pressure Systems
5. Air masses meet along sharp boundaries or fronts

36. Fronts
5. Air masses meet along sharp boundaries or fronts

37. Warm Fronts
5. Air masses meet along sharp boundaries or fronts

38. Warm Fronts
•
•
•
•
Gradual Onset
Warm Air over Cool Air
Little Turbulence
Weather Rarely Violent
5. Air masses meet along sharp boundaries or fronts

39. Cold Fronts
5. Air masses meet along sharp boundaries or fronts

40. Cold Fronts
•
•
•
•
•
•
Abrupt Onset
Cold Air Lifting Warm Air
Considerable Turbulence
Weather Sometimes Violent
Thunderstorms Common
Can Spawn Tornadoes
5. Air masses meet along sharp boundaries or fronts

41. Old Low Pressure Systems
5. Air masses meet along sharp boundaries or fronts

42. Occluded Fronts
5. Air masses meet along sharp boundaries or fronts

43. Occluded Fronts
• Two fronts merge
• Any two types of front can occlude
• Most common: Cold Front overtakes Warm
Front
• Starts off like a warm front, finishes like a
cold front
5. Air masses meet along sharp boundaries or fronts

44. Weather Prediction
5. Air masses meet along sharp boundaries or fronts

45. Weather Prediction
5. Air masses meet along sharp boundaries or fronts

46. Weather Prediction
5. Air masses meet along sharp boundaries or fronts

47. Chaos
x

48. Chaos Theory
Does Not Mean:
•
•
•
•
•
Cloned Dinosaurs will run amok
Systems do not follow physical laws
Systems behave with wild unpredictability
Systems do not have limits
Phenomena cannot be predicted
6. Weather is inherently chaotic and that limits our ability to forecast it

49. Chaos Theory
Does Mean:
• Small differences compound over time
• There are limits to how accurately
phenomena can be predicted
• Examples:
– Weather
– The Planets
– Traffic
6. Weather is inherently chaotic and that limits our ability to forecast it