9/22/20101Geography 1001:Climate & VegetationLecture 8: Atmospheric CirculationSeptember 21, 2010Professor Holly BarnardOutline • Wind drivers– Pressure gradient– Coriolis– friction• High and Low Pressure Systems– Cyclone– Anticyclone • Global Circulation– Trade winds– westerliesGlobal wind direction: in a Non-rotating earthGlobal wind direction9/22/20102Driving Forces Withinthe Atmosphere• Gravity vertical winds• Pressure Gradient Force; PGF• Coriolis Acceleration• Friction Horizontal windsPressure GradientFigure 6.7PGF Key detail!• PGF• Coriolis– angular momentum– momentum = mass x velocity• FrictionCan get the wind going from ZeroOnly on objects already moving9/22/20103Wind Pattern DevelopmentCoriolis ForcePressure + Coriolis =Geostrophic WindWind Pattern Development9/22/20104Pressure + Coriolis + FrictionFigure 6.8Wind Pattern DevelopmentCyclones and AnticyclonesHurricane Francis (2004)9/22/20105Atmospheric Circulation• Primary (1º) Circulation: general worldwide circulation• Secondary (2º) Circulation: high and low pressure systems• Tertiary (3º) Circulation: local winds and temporal weather patterns Primary High-Pressure and Low-Pressure Areas• Equatorial low-pressure trough– 10°N to 10°S; warm, moist air• Polar high-pressure cells– Weak pressure cells– 90°N & 90°S; cold, dry air• Subtropical high-pressure cells– 20° to 35° N & S; hot, dry air• Subpolar low-pressure cellsAtmospheric Patterns of Motion1. Hadley Cells2. Ferrel Cells3. Polar CellsEquatorial low-pressure trough• Inter-Tropical Convergence Zone (ITCZ)– Low winds (low pressure gradient, vertical ascent)– doldrums• Trade winds– Return of Hadley cell circulation– Subtropical highs to equatorial low9/22/20106Seasonal Pressure and Precipitation PatternsSubtropical high-pressure cells• Westerlies• Bermuda high• Azores high• Pacific highFigure 6.14Subpolar low-pressure cells• Aleutian low• Icelandic low• Polar front9/22/20107Jet streams form at convergence of cellsGlobal Atmospheric Circulation ModelGeneral Atmospheric CirculationFigure 6.13General Atmospheric CirculationFigure 6.139/22/20108Ocean Circulation Patterns• Frictional drag of wind drives ocean currents.– Coriolis also has an effect but not as strong.• Most ocean circulation is driven by air circulating around high pressure cells• Currents bring cold water from poles toward equator along west coasts andbring warm water from tropics along east coasts• Ocean circulation cell called a “gyre”Ocean CirculationOcean CirculationSummary• Wind drivers– Pressure gradient– Coriolis (deflects to the right in N Hemisphere)– Friction (drag on surface)• High and Low Pressure Systems– Cyclone (counterclockwise rotation)– Anticyclone (clockwise rotation)• Global Circulation– Trade winds (east to west - equatorial)– Westerlies (west to east –
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