1. ATM OCN 100 Summer 2003 1 ATM OCN 100 – Summer 2004 LECTURE 2B ATMOSPHERIC PRESSURE: A Fundamental Weather Element (con’t.) u A. Practical Problem u B. Basic Concepts u C. Barometry u D. Climatology of Atmospheric Pressure u E. Altimetry

2. ATM OCN 100 Summer 2003 2 MADISON’S CURRENT WEATHER Madison Weather at Madison Weather at 1000 AM CDT MON 23 JUN 2003 Temperature: 25.6°C (78°F) Dewpoint: 15.0°C (59°F) [RH = 52%] Pressure (altimeter): 29.93 inches Hg (1013.6 mb) [Sea-level pressure: 1012.9 mb] Winds: from the S (170 degrees) at 13 MPH (11 knots; 5.7 m/s) Visibility: 10 or more miles (16+ km) Ceiling: at least 12,000 feet AGL Clouds: sky clear below 12,000 feet AGL Present Weather: no significant weather observed

3. ATM OCN 100 Summer 2003 3 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radar Isobars

4. ATM OCN 100 Summer 2003 4 Observations for MADISON DANE RGNL, WI (MSN) 1453Z 18 Jun 2003 to 1353Z 19 Jun 2003 u http://weather.uwyo.edu/surface/met eogram/ http://weather.uwyo.edu/surface/met eogram/ http://weather.uwyo.edu/surface/met eogram/ u http://weather.uwyo.edu/cgi- bin/wyowx.fcgi?TYPE=sfgram&DATE =current&HOUR=current&UNITS=A& STATION=MSN http://weather.uwyo.edu/cgi- bin/wyowx.fcgi?TYPE=sfgram&DATE =current&HOUR=current&UNITS=A& STATION=MSN http://weather.uwyo.edu/cgi- bin/wyowx.fcgi?TYPE=sfgram&DATE =current&HOUR=current&UNITS=A& STATION=MSN

5. ATM OCN 100 Summer 2003 5 Deadly Tornado in Nebraska

6. ATM OCN 100 Summer 2003 6 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radar Isobars

7. ATM OCN 100 Summer 2003 7 Current Surface Winds with Streamlines & Isotachs (“iso” = equal & “tach” = speed) H H L Streamlines

8. ATM OCN 100 Summer 2003 8 Current Surface Winds with Streamlines & Isotachs (“iso” = equal & “tach” = speed) H Streamlines H L L H L

9. ATM OCN 100 Summer 2003 9 Clouds Current Visible Satellite Image

10. ATM OCN 100 Summer 2003 10 Current Temperatures ( ° F) & Isotherms (“iso” = equal +”therm” = temperature)

11. ATM OCN 100 Summer 2003 11 Tomorrow AM Forecast Map

12. ATM OCN 100 Summer 2003 12 The Summer Solstice was Saturday @ 2:10 PM CDT See Fig. 3.10 Moran (2002)

13. ATM OCN 100 Summer 2003 13 ATM OCN 100 – Summer 2004 LECTURE 2B ATMOSPHERIC PRESSURE: A Fundamental Weather Element (con’t.) u A. Practical Problem u B. Basic Concepts u C. Barometry u D. Climatology of Atmospheric Pressure

14. ATM OCN 100 Summer 2003 14 LATEST RADAR

15. ATM OCN 100 Summer 2003 15

16. MADISON’S CURRENT WEATHER Madison Weather at 1000 AM CDT MON 24 JUN 2002 Updated twice an hour at :05 and :25 Sky/Weather: SUNNY Temperature: 81 F (27 C) Dew Point: 64 F (17 C) Relative Humidity: 56% Wind: SW6 MPH Barometer: 30.11F (1019.6 mb)

17. ATM OCN 100 Summer 2003 17 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radar Isobars

18. ATM OCN 100 Summer 2003 18 Rule of Thumb u The tighter the packing of the isobars, the stronger the winds!

19. ATM OCN 100 Summer 2003 19 Current Surface Winds with Streamlines & Isotachs (“iso” = equal & “tach” = speed) H L L H L H

20. ATM OCN 100 Summer 2003 20 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radar Isobars

21. ATM OCN 100 Summer 2003 21 Current Temperatures ( ° F) & Isotherms (“iso” = equal +”therm” = temperature)

22. ATM OCN 100 Summer 2003 22 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radar

23. ATM OCN 100 Summer 2003 23 CURRENT IR GABRIELLE

24. ATM OCN 100 Summer 2003 24 Tomorrow AM Forecast Map

25. ATM OCN 100 Summer 2003 25

26. 26 Happy Summer Solstice Today at (2:38 AM CDT) See Fig. 2.10 Moran & Morgan (1997)

27. ATM OCN 100 Summer 2003 27 Announcements u Homework 1 –Get from our webpage F http://www.aos.wisc.edu/~hopkins/aos100/homework/ –Due Thursday 26June 2003

28. ATM OCN 100 Summer 2003 28 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radar Isobars

29. ATM OCN 100 Summer 2003 29 Did You know that – Sunday is summer solstice (7:57 PM CDT)? See Fig. 3.10 Moran (2002)

30. ATM OCN 100 Summer 2003 30 ATM OCN 100 – Summer 2004 LECTURE 2B ATMOSPHERIC PRESSURE: A Fundamental Weather Element u A. PRACTICAL PROBLEM – The pump problem: Why can’t water be lifted more than 30 ft?

31. ATM OCN 100 Summer 2003 31 B. BASIC CONCEPTS: ATMOSPHERIC PRESSURE

32. ATM OCN 100 Summer 2003 32 B. BASIC CONCEPTS: ATMOSPHERIC PRESSURE u Fundamental concepts u Importance of air pressure

33. ATM OCN 100 Summer 2003 33 B. BASIC CONCEPTS: ATMOSPHERIC PRESSURE

34. ATM OCN 100 Summer 2003 34 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radar Isobars

35. ATM OCN 100 Summer 2003 35 Explaining Differences in Air Pressure u Low Pressure u High Pressure LH

36. ATM OCN 100 Summer 2003 36 Isobars - - lines of equal barometric pressure - use sea level corrected pressure H H L

37. ATM OCN 100 Summer 2003 37 Display of Pressure Differences on a Weather Map - Isobars HL

38. ATM OCN 100 Summer 2003 38 Announcement u Homework 1 is due 1 week from today (Thurs. 28 June 2001)

39. ATM OCN 100 Summer 2003 39 C. BAROMETRY u The Science of Barometry – How is air pressure measured? u Types of Barometers – Mercury Barometers – Aneroid Barometers where... where...

40. ATM OCN 100 Summer 2003 40 MERCURY BAROMETERS u Torricelli invention in 1643; u A self-supported column of mercury; u A balance between weight of atmosphere & weight of mercury in tube.

41. ATM OCN 100 Summer 2003 41 ATM OCN 100 - Fall 2001 LECTURE 4 ATMOSPHERIC PRESSURE: A Fundamental Weather Element (con’t.) u A. Practical Problem u B. Basic Concepts u C. Barometry

42. ATM OCN 100 Summer 2003 42 Principles of a Mercury Barometer See Fig 5.1, Moran (2002)

43. ATM OCN 100 Summer 2003 43 MERCURY BAROMETERS (con’t.)

44. ATM OCN 100 Summer 2003 44 ANEROID BAROMETERS u Perfected in 1840’s; u An evacuated canister; u A balance between force of atmosphere & force of spring inside canister. See Fig 5.2 A, Moran (2002)

45. ATM OCN 100 Summer 2003 45 Aneroid Barometer See Fig 5.2 B, Moran (2002)

46. ATM OCN 100 Summer 2003 46 BAROGRAPH See Fig 5.3, Moran (2002)

47. ATM OCN 100 Summer 2003 47 A pressure trace from a barograph See Fig. 5.6, Moran (2002)

48. ATM OCN 100 Summer 2003 48 C. BAROMETRY (con’t.) u Placement of Barometers u Pressure Units – Inches or millimeters of mercury; – Pounds per square inch or millibars. u The Millibar – Traditional meteorological pressure unit (a metric unit of force per unit area); – Sea level pressure  1000 millibars (or 1 bar).

49. ATM OCN 100 Summer 2003 49 C. BAROMETRY (con’t.) u Placement of Barometers u Pressure Units – Inches or millimeters of mercury; – Pounds per square inch or millibars. u The Millibar – Traditional meteorological pressure unit (a metric unit of force per unit area); – Sea level pressure  1000 millibars (or 1 bar).

50. ATM OCN 100 Summer 2003 50 D. CLIMATOLOGY of ATMOSPHERIC PRESSURE u Spatial variation in Sea-level air pressure – Typical range: 900 mb to 1060 mb. u Temporal variation of Sea-level air pressure – Typically less than 1 mb per 3 hours.

51. ATM OCN 100 Summer 2003 51 D. CLIMATOLOGY of ATMOSPHERIC PRESSURE u Spatial variation in Sea-level air pressure – Extremes – Typical range: 980 mb to 1040 mb such as … such as …

52. ATM OCN 100 Summer 2003 52 Average Sea Level Air Pressure & Atmospheric Circulation

53. ATM OCN 100 Summer 2003 53 D. CLIMATOLOGY of ATMOSPHERIC PRESSURE (con’t.) u Temporal variation of Sea-level air pressure – Typically less than 1 mb per 3 hours, but…

54. ATM OCN 100 Summer 2003 54 A pressure trace from a barograph See Fig. 5.6, Moran (2002) 5-6 mb in 3 hrs

55. ATM OCN 100 Summer 2003 55 D. CLIMATOLOGY of ATMOSPHERIC PRESSURE (con’t.) u Why Do Variations occur in Air Pressure? u Piling On or taking off by: – Local Heating (convection) – Dynamic Effects

56. ATM OCN 100 Summer 2003 56 AIR PRESSURE CLIMATOLOGY (con’t.)

57. ATM OCN 100 Summer 2003 57 Vertical Cross-Section of Air Converging Aloft See Fig. 8.20, Moran (2002) H

58. ATM OCN 100 Summer 2003 58 Map View of Flow around High Pressure Fig. 1.3A, Moran (2002)

59. ATM OCN 100 Summer 2003 59 Vertical Cross-Section of Air Diverging Aloft Fig. 8.21, Moran (2002) L

60. ATM OCN 100 Summer 2003 60 Map View of Flow around Low Pressure Fig. 1.3B, Moran (2002)

61. ATM OCN 100 Summer 2003 61 D. CLIMATOLOGY of ATMOSPHERIC PRESSURE (con’t.) u Vertical Variations in air pressure u The higher up, the lower the pressure. – Why?

62. ATM OCN 100 Summer 2003 62 AIR PRESSURE CLIMATOLOGY (con’t.) Vertical Variation

63. ATM OCN 100 Summer 2003 63 Pressure falls Pressure @ top of building ~ 5-6 mb less than @ street level So how tall is the building?

64. ATM OCN 100 Summer 2003 64 AIR PRESSURE CLIMATOLOGY (con’t.) See Fig. 5.4, Moran (2002)

65. ATM OCN 100 Summer 2003 65 AIR PRESSURE CLIMATOLOGY (con’t.) See Fig. 5.4 Moran (2002) 50% of surface

66. ATM OCN 100 Summer 2003 66 AIR PRESSURE CLIMATOLOGY (con’t) AIR PRESSURE CLIMATOLOGY (con’t) u Variation of air pressure with height – Lower atmosphere: 1mb per 10 m height rise ; – or

67. ATM OCN 100 Summer 2003 67 E. ALTIMETRY u Background – Pressure Altimeters Aneroid Barometers that indicate altitude rather than pressure u Computation

68. ATM OCN 100 Summer 2003 68 AIR PRESSURE CLIMATOLOGY (con’t.) See Fig. 5.4, Moran (2002) 1 mb drop for 10 meter height rise

69. ATM OCN 100 Summer 2003 69 ALTIMETRY (con’t.) u Sources of error –Standard or model atmosphere does not always apply!

70. ATM OCN 100 Summer 2003 70 ALTIMETRY u Background – Pressure Altimeters Aneroid Barometers that indicate altitude rather than pressure u Computation u Sources of error –Standard or model atmosphere does not always apply!

71. ATM OCN 100 Summer 2003 71 Altimetry & Air Temperature (see Figure, pg. 107, Moran (2002)) Cold Warm

72. ATM OCN 100 Summer 2003 72 Flying with a pressure altimeter See Fig, pg. 107, Moran (2002) Cold Warm

73. ATM OCN 100 Summer 2003 73 Sea-Level Pressure Reduction u Desire – –To view only horizontal pressure variations on weather map; u Need – –To eliminate effects of topography from station pressure.

74. ATM OCN 100 Summer 2003 74 Sea-Level Pressure Reduction (con’t.) u Result –Assume that pressure increases at fixed rate down to sea level. Station Pressure Sea level Pressure readings ≈ 1 mb per 10 m

75. ATM OCN 100 Summer 2003 75 Isobars - - lines of equal barometric pressure - use sea level corrected pressure H H L

76. ATM OCN 100 Summer 2003 76 Current Surface weather map 24 June 2003

77. ATM OCN 100 Summer 2003 77 Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radar Isobars

78. ATM OCN 100 Summer 2003 78 Atmosphere in Perspective (UCAR)