1. Class #27: Monday November 1 Small-scale winds 1Class #27: Monday, November 1, 2010

2. Review for test #3 Chapter 9, pp. 251-272; skip Box 9.2 on pp. 262-3 Chapter 10, pp. 276-302; skip pages 304-308 Chapter 11, all of pages 311-347 Chapter 12, pages 351-370 Class #27: Monday, November 1, 20102

3. Small-scale Winds Subsynoptic-scale weather Weather phenomena that develop and change across distances you can see (a few tens of miles or less) Coriolis force usually not important Balance of forces between horizontal pressure gradient and friction Geography and topography are crucial Class #26: Friday, October 29, 20103

4. Friction, eddies, and turbulence Molecular viscosity is friction near the ground Eddies are viscosity within the atmosphere Eddies are swirls of air that arise as the wind blows around obstacles Eddies also arise from daytime heating The atmosphere itself also produces eddies of all sizes The eddies are also called turbulent eddies Class #26: Friday, October 29, 20104

5. Turbulence Is the irregular almost random pattern of wind Is bumpiness due to small-scale changes in the wind Has no precise definition At smaller scales, winds are slowed down and made irregular, or turbulent, by the effect of eddies Class #26: Friday, October 29, 20105

6. Turbulence Acts like a brake on the pressure gradient force which sets air in motion from high towards low pressure At the smallest scales, true molecular friction robs the eddies of the energy they take from the wind Class #27: Monday, November 1, 20106

7. Fig. 12-1, p. 352

8. Clear-Air Turbulence (CAT) Eddies in the upper troposphere are about the same size as turbulent eddies Aircraft avoid turbulence they can see: – Microbursts – Lenticular clouds – Parallel lines of clouds near mountains Clear-air turbulence is usually invisible Keep your seat belt fastened, CAT can kill Class #26: Friday, October 29, 20108

9. Box 12-1, p. 353

10. Survey of small-scale winds Fig. 12-2, p. 354

11. Mt. Washington, a windy place Mt. Washington, NH, is an isolated mountain peak—winds blow over, not around the peak At a height of 6288 feet, has persistent clouds, heavy snow, cold temperatures and record- setting high winds Record wind: 231 mph set here in 1934, a record for surface wind Winds exceed hurricane force on average 104 days per year Class #27: Monday, November 1, 201011

12. Box 12-2, p. 355

13. Coastal Fronts Common in New England and along the east coast of the US Cold air near mountains; warmer air offshore can lead to a miniature stationary front Heavy snow—rain separated by only a few km Stubborn entrenchment of cold air pinned against high mountains is called cold air damming: accompanied by freezing rain Class #27: Monday, November 1, 201013

14. Gravity waves Alternating patterns of high and low pressure maintained by gravity Sometimes form long straight lines of clouds Form when wind blows over a mountain or a thunderstorm Wind changes in the jet stream can send out ripples of waves Are very difficult to forecast Class #27: Monday, November 1, 201014

15. Fig. 12-3, p. 357

16. Fig. 12-4, p. 357

17. Fig. 12-5, p. 358

18. 25 years of strong gravity waves Fig. 12-6, p. 359

19. Lake Breezes Resemble the sea breeze: the water is cold compared to the land and a wind blows from the water to the land The boundary between the lake breeze and the land air can be a focal point for thunderstorm development Class #27: Monday, November 1, 201019

20. Fig. 12-7, p. 359

21. Derechos Straight-line winds of up to 150 mph forming an hours long windstorm along a line of severe thunderstorms Storms typically form along a stationary front in summer Storms form a bow echo Responsible for 40% of all thunderstorm injuries and deaths Cause extensive property and tree damage Class #27: Monday, November 1, 201021

22. Fig. 12-8, p. 360

23. Derechos from 1994 to 2003 Fig. 12-9, p. 360

24. Blue Northers Are fast-moving dry cold fronts that sweep across the plains to Texas Northerly winds occur behind the front No clouds accompany the fronts A sharp temperature drop marks the front Class #27: Monday, November 1, 201024

25. Snow fences and windbreaks Help slow the wind like speed bumps do to traffic on a road Cause turbulent eddies to develop Snow fences keep snow from blowing across land and roadways Windbreaks keep soil from blowing across land and roadways Class #27: Monday, November 1, 201025

26. Box 12-3, p. 361

27. Dust storms and the Dust Bowl A pressure gradient and dry ground are all that are needed for a dust storm Dry line thunderstorms with downbursts Dry fronts like blue northers The dry slot of an extratropical cyclone Drought in the 1930s: 14 dust storms in 1932 and 38 in 1933 Soil conservation efforts, wetter conditions prevent dust storms Class #27: Monday, November 1, 201027

28. Box 12-4, p. 362

29. Heat bursts Originate as high updrafts Sinking air warms at DALR as it is compressed Like a hot microburst, air splashes against the ground an spreads out Last about 30 minutes, have winds of 41 mph on average, and can cause damage Temperatures rise and dew point falls Captured by mesonetworks Class #27: Monday, November 1, 201029

30. Fig. 12-10, p. 363

31. Fig. 12-11, p. 364

32. Chinooks Warm dry winds on the downslope side of a mountain range Air warms at the DALR as it descends Air arrives at the surface warm and dry Can raise the air temperature extremely rapidly Have different names in different parts of the world Class #27: Monday, November 1, 201032

33. Mountain/Valley winds and windstorms Upslope winds during the day when the slopes are warmed Downslope winds at night when the slopes cool Usually gentle; when strong are called katabatic winds Any strong pressure gradient can cause funneling of the wind in passes and cause a windstorm with property damage Class #27: Monday, November 1, 201033

34. Fig. 12-12, p. 365

35. Fig. 12-12a, p. 365

36. Fig. 12-12b, p. 365

37. Fig. 12-13, p. 365

38. Dust devils Thin, rotating columns of air Created by solar heating Unstable air rises and creates a tiny low- pressure center Form under clear skies Seldom cause damage Class #27: Monday, November 1, 201038

39. Fig. 12-14, p. 366

40. Lenticular clouds Formed when moist air rises on the crest of a gravity wave, gets saturated Look like lenses Stay in the same place Are a sign of turbulence nearby and beneath the cloud, in spite of its smooth appearance Class #27: Monday, November 1, 201040

41. Fig. 12-15, p. 367

42. Beneath a lenticular cloud Fig. 12-16, p. 368

43. Santa Ana Winds Another downslope wind Caused by pressure gradient of an anticyclone over the Rockies and friction Forces already dry air down the Coast Range or the San Gabriel mountains and out to the ocean Most common in autumn Temperature increases and dew point decreases Class #27: Monday, November 1, 201043

44. Santa Ana winds (continued) Occur in a heavily populated area Cause extreme fire danger Similar winds are observed at other locations in other parts of the world Class #27: Monday, November 1, 201044

45. Fig. 12-17, p. 368

46. Von Kármán vortex sheet A long interlocking chain of ripples downwind of a mountain Caused when wind flows around rather than over a mountain Air closest to the mountain is slowed; farther away air is deflected Wind shear causes deflected air to roll up into interlocking pairs of vortices, one cyclonic and one anticyclonic; not dangerous Class #27: Monday, November 1, 201046

47. Fig. 12-18, p. 369

48. Fig. 12-19, p. 370