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Class #27: Monday November 1

Class #27: Monday November 1. Small-scale winds. 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. Small-scale Winds. Subsynoptic-scale weather

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Class #27: Monday November 1

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

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  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, 2010

  47. Fig. 12-18, p. 369

  48. Fig. 12-19, p. 370

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