Transcript Class #23: Friday October 23

Class #27: Monday
November 1
Small-scale winds
Class #27: Monday, November 1, 2010
1
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
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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
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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
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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
Class #26: Friday, October 29, 2010
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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
Class #26: Friday, October 29, 2010
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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 recordsetting 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
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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
Class #27: Monday, November 1, 2010
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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
Class #27: Monday, November 1, 2010
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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
Class #27: Monday, November 1, 2010
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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
Class #27: Monday, November 1, 2010
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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
Class #27: Monday, November 1, 2010
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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 lowpressure 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