Chapter 12Chapter 12

Small-Scale WindsSmall-Scale Winds

Figure CO: Chapter 12, Small-Scale Winds--Fog over Golden Gate Bridge

© Andy Dean Photography/ShutterStock, Inc.

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

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

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

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

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

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

Figure B01: Photo of wave clouds breaking

© Kay Ekwall, www.mtshastaphotography.com

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

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

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

Figure 03: Wind flow

Source: SSEC, University of Wisconsin-Madison

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

Figure 04: Lines of clouds caused by gravity waves in the lee of the Appalachian Mountains

Courtesy of SSEC, University of Wisconsin-Madison

Figure 05: Water vapor image over Alabama

Courtesy of CIRA/Colorado State University and NOAA

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

Figure 07: Gravity wave climatology

Adapted from Koppel, L., et al., Monthly Weather Review, January 2000: 58

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

Figure 08: Lake breeze

Courtesy of SSEC, University of Wisconsin-Madison

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

Figure 09: Radar of derecho

Courtesy of NOAA

Figure 10: A climatology of derechos

Modified from Coniglio, M. C., and D. J. Stensrud, Wea. Forecasting 19 (2004): 595-605

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

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

Figure B02: Snow acts as a blanket in winter

Courtesy of Steven Ackerman

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

Figure B03: Dust storm

Courtesy of NOAA's National Weather Service (NWS) Collection

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

Source: Oklahoma Climatological Survey & The Oklahoma Mesonetwork

Figure 11A: Heat burst, 11B: Heat burst, 11C: Heat burst

Figure 12: Temperature and dew point plots for heat burstSource: NOAA

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

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

Figure 13: Mountain/valley breezes

Figure 14: Winds in the Boulder, Colorado, windstorm of February 2, 1999

Source: University Corporation for Atmospheric Research

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

Figure 15: Dust devil

Courtesy of NASA

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

Figure 16: Lenticular cloud

Courtesy of Cynthia Stoneburner

Figure 17: Wave cloud diagram

Figure 18: Flight turbulence

Adapted from Lester, P. Turbulence. Jeppesen, 1994.

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

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

Figure 19: Santa Ana Winds

Courtesy of JPL/NASA

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

Figure 20: von Kármán vortex

Courtesy of NASA/EROS, USGS

Figure 21: Global wind distribution