2 global cirkulation

Visualizing Physical Geography Copyright 2008 John Wiley and Sons Publishers Inc.

Chapter 5Winds and Global Circulation



Atmospheric PressureAtmospheric pressure: pressure exerted by the atmosphere because of the force of gravity acting upon the overlying column of airAtmospheric pressure at a single location varies slightly from day to dayBarometer: instrument that measures atmospheric pressure



Atmospheric PressureAir Pressure and AltitudeAtmospheric pressure decreases with altitude



Local Wind PatternsWinds are identified by the direction from which the wind comesWind vane?Anemometer?



Local Wind PatternsPressure GradientsWind is caused by differences in atmospheric pressure from one place to anotherAir tends to move from regions of high pressure to regions of low pressure

Pressure gradient: change of atmospheric pressure measured along a line at right angles to the isobarsIsobars ?


Pressure Gradients develop because of unequal heating in the atmosphere

Local Wind PatternsLocal WindsSanta Ana windsSea and land BreezesMountain and valley winds

Cyclones and AnticyclonesCoriolis Effect: effect of the Earths rotation that acts like a force to deflect a moving object on the Earths surface to the right in the northern hemisphere and to the left in the southern hemisphereActs at right angles to direction of motionDeflects to right in the northern hemisphere, left in the southern hemisphereNo deflection at equator, maximum deflection at poles

Cyclones and AnticyclonesCoriolis EffectBalance of Forces on a parcel of surface air:Pressure GradientCoriolis EffectStrength increases with speed of motionStrength decreases with latitudeFrictionExerted by ground surfaceProportional to wind speedActs opposite to direction of motion

Cyclones and AnticyclonesCyclone: center of low atmospheric pressureAir spirals inward (convergence) and upwardAssociated with cloudy, rainy weatherAnticyclone: center of high atmospheric pressureAir spirals downward and outward (divergence)Associated with fair weather


Global Wind and Pressure PatternsHadley Cell: low-latitude atmospheric circulation cell with rising air over the equatorial trough and sinking air over the subtropical high-pressure beltsIntertropical Convergence Zone (ITCZ): zone of convergence of air masses along the equatorial troughSubtropical high-pressure belts: belts of persistent high atmospheric pressure centered at about lat. 30 N and SPolar Front: front lying between cold polar air masses and warm tropical air masses



Global Wind and Pressure PatternsITCZ and Monsoon CirculationITCZ and Hadley cells shift with the seasonsShift is very large in Asia


ITCZ and Monsoon CirculationMonsoon: seasonal reversal of winds

Winds AloftPressure decreases less rapidly with height in warmer air than in colder airThere is a temperature gradient from the equator to the polesSo, at high altitudes there is a pressure gradient from the equator to the polesstrong winds at high altitudes

Winds AloftThe Geostrophic WindForces acting on an upper air parcel:1. Pressure gradient force:Moves from high pressure to low pressure2. Coriolis forceDeflects to right in NH, to left in SH Pressure gradient force balances coriolis forceWind blows parallel to isobarsGeostrophic Wind: wind at high levels above the Earths surface blowing parallel with a system of straight parallel isobars

Winds AloftGlobal Circulation at Upper Levels:

Winds AloftRossby Waves, Jet Streams, and the Polar FrontRossby waves: horizontal undulations in the flow path of the upper-westerlies; upper-air wavesPolar front: zone where cold polar air meets warm tropical airRossby waves:Arise in the polar frontContribute to variable weather in mid-latitudes

Flow of air along front smooth for days or weeks. Undulations begin and Become stronger. RossbyWave forms. Warm airPushed poleward, cold air southWaves stronger. Tongue of cold airBrough south and warm north. Tongue pinched off. Pool of cold air farthersouth than originally. Become cyclones of cold air Persist for days of weeks.


Winds AloftJet streams: high-speed air flow in narrow bands within the upper-air westerlies and along certain other global latitude zones at high levels


Polar-front jet streamWesterly; associated with Rossby wavesSubtropical jet streamWesterly; above subtropical high-pressure cellsTropical easterly jet streamRuns east to westSummer onlyDevelops in Asia

Ocean CurrentsOceanic circulation:Currents exchange heat between high and low latitudesOcean current: persistent, dominantly horizontal flow of water


Ocean CurrentsLarge-Scale Circulation of Ocean Waters


Ocean CurrentsCurrent PatternsDuring an El Nio event:Upwelling along Peruvian coast ceasesTrade winds weakenWeak equatorial eastward current develops

Moisture and temperature patterns alter

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2 global cirkulation

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