Class #27: Monday November 1
Small-scale winds
1Class #27: Monday, November 1, 2010
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 2
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 3
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
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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
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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
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Fig. 12-1, p. 352
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
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Box 12-1, p. 353
Survey of small-scale winds
Fig. 12-2, p. 354
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
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Box 12-2, p. 355
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
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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
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Fig. 12-3, p. 357
Fig. 12-4, p. 357
Fig. 12-5, p. 358
25 years of strong gravity waves
Fig. 12-6, p. 359
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
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Fig. 12-7, p. 359
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
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Fig. 12-8, p. 360
Derechos from 1994 to 2003
Fig. 12-9, p. 360
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
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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
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Box 12-3, p. 361
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
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Box 12-4, p. 362
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
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Fig. 12-10, p. 363
Fig. 12-11, p. 364
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
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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
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Fig. 12-12, p. 365
Fig. 12-12a, p. 365
Fig. 12-12b, p. 365
Fig. 12-13, p. 365
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
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Fig. 12-14, p. 366
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
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Fig. 12-15, p. 367
Beneath a lenticular cloud
Fig. 12-16, p. 368
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
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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
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Fig. 12-17, p. 368
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
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Fig. 12-18, p. 369
Fig. 12-19, p. 370