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Lecture 5: General Circulation of the Atmosphere

Lecture 5: General Circulation of the Atmosphere. EarthsClimate_Web_Chapter.pdf , p. 15-22. Questions? 1. Why do winds blow around a high or a low pressure center?. Why does a low area mean more precipitation ? Why does a high mean a dry area?.

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Lecture 5: General Circulation of the Atmosphere

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  1. Lecture 5: General Circulation of the Atmosphere EarthsClimate_Web_Chapter.pdf, p. 15-22 Questions? 1. Why do winds blow around a high or a low pressure center? • Why does a low area mean more precipitation? • Why does a high mean a dry area?

  2. The Global Energy Budget: Driver of Atmospheric Motion A balance exists between the incoming solar and outgoing longwave energy averaged over the globe and the year However, the tilt of the Earth means this balance is not maintained for each latitude SURPLUS DEFICIT

  3. To compensate for this energy • imbalance, winds in the • atmosphere and currents in the • oceans transport cold air and • water toward the equator • About 1/3 of this transport • occurs from the evaporation of • tropical waters and subsequent • transport into high latitudes, • where it condenses and • releases latent heat • About 1/3 occurs from the • poleward transport of warm • waters by ocean currents • The remaining 1/3 occurs from • middle latitude cyclones and • anticyclones

  4. Global Wind Patterns Polar easterlies Westerlies Hadley Cell Hadley Cell Westerlies Polar easterlies

  5. Atmospheric Pressure The pressure at the base of this column of air results from theweight of the gasses above. The earth's atmosphere has a greater density of gases at its base due to gravity.

  6. Forces & Motion Air responds similarly as water to this force, moving from higher pressure to lower pressure. Pressure forces are only one influence on the movement of atmospheric air.

  7. PGF vs. Cyclonic Winds Pressure gradient force (PGF) winds acting alonewould head directly into low pressure. Surface observations of winds, such as the cyclonic flow around this low, reveal that PGF winds are deflected by other forces.

  8. Playing catch on a merry-go-round Clockwise Blue, Yellow,Red, Green Straight-line, watch from above Curved to the left, watch by the green on board

  9. Coriolis Force Counterclockwise Earth's rotation transforms straight line motion for an outside viewer in space into curved motion for an viewer on Earth. The Coriolis force explains this apparent curvature of winds to the right due to rotation. Its magnitude increases with wind velocity and earth's latitude.

  10. Cyclones and Anticyclones Three forces: PGF Coriolis Frictional

  11. Vertical Air Motion Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules. These updrafts may cause cloud formation. Likewise, diverging air molecules from a high pressure area are replenished by downward winds.

  12. Northern Hemisphere Flow Winds blow counterclockwise around low pressure systems in the Northern Hemisphere, but clockwise around lows in the Southern Hemisphere. Figure 9.27A

  13. Coriolis Force and Nature’s Greatest Storms

  14. Hurricane Movement • Formation • Low-pressure areas (easterly waves) • Warm, moisture, unstable area • Ocean surf. temp. > 26–27°C • Low wind shear • Sufficient Coriolis force (5-10 degrees away from the equator) • Strong upper-level anticyclone. • Their movement is guided by winds around subtropical high pressure systems.

  15. R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

  16. R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

  17. R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

  18. R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

  19. Can you answer these questions at the end of this lecture? • What causes atmospheric surface pressure gradient and change? • What atmospheric forces drive winds? • Why is cyclonic flow associated with cloudy and rainy weather, whereas anticyclonic flow is associated with clear and cooler weather?

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