Chapter 9

Small-scale winds interacting with the environment Scales of motion

Micro, meso, synoptic

Friction and Turbulence in Boundary LayerLaminar flowIrregular turbulent motionPlanetary boundary layer (PBL)Depth of PBL

Small-scale winds interacting with the environment Observations: Eddies & Air Pockets

Eddies on leeward side of solid objectRoll eddies, mountain wave eddy (clear air

turbulence)Increase wind speed/shear deforms layer

into wave and air pocket.

Force of the WindBridges and hills or rises can modify wind,

increasing the force at specific locations.

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)

Determining wind speed and direction Influence of Prevailing Winds

Prevailing most frequently observed direction during a given time period

Impact human and natural landscapeWind rose

Topic: Wind PowerTurbines need moderate, steady windsOnly a few percent of nation’s total energy

needs generated by wind power in the first half of the century

Local Winds

Thermal circulationHeating 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.

Local Winds

Sea and Land BreezeUneven heating of land and waterDay: land hot, water cold = sea breezeNight: water hot, land cold = land breezeSea breeze front, sea breeze convergence

Local Winds and WaterLocal winds will change speed and direction

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

Local Winds

Seasonally Changing Winds: The MonsoonArabic for seasonalWinds change direction seasonably causing

extreme dry and wet seasonEastern and southern Asia, North America

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 windsCold wind rushes down elevated slopes,

usually 10 kts or less but can reach hurricane strength

Local Winds

Chinook/Foehn WindsDry warm descending on the leeward side of

a orographic barrierEastern slope of Rockies (chinook), Europe

(foehn), Argentina (zonda)

Focus: Snow EatersThirsty wind on east side of Rockies that eat

or melt snow due to rapid change in temperature

Local Winds

Santa Anna WindsWarm dry that blows from east or northeast

don canyons into S. CaliforniaVery fast, desiccates vegetation, providing

fuel for fires

Desert windsDust storms, sand storms, dust devil,

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