Chapter 6

1. Chapter 6 Pressure, and the forces that explain the wind

2. Dz area Hydrostatic balance: The upward pressure gradient force is equal and opposite to the gravity

4. aneroid barometer mercury barometer aneroid barograph

5. What is the typical SLP?How much does it vary ?

6. 780 Average air pressure in Laramie

8. We need to reduce station pressure to a standard height, for instance sea level Why? Because winds are driven by horizontal pressure differences

9. Isobars and pressure patterns

10. Where are you more likely to find a pressure value of 994 mb? At A or B ?

11. Becoming acquainted with contouring and frontal analysis • http://cimss.ssec.wisc.edu/wxwise/contour/index.html • http://cimss.ssec.wisc.edu/wxwise/fronts/fronts.html Defining patterns on a surface weather chart • lows and highs • trofs and ridges • saddle

13. ridge trof

14. examine the current weather analysis

15. What drives the wind?

16. Pressure gradient force (PGF) and wind

17. here, Dx=100 km and Dp=4 mb

18. The PGF is directed from high to low pressure, and is stronger when the isobars are more tightly packed

19. in reality, winds do not blow from high to low, at least not along the shortest path … so there must be other force(s)

20. Coriolis force Geostrophic wind balance: a balance between the PGF and the Coriolis force link

21. L

22. Buys-Ballot law • When you face downwind, the low will be on your left • Vice versa in the southern hemisphere you (seen from above)

23. The geostrophic wind blows along the isobars (height contours), counterclockwise around lows (in the NH), and at a speed inversely proportional to the spacing between the isobars (height contours)

25. in the southern hemisphere, the low is on your right when you look downwind L

26. There is a third force, important only near the ground Friction slows the wind

27. 1008 1004 1000 • Interplay between 3 forces • Pressure gradient force • Coriolis force • Friction (near the ground) • Check out how they affect the wind! Guldberg-Mohn balance

28. Trajectories spiral out of a high, and into a low • ~ 10° over oceans • ~ 30° over land • > 30° near mountainous terrain

30. finally, a fourth force: centrifugal force CFF PGF Coriolis PGF Coriolis CFF faster-than-geostrophic wind (supergeostrophic) slower-than-geostrophic wind (subgeostrophic)

31. The jet stream wind is subgeostrophic in trofs, and supergeostrophic in ridges slow fast fast slow

32. Where does the air, spiraling into a low, end up? height

33. subsidence leads to clear skies rising motion leads to cloudiness and precipitation

34. Fig. 10.11

35. 300 mb height, 9 Nov 1975, 7 pm Find the trofs Fig. 10.13

36. fast upper-level divergence, low-level convergence slow surface low 300 mb height, 9 Nov 1975, 7 pm Fig. 10.13

37. Today’s surface weather analysis http://www.rap.ucar.edu/weather/surface/sfc_den.gif http://weather.uwyo.edu/surface/front.html Today’s upper-air maps http://weather.uwyo.edu/upperair/uamap.html

38. Upper-level winds,and upper-level charts

39. Upper level charts are NOT plotted at constant height, eg 18,000 ft. Rather, they display the topography of a pressure surface, eg 500 mb

40. Pressure Approximate Height Approximate Temperature* 1013 mb 0 m (sea level) 0 ft 15 °C 59 °F 1000 mb 100 m 300 ft 15 °C 59 °F 850 mb 1500 m 5000 ft 5 C 41 F 700 mb 3000 m 10000 ft -5 C 23 F 500 mb 5000 m 18000 ft -20 C -4 F 300 mb 9000 m 30000 ft -45 C -49 F 200 mb 12000 m 40000 ft -55 C -67 F 100 mb 16000 m 53000 ft -56 C -69 F Approximate conversion of pressure level to altitude

41. 1000 mb – near sea level

42. 850 mb - ~5,000 ft

43. 700 mb - ~10,000 ft

44. 500 mb - ~18,000 ft

45. 300 mb - ~30,000 ft

46. 200 mb ~ 40,000 ft

47. pressure at a fixed height (sea level)

48. elevation of the 1000 mb surface

49. contours: sea-level pressurecolor fill: 1000 mb height

50. Why do isobar and height contour charts look (almost) the same? 1560 m height high low 1500 m pressure surface sea level New York Boston Pressure decreases with height at about 10 mb every 100 m