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Week 7

Week 7. Today (Oct 6) - Midterm Corrections - Term Paper Topics - Air Pressure and Winds (Chp 6) Wednesday (Oct 8) - Atmospheric Circulations (Chp 7) Next Week (Oct 13, 15) - El Nino/La Nina - Remote Sensing - Atmospheric Optics. Midterm #1. Class Average 69% =>.

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Week 7

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  1. Week 7 Today (Oct 6) - Midterm Corrections - Term Paper Topics - Air Pressure and Winds (Chp 6)Wednesday (Oct 8) - Atmospheric Circulations (Chp 7) Next Week (Oct 13, 15) - El Nino/La Nina - Remote Sensing - Atmospheric Optics

  2. Midterm #1 Class Average 69% => Example: missed 8 would be 76 but add 6 for an 82. Add 6 points to your score

  3. Midterm Corrections (HW#6) • Correct ALL missed questions on Midterm • Three parts (All mandatory. See example) • Correct answer for question • 2 or 3 short sentence explanation of why this is the right answer • Source of correct answer • Typed and double-spaced (NO EMAIL) • 10 pts max possible • DUE: Mon. Oct 13 at beginning of class

  4. Midterm Corrections (HW#6) Correction Example: - # 39. C, Scattering • The sky is blue because of Rayleigh scattering. The size of oxygen and nitrogen molecules selectively scatter the shorter blue wavelengths. It is this prominent scattered blue light that our eyes see. • Ahrens text: pg 405 (or Atmospheric Optics PowerPoint, slide # 62)

  5. Term Paper Topics • Be sure focus is on meteorology • Biographies, wildfires, health • No annotated lists • Clouds, extremes

  6. Air Pressure and Winds

  7. Weight of Air Pressure = Force / Area Force = Weight of overlying column of air = mass x gravity

  8. Pressure • The steady exertions of atoms and molecules, exchanging momentum with the walls of a container are “Pressure”.

  9. Atmospheric Pressure • More air near the surface then number of molecules decreases with height • Air pressure, Air Density and Air temperature are all interrelated. • If one changes then the other 2 will change

  10. Pressure Changes • Horizontal: Changes ~ 1 mb over 6000 meters • Vertical: 1 mb over 10 meters (600 X greater) • Vertical atmospheric motions are most important • Vertical pressure and temperature changes are much more dramatic

  11. Two columns of air– same temperaturesame distribution of mass 500 mb 1000 mb 1000 mb

  12. Cool the left column; warm the right column New 500 mb Low Now 480 mb High Original 500 mb Now 520 mb New 500 mb High Low 1003 mb 997 mb

  13. What have we just observed? • Differential heating of uniform atmosphere • Different rates of expansion in the air • Results in horizontal pressure differences • Pressure differences caused flow of air • Atmosphere converting heating into motion

  14. Station Pressure v. Sea Level Pressure

  15. Pressure Maps • a) Surface map has altitude-adjusted station pressures to construct sea level pressure contours • b) Upper air map has constant pressure level delineated by height above sea level

  16. Primary Levels 1000 mb = Surface 850 mb = 5,000’ 700 mb = 10,000’ 500 mb = 18,000’ (middle of the atmosphere) 300 mb = 30,000’

  17. But contour lines are usually not straight. Ridges (elongated highs) occur where air is warm Troughs (elongated lows occur where air is cold Temperature gradients => pressure gradients Isobars usually decrease in value from south to north (cooler temperatures) Troughs and Ridges

  18. Near the surface in the N Hemisphere winds blow counterclockwise around and in toward the center of low pressure areas clockwise around and outward from the center of high pressure areas Surface pressure and winds Why doesn’t the wind blow directly from high to low pressure?

  19. Upper Level Pressure Patterns • Winds parallel to the pressure/height contours Ridge Trough

  20. Forces and winds • Differences in pressure produce fluid movement

  21. Forces Controlling the Wind • Four forces act simultaneously to cause the wind • Pressure Gradient Force • Coriolis Force • Centripetal Force • Friction Force

  22. Pressure Gradient Force • Magnitude • Inversely proportional to distance • Closer together = stronger force • Direction • Directed toward lower pressure and perpendicular to isobars

  23. Pressure Gradient Force

  24. Coriolis Force

  25. Coriolis Force Apparent force due to earth’s rotation • Magnitude • Dependent on latitude and speed of air parcel • Higher latitude = larger Coriolis force • The faster speed = larger Coriolis force • Direction • To the right he Northern Hemisphere • Does NOT change speed

  26. Coriolis Force

  27. Coriolis Force • Acts to right in northern hemisphere • Stronger (i.e. more deviation) for faster wind

  28. 994 mb 996 mb 998 mb Higher Pressure Geostrophic Wind • Geostrophic wind is flow in a straight line in which the pressure gradient force balances the Coriolis force. PGF=CF Lower Pressure

  29. Geostrophic Wind • Wind speed constant if isobars are straight • Speed is proportional to Pressure Gradient • Bernoulli Effect • Same as nozzle on water hose

  30. Geostrophic flow • With the inclusion of the Coriolis Force, air flows parallel to isobars of constant pressure.

  31. Centripetal Force • Object on a curved path has an apparent inward force: centripetal force • Magnitude • depends upon the radius of curvature of the curved path taken by the air parcel • depends upon the speed of the air parcel • Direction • at right angles to the direction of movement

  32. Friction near Earth’s surface • Friction of the ground slows wind down • Magnitude depends on • Speed of the air parcel • Roughness of the terrain • How uniform the wind field is • Direction • Always opposite to air movement • Importance of friction layer (aka PBL = Planetary Boundary Layer) • Approx. lowest 3,000 ft of the atmosphere

  33. Frictional Effects • AGAIN Friction only slows wind speed, does not change wind direction • Therefore, in the Northern Hemisphere • Wind speed decreased by friction • Coriolis force thus decreased and thus will not quite balance the pressure gradient force • Force imbalance (PGF > CF) pushes wind in toward low pressure • Angle at which wind crosses isobars depends on surface roughness • Average ~ 30 degrees

  34. Frictional Effects • Retards wind speed near the surface • Lowers the Coriolis Force • Therefore, wind direction is altered from parallel to crossing isobars.

  35. Cyclonic & Anticyclonic Winds

  36. Isobar Surface Map

  37. Fig. 7.19, p. 190

  38. Fig. 7.19, p. 190

  39. Wind Flow

  40. Winds and vertical air motion • Surface winds blow • Toward low pressure (convergence) • Outward from high pressure (divergence) • Vertical movement to compensate • Surface convergence leads to divergence aloft • Surface divergence leads to convergence aloft VERY IMPORTANT CONCEPT

  41. Winds and vertical air motion VERY IMPORTANT CONCEPT

  42. Naming Winds • Named for direction of origin • North wind comes from the north • Seabreeze comes from the sea • Exceptions: offshore/onshore • upslope/downslope

  43. Measuring Winds • Instruments • Wind vanes • Anemometers • Combo • Aerovane • Wind sock • Profilers • Radar

  44. Wind Measurements • Speeds • Sustained: 2 minute average in past 10 minutes • Gusts: greatest 5-second speed in past 10 minutes • Peak: greatest 5-second speed since last observation • Direction • 2 minute average direction • +/- 10 degrees

  45. Wind Direction • Directional names • (16-point compass)

  46. Table 7.2, p. 196

  47. Winds and Upwelling Ekman Transport

  48. Winds and Upwelling

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