Understanding Wind Patterns and Atmospheric Forces

1. Weather 101 and beyond Edward J. Hopkins Dept. of Atmospheric & Oceanic Sciences Univ. of Wisconsin-Madison Midwest Hot Air Balloon Safety Seminar “Hot Aireventure” Oshkosh 3 March 2001

2. Concerns of Balloonists • The Weather • The Terrain (or Surface)

4. Quiz • Ballooning (Fair) Weather is associated with High or Low Pressure? • Which way do winds blow around: High pressure? • Low pressure?

7. WIND • What is Wind? • Why the wind? • Review of basic concepts

8. ASOS Wind InstrumentsWind Vane (left) & Cup Anemometer (right)

9. Aerovane Measures wind speed & direction

10. BEAUFORT WIND FORCE SCALE[Modern version, Source: Federal Meteorological Handbook I]

11. BEAUFORT WIND FORCE SCALE(con’t.)

12. The wind responds to a Difference in air pressure

13. BASIC CONCEPTS Air Pressure (con’t.)

14. Low Pressure High Pressure Explaining Differences in Air Pressure

15. Display of Pressure Differences on a Weather Map - Isobars

16. Isobars -- lines of equal barometric pressure- use sea level corrected pressure

17. AIR PRESSURE in the Vertical (con’t.)

18. As a Sidebar… Altimetry • Since pressure decreases at a “reasonably” known rate of1 mb decrease per 10 meter rise or 0.01 inch of Hg per 10 feet, • then… pressure altimeters are barometers made to read in altitude. • But...

19. Pressure Change with height depends upon Temperature of column- (It is really the density!)

20. WHY THE WIND? (con’t.) • Reasons for Atmospheric Motions: • Buoyancy Effects or Dynamic Effects

22. Daily Heating Heat Gain Heat Loss Daylight Nighttime

23. January Temperatures - Madison, WI (1981-90) Nighttime Daylight Nighttime

24. January Wind Speeds - Madison, WI (1981-90) Nighttime Daylight Nighttime

25. July Temperatures - Madison, WI (1981-90) Daylight Nighttime Nighttime

26. July Wind Speeds - Madison, WI (1981-90) Daylight Nighttime Nighttime

27. ENERGY TRANSPORT: CONVECTION

28. LAPSE CONDITIONSTemperature decreases with height

29. ISOTHERMAL CONDITIONSTemperature remains constant with height

30. INVERSION CONDITIONSTemperature increases with height

31. U.S. STANDARD ATMOSPHERESee Fig. 1.9 Moran & Morgan (1997) Thermosphere Mesopause Mesosphere Stratopause Stratosphere Tropopause Troposphere

33. WHY THE WIND? (con’t.) • Reasons for Atmospheric Motions: • Buoyancy Effects or Dynamic Effects

34. Air Converging Aloft

36. Air Diverging Aloft

38. The Surface • The “Obvious” • Obstacles to take-off and landing (e.g., trees, power lines, animals) • The Surface and the Winds • Affects the Boundary Layer wind flow • Can produce local wind regimes

39. Boundary Layer • Where we live • Extends from surface to approximately 3000 ft. (1000 m) • Consists of • Surface Boundary Layer (30 to 60 ft. & includes Anemometer Level) • Ekman or Spiral Layer(above 60 ft. to Free Atmosphere)

40. Relative Surface RoughnessSource: Stull, 1995

41. B. EXPLANATIONS of ATMOSPHERIC MOTION • Practical Problems • Historical Concepts • Forces of Motion & Newton's Laws

42. An example of an equation of motionNASA

43. PRESSURE GRADIENT FORCE

44. PRESSURE GRADIENT FORCE(con’t.)

45. ASSUMPTIONS For convenience, assume that: • Winds are nearly horizontal; • Atmosphere is in nearly “hydrostatic balance”i.e., air parcels do not accelerate upward or downward;

46. HYDROSTATIC BALANCE CONCEPTSee Fig. 9.11 Moran & Morgan (1997)

47. HORIZONTAL PRESSURE GRADIENT FORCE(con’t.)Direction is from High to Low pressure!

48. HORIZONTAL PRESSURE GRADIENT FORCE (con’t.)See Fig. 9.1 Moran & Morgan (1997)Magnitude depends on isobar spacing!

49. LOCAL WINDSFLOW RESPONDING TO PRESSURE GRADIENT FORCE - LOCAL WINDS • Assumptions: • Only Pressure gradient force operates; • Results from temperature differences • Acts for short time & short distances. • Examples: • Sea-Land Breeze Circulation • Mountain-Valley Breeze Circulation • City-Country Circulation

50. Sea (Lake) Breeze(Graphics from UIUC WW2010)