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

Review for test #3 Chapter 9, pp ; skip Box 9.2 on pp Chapter 10, pp ; skip pages Chapter 11, all of pages Chapter 12, pages Class #27: Monday, November 1, 20102

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, 20103

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, 20104

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, 20105

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, 20106

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, 20108

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 Class #27: Monday, November 1,

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,

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,

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 Class #27: Monday, November 1,

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,

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 Class #27: Monday, November 1,

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,

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,

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 Class #27: Monday, November 1,

Fig , p. 363

Fig , 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 Class #27: Monday, November 1,

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,

Fig , p. 365

Fig a, p. 365

Fig b, p. 365

Fig , 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 Class #27: Monday, November 1,

Fig , 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 Class #27: Monday, November 1,

Fig , p. 367

Beneath a lenticular cloud Fig , 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 Class #27: Monday, November 1,

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,

Fig , 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 Class #27: Monday, November 1,

Fig , p. 369

Fig , p. 370