Class #6, Wednesday, July 14, 2010 1

Class #6: Wednesday, July 14 Small scale and local winds

Chapter 9

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Wind: small scale and local systems

Chapter 9

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Small-scale winds interacting with the environment

• Scales of motion– Micro, meso, synoptic

• Friction and Turbulence in Boundary Layer– Laminar flow– Irregular turbulent motion– Planetary boundary layer (PBL)– Depth of PBL

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Small-scale winds interacting with the environment

• Observations: Eddies & Air Pockets– Eddies on leeward side of solid object– Roll eddies, mountain wave eddy (clear air

turbulence)– Increase wind speed/shear deforms layer

into wave and air pocket.• Force of the Wind– Bridges and hills or rises can modify wind,

increasing the force at specific locations.

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Small-scale winds interacting with the environment

• Microscale winds blowing over the Earth’s surface– Wind erosion, desert pavements, sand

ripples– Snow ripples, snow dunes, snow rollers,

snow fences– Windbreak, shelter belt

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Determining wind speed and direction

• Wind characterized by direction, speed, and gustiness

• Wind direction describes the direction from which it is blowing

• Topic: Pedaling in the Wind– As wind blows against an object it exerts force (V2

or P x A)

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Determining wind speed and direction

• Influence of Prevailing Winds– Prevailing most frequently observed direction during a

given time period– Impact human and natural landscape– Wind rose

• Topic: Wind Power– Turbines need moderate, steady winds– Only a few percent of nation’s total energy needs

generated by wind power in the first half of the century

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Determining wind speed and direction

• Wind Measurements– Wind vane– Pressure plate anemometer– Cup anemometer– Aerovane– Rawinsonde– Wind soundings

• Observations: Winds from space– QuickScat: scatterometer

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Local Winds

• Thermal circulation– Heating and cooling of the atmosphere above the

ground create cold, core high and warm, core low pressure cells.

– Wind travels from high to low and rises until it cools and begins to sink.

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Local Winds

• Sea and Land Breeze– Uneven heating of land and water– Day: land hot, water cold = sea breeze– Night: water hot, land cold = land breeze– Sea breeze front, sea breeze convergence

• Local Winds and Water– Local winds will change speed and direction as

they cross a large body of water due to less friction, greater speed and greater Coriolis effect

Class #6, Wednesday, July 14, 2010 22Fig. 9-22, p. 239

Class #6, Wednesday, July 14, 2010 23Fig. 9-23, p. 239

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Class #6, Wednesday, July 14, 2010 26Stepped Art

Fig. 9-25, p. 241

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Local Winds

• Seasonally Changing Winds: The Monsoon– Arabic for seasonal– Winds change direction seasonably causing

extreme dry and wet season– Eastern and southern Asia, North America

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Local Winds

• Mountain and Valley Breeze– On mountain slopes, warm air rises during the day

creating a valley breeze; during night nocturnal drainage of cool air creating a mountain breeze

– Associated with cumulus clouds in the afternoon• Katabatic winds– Cold wind rushes down elevated slopes, usually 10

kts or less but can reach hurricane strength

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Local Winds

• Chinook/Foehn Winds– Dry warm descending on the leeward side of a

orographic barrier– Eastern slope of Rockies (chinook), Europe

(foehn), Argentina (zonda)• Focus: Snow Eaters– Thirsty wind on east side of Rockies that eat or

melt snow due to rapid change in temperature

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Local Winds

• Santa Anna Winds– Warm dry that blows from east or northeast don

canyons into S. California– Very fast, desiccates vegetation, providing fuel for

fires• Desert winds– Dust storms, sand storms, dust devil, haboob

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Wind: small scale and local systems

Chapter 9