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  • 5

    1. 1. Chapter 5: Atmospheric Pressure, Winds, and Circulation Patterns Physical Geography Ninth Edition Robert E. Gabler James. F. Petersen L. Michael Trapasso Dorothy Sack
    2. 2. Pressure <ul><li>Atmospheric Pressure </li></ul><ul><ul><li>Variations in pressure create atmospheric circulation (including wind). </li></ul></ul>
    3. 3. Pressure <ul><li>Mercury Barometer </li></ul><ul><ul><li>Standard Sea level pressure is 1013.2 mb. (29.92 inches of mercury) </li></ul></ul><ul><ul><li>When air pressure increases, what happens to the mercury in the tube? </li></ul></ul>
    4. 4. 5.1 Variations in Atmospheric Pressure <ul><li>Vertical Variation </li></ul><ul><ul><li>Air Pressure and air density decrease rapidly with altitude (height). </li></ul></ul><ul><ul><li>By approximately how much does density drop between 0 and 100 km? </li></ul></ul>
    5. 5. 5.1 Variations in Atmospheric Pressure <ul><li>Horizontal Variations </li></ul><ul><ul><li>Determined by thermal (temp) or dynamic (motion of atmosphere) conditions. </li></ul></ul><ul><ul><li>Thermal </li></ul></ul><ul><ul><ul><li>Warm/hot air is less dense and wants to rise. This creates low pressure near the equator. </li></ul></ul></ul><ul><ul><ul><li>Cold air is more dense and wants to sink, creating high pressure, near the poles. </li></ul></ul></ul><ul><ul><li>Dynamic: </li></ul></ul><ul><ul><ul><li>High pressure in the subtropics. </li></ul></ul></ul><ul><ul><ul><li>Low pressure in the subpolar regions (e.g. 40-60 o N and S) </li></ul></ul></ul>
    6. 6. 5.2 Basic Pressure Systems <ul><li>Low (Cyclone) = L </li></ul><ul><ul><li>Air is ascending (rising) </li></ul></ul><ul><ul><li>Low pressure </li></ul></ul><ul><li>High (anticyclone) = H </li></ul><ul><ul><li>Air is descending (subsidence) </li></ul></ul><ul><ul><li>High Pressure </li></ul></ul>
    7. 7. 5.3 How is Temp related to the Density of Air <ul><li>Convergence: wind going toward center (L) </li></ul><ul><li>Divergence: wind going away from center (H) </li></ul>
    8. 8. <ul><li>Mapping Pressure Distribution </li></ul><ul><ul><li>Adjust to sea level pressure </li></ul></ul><ul><ul><li>Isobars: lines of equal pressure </li></ul></ul><ul><ul><li>strong pressure gradient (isobars close together causes stronger winds </li></ul></ul><ul><ul><li>Weak pressure gradient (isobars farther apart) causes weaker winds </li></ul></ul>5.3 How is Temp related to the Density of Air
    9. 9. <ul><li>Wind is the horizontal movement of air due to pressure differences. </li></ul><ul><li>Pressure Gradient </li></ul><ul><ul><li>Where on this figure would winds be the strongest? </li></ul></ul>5.4 Wind
    10. 10. <ul><li>Coriolis Effect </li></ul><ul><ul><li>Apparent deflection of the wind </li></ul></ul><ul><ul><li>N. hem: wind is deflected to the right </li></ul></ul><ul><ul><li>S. hem: wind is deflected to the left. </li></ul></ul>5.4 Wind
    11. 11. <ul><li>Friction and Wind </li></ul><ul><ul><li>Upper Levels (no friction): Ground has little effect. Wind is parallel to isobars. This is a geostrophic wind. </li></ul></ul><ul><ul><li>At or near surface, friction slows the wind and reduces the Coriolis force. Surface wind blows across isobars. </li></ul></ul>5.4 Wind
    12. 12. <ul><li>Wind Terminology </li></ul><ul><ul><li>Winds are named for where they come from </li></ul></ul><ul><ul><ul><li>Wind from northeast is called NE wind </li></ul></ul></ul><ul><ul><li>Windward </li></ul></ul><ul><ul><li>Leeward </li></ul></ul>5.4 Wind
    13. 13. <ul><li>Anticyclone (H) – wind moves away from center in a clockwise spiral in N. hem. </li></ul><ul><li>Wind goes form high to low pressure </li></ul><ul><li>Cyclone (L) – wind moves towards center in a counterclockwise spiral in N. hem </li></ul>5.4 Wind
    14. 14. <ul><ul><li>What do you think might happen to the diverging </li></ul></ul><ul><ul><li>air of an anticyclone if there is a cyclone nearby? </li></ul></ul>5.4 Wind
    15. 15. <ul><li>Global Pressure Belts </li></ul><ul><ul><li>Equator (trough or L) </li></ul></ul><ul><ul><li>30 o N and S – subtropical High </li></ul></ul><ul><ul><li>Subpolar low (L) </li></ul></ul><ul><ul><li>Polar high (H) </li></ul></ul><ul><ul><li>This idealized pressure pattern is affected by landmasses and topography. </li></ul></ul>5.5 Global Pressure Belts
    16. 16. <ul><li>Seasonal Variations in the Pressure pattern </li></ul><ul><ul><li>Shift northward in July and southward in January due to location of sun’s direct rays. </li></ul></ul><ul><ul><li>January : </li></ul></ul><ul><ul><ul><li>Icelandic Low </li></ul></ul></ul><ul><ul><ul><li>Aleutian low </li></ul></ul></ul><ul><ul><li>July </li></ul></ul><ul><ul><ul><li>Bermuda/Azores High </li></ul></ul></ul><ul><ul><ul><li>Pacific High </li></ul></ul></ul>5.5 Global Pressure Belts
    17. 17. <ul><ul><li>What is the difference between the January and July average sea-level pressure at your location? Why do they vary? </li></ul></ul>5.5 Global Pressure Belts
    18. 18. 5.6 Global Surface Wind Systems
    19. 19. <ul><li>Latitudinal Zones </li></ul><ul><ul><li>Trade Winds (5 o -25 o ) </li></ul></ul><ul><ul><li>Doldrums </li></ul></ul><ul><ul><ul><li>A zone of calm and weak winds </li></ul></ul></ul><ul><ul><ul><li>Trade winds converge </li></ul></ul></ul><ul><ul><ul><li>ITCZ (Intertropical Convergence Zone) </li></ul></ul></ul><ul><ul><ul><li>ITCZ = region with high precip. and cloud cover. </li></ul></ul></ul>5.6 Global Surface Wind Systems
    20. 20. <ul><li>Subtropical Highs </li></ul><ul><li>Westerlies </li></ul><ul><li>Polar Winds </li></ul><ul><li>Polar Front </li></ul>5.6 Global Surface Wind Systems
    21. 21. <ul><li>Effects of Seasonal Migration greatest at transition zone </li></ul><ul><ul><li>5 o -15 o (ITCZ and subtropical high) </li></ul></ul><ul><ul><li>30 o -40 o (subtropical high and polar front) </li></ul></ul>5.6 Global Surface Wind Systems
    22. 22. <ul><li>Longitudinal Differences in Winds </li></ul>5.6 Global Surface Wind Systems
    23. 23. <ul><li>Jet Stream – very strong, narrow band of winds embedded within the upper air westerlies </li></ul>5.7 Upper Air Winds and Jet Streams
    24. 24. <ul><li>Polar front Jet Stream </li></ul><ul><li>Subtropical Jet Stream </li></ul><ul><li>Which jet stream is most likely to affect you in January? </li></ul>5.7 Upper Air Winds and Jet Streams
    25. 25. 5.7 Upper Air Winds and Jet Streams
    26. 26. <ul><li>Rossby waves </li></ul>5.7 Upper Air Winds and Jet Streams
    27. 27. <ul><li>Monsoons: seasonal shift of the winds </li></ul><ul><ul><li>Low pressure (summer) - wet </li></ul></ul><ul><ul><li>High pressure (winter) - dry </li></ul></ul>5.8 Subglobal Surface Wind Systems
    28. 28. <ul><li>Local Warming Winds </li></ul><ul><ul><li>Air is compressed and heated as it moves downslope </li></ul></ul><ul><ul><li>Chinooks </li></ul></ul><ul><ul><li>Santa Ana </li></ul></ul><ul><li>Local Drainage Winds </li></ul><ul><ul><li>Katabatic </li></ul></ul>5.8 Subglobal Surface Wind Systems
    29. 29. <ul><li>Land-Sea Breeze </li></ul><ul><ul><li>Diurnal (daily reversal of wind) </li></ul></ul><ul><ul><li>Differential heating between land and water </li></ul></ul>5.8 Subglobal Surface Wind Systems
    30. 30. <ul><li>Mountain Breeze-valley breeze </li></ul><ul><ul><li>Diurnal </li></ul></ul>5.8 Subglobal Surface Wind Systems
    31. 31. <ul><li>Ocean Currents </li></ul><ul><li>Gyres: major surface currents </li></ul><ul><li>Warm currents </li></ul><ul><ul><li>Gulf Stream </li></ul></ul><ul><ul><li>Kuroshio Current </li></ul></ul>5.9 Ocean-Atmosphere Relationships
    32. 32. <ul><li>How does this map of ocean currents help explain the mild winters in London, England? </li></ul>5.9 Ocean-Atmosphere Relationships
    33. 33. <ul><li>El Niño: weak warm countercurrent that replaces cold coastal waters off the coast of Peru (equatorial Pacific) . </li></ul>5.9 Ocean-Atmosphere Relationships
    34. 34. <ul><li>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. </li></ul><ul><li>La Niña – opposite of ENSO </li></ul>5.9 Ocean-Atmosphere Relationships
    35. 35. <ul><li>El Niño and Global Weather </li></ul>5.9 Ocean-Atmosphere Relationships
    36. 36. <ul><li>North Atlantic Oscillation (NAO): relationship between Azores High and Icelandic Low. </li></ul><ul><li>+ NAO = larger pressure difference between Azores and Icelandic. Eastern US may be mild and wet during winter. </li></ul>5.9 Ocean-Atmosphere Relationships
    37. 37. Physical Geography End of Chapter 5: Atmospheric Pressure, Winds, and Circulation Patterns