1. Chapter 12 Small-Scale Winds

2. Figure CO: Chapter 12, Small-Scale Winds--Fog over Golden Gate Bridge © Andy Dean Photography/ShutterStock, Inc.

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

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

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

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

7. Figure 01: The relationship among eddies, turbulence, and wind gusts

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

9. Figure B01: Photo of wave clouds breaking © Kay Ekwall, www.mtshastaphotography.com

10. Figure 02: Geographic summary of small-scale winds across the contiguous U.S.

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

12. 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

13. Figure 03: Wind flow Source: SSEC, University of Wisconsin-Madison

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

15. Figure 04: Lines of clouds caused by gravity waves in the lee of the Appalachian Mountains Courtesy of SSEC, University of Wisconsin-Madison

16. Figure 05: Water vapor image over Alabama Courtesy of CIRA/Colorado State University and NOAA

17. Figure 06: Automated observations of wind and pressure at Birmingham, AL Source: Bradshaw, John T., et al., The Alabama gravity wave event of February 22, 1998. NOAA, 1998. Retrieved February 28, 2011, from http://www.srh.noaa.gov/bmx/n=research_02221998

18. Figure 07: Gravity wave climatology Adapted from Koppel, L., et al., Monthly Weather Review, January 2000: 58

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

20. Figure 08: Lake breeze Courtesy of SSEC, University of Wisconsin-Madison

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

22. Figure 09: Radar of derecho Courtesy of NOAA

23. Figure 10: A climatology of derechos Modified from Coniglio, M. C., and D. J. Stensrud, Wea. Forecasting 19 (2004): 595-605

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

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

26. Figure B02: Snow acts as a blanket in winter Courtesy of Steven Ackerman

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

28. Figure B03: Dust storm Courtesy of NOAA's National Weather Service (NWS) Collection

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

30. Source: Oklahoma Climatological Survey & The Oklahoma Mesonetwork Figure 11A: Heat burst, 11B: Heat burst, 11C: Heat burst

31. Figure 12: Temperature and dew point plots for heat burst Source: NOAA

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

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

34. Figure 13: Mountain/valley breezes

35. Figure 14: Winds in the Boulder, Colorado, windstorm of February 2, 1999 Source: University Corporation for Atmospheric Research

36. 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

37. Figure 15: Dust devil Courtesy of NASA

38. 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

39. Figure 16: Lenticular cloud Courtesy of Cynthia Stoneburner

40. Figure 17: Wave cloud diagram

41. Figure 18: Flight turbulence Adapted from Lester, P. Turbulence. Jeppesen, 1994.

42. 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

43. 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

44. Figure 19: Santa Ana Winds Courtesy of JPL/NASA

45. 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

46. Figure 20: von Kármán vortex Courtesy of NASA/EROS, USGS

47. Figure 21: Global wind distribution