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  • 1. Chapter 5: AtmosphericPressure and WindMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  • 2. © 2011 Pearson Education, Inc.Atmospheric Pressure and Wind• The Impact of Pressure and Wind on theLandscape• The Nature of Atmospheric Pressure• The Nature of Wind• Vertical Variations in Pressure and Wind• The General Circulation of theAtmosphere• Modifications of the General Circulation2
  • 3. © 2011 Pearson Education, Inc.Atmospheric Pressure and Wind• Localized Wind Systems• El Niño-Southern Oscillation• Other Multiyear Atmospheric and OceanicCycles3
  • 4. © 2011 Pearson Education, Inc.The Impact of Pressure and Windon the Landscape• Atmospheric pressure indirectly affects thelandscape• Changes manifest primarily by changes in wind andtemperature• Wind has a visible component to its activity• Severe storm winds can drastically affect thelandscape4
  • 5. © 2011 Pearson Education, Inc.The Nature of AtmosphericPressure• Gas moleculescontinuously in motion• Force exerted by gasmolecules is calledatmospheric pressure• Force exerted on everysurface the gas touches• Pressure isapproximately 14 lbsper square inch5Figure 5-1
  • 6. © 2011 Pearson Education, Inc.The Nature of AtmosphericPressure• Factors influencingatmospheric pressure– Density—at higherdensity, particles arecloser and collide morefrequently, increasingpressure– Temperature—warmerparticles move fasterand collide morefrequently, increasingpressure6Figure 5-3
  • 7. © 2011 Pearson Education, Inc.The Nature of AtmosphericPressure• Dynamic influences on air pressure– Strongly descending air, a dynamic high– Very cold surface conditions, a thermal high– Strongly ascending air, a dynamic low– Very warm surface conditions, a thermal low• Dynamic influences work in tandem with influencesfrom density to affect air pressure7
  • 8. © 2011 Pearson Education, Inc.The Nature of AtmosphericPressure• Mapping pressure withisobars– Pressure measured witha barometer– Typical units aremillibars or inches ofmercury– Contour pressure valuesreduced to sea level– Shows highs and lows,ridges and troughs8Figure 5-4
  • 9. © 2011 Pearson Education, Inc.The Nature of Wind• Origination of wind– Uneven heating ofEarth’s surface createstemperature andpressure gradients– Direction of wind resultsfrom pressure gradient– Winds blow from highpressure to low pressure9Figure 5-5
  • 10. © 2011 Pearson Education, Inc.The Nature of Wind• Forces which govern the wind– Pressure gradient force• Characterized by wind moving from high to low pressure,always• Winds blow at right angles to isobars– Coriolis force• Turns wind to the right in the Northern Hemisphere, leftin Southern Hemisphere• Only affects wind direction, not speed, though fasterwinds turn more– Friction• Wind is slowed by Earth’s surface due to friction, doesnot affect upper levels10
  • 11. © 2011 Pearson Education, Inc.The Nature of Wind• Force balances– Geostrophic balance• Balance between pressuregradient force and Coriolis• Winds blow parallel toisobars– Frictional balance• Winds blow slightly towardslow pressure and slightlyaway from high pressure• Winds slowed by frictionweaken Coriolis, so pressuregradient force is strongerand turns the winds11Figure 5-6
  • 12. © 2011 Pearson Education, Inc.The Nature of Wind• Anticyclones and cyclones12Figure 5-8
  • 13. © 2011 Pearson Education, Inc.The Nature of Wind• Vertical motions– Surface convergence andlow pressure indicate risingmotion– Surface divergence andhigh pressure indicatesinking motion– Rising motion results inclouds and storms– Sinking motion results insunny skies13Figure 5-9
  • 14. © 2011 Pearson Education, Inc.The Nature of Wind• Wind speed– Tight pressure gradients(isobars close together)indicate faster wind speeds– Wind speeds are gentle onaverage14Figure 5-11Figure 5-10
  • 15. © 2011 Pearson Education, Inc.Vertical Variations in Pressureand Wind• Atmospheric pressuredecreases rapidly withheight• Atmospheric surfacepressure centers leanwith height• Winds aloft are muchfaster than at the surface• Jet streams15
  • 16. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere16• Atmosphere is in constant motion• Major semipermanent conditions of wind andpressure—general circulation• Principal mechanism for longitudinal andlatitudinal heat transfer• Second only to insolation as a determinationfor global climate
  • 17. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• Simple example: A non-rotating Earth– Strong solar heating at equator– Little heating at poles– Thermal low pressure formsover equator– Thermal high forms over poles– Ascending air over equator– Descending air over poles– Winds blow equatorward atsurface, poleward aloft17Figure 5-12
  • 18. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• Observed general circulation– Addition of Earth’s rotationincreases complexity ofcirculation– One semipermanentconvective cell near theequator– Three latitudinal wind beltsper hemisphere– Hadley cells18Figure 5-14
  • 19. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• Seasonal differences inthe general circulation19Figure 5-15
  • 20. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• Components of thegeneral circulation– Subtropical highs• Persistent zones of highpressure near 30° latitudein both hemispheres• Result from descending airin Hadley cells• Subsidence is commonover these regions• Regions of world’s majordeserts• No wind, horse latitudes20Figure 5-16
  • 21. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Trade winds• Diverge from subtropicalhighs• Exist between 25°N and25°S latitude• Easterly winds:southeasterly in SouthernHemisphere, northeasterlyin Northern Hemisphere• Most reliable of winds• “Winds of commerce”21Figure 5-17
  • 22. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Trade winds (cont.)• Heavily laden withmoisture• Do not produce rainunless forced to rise• If they rise, theyproduce tremendousprecipitation and stormconditions22Figure 5-20
  • 23. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Intertropical ConvergenceZone (ITCZ)• Region of convergenceof the trade winds• Constant rising motionand storminess in thisregion• Position seasonallyshifts (more over landthan water)• Doldrums23Figure 5-21
  • 24. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Westerlies• Form on poleward sidesof subtropical highs• Wind system of themidlatitudes• Two cores of high winds– jet streams• Rossby waves24Figure 5-22Figure 5-24
  • 25. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Polar highs• Thermal highs that develop over poles due toextensive cold conditions• Winds are anticyclonic; strong subsidence• Arctic desert– Polar easterlies• Regions north of 60°N and south of 60°S• Winds blow easterly• Cold and dry25
  • 26. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere– Polar front• Low pressure areabetween polar high andwesterlies• Air mass conflict betweenwarm westerlies and coldpolar easterlies• Rising motion andprecipitation• Polar jet stream positiontypically coincident withthe polar front26Figure 5-25
  • 27. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• The seven components of the general circulation27Figure 5-26
  • 28. © 2011 Pearson Education, Inc.The General Circulation of theAtmosphere• Vertical wind patterns ofthe general circulation– Most dramaticdifferences in surfaceand aloft winds is intropics– Antitrade winds28Figure 5-28
  • 29. © 2011 Pearson Education, Inc.Modifications of the GeneralCirculation• Seasonal modifications– Seven generalcirculation componentsshift seasonally– Components shiftnorthward duringNorthern Hemispheresummer– Components shiftsouthward duringSouthern Hemispheresummer29Figure 5-29
  • 30. © 2011 Pearson Education, Inc.Modifications of the GeneralCirculation• Monsoons– Seasonal wind shift of up to180°– Winds onshore duringsummer– Winds offshore duringwinter– Develop due to shifts inpositions of ITCZ andunequal heating of land andwater30Figure 5-30
  • 31. © 2011 Pearson Education, Inc.Modifications of the GeneralCirculation• Major monsoonsystems31Figure 5-32
  • 32. © 2011 Pearson Education, Inc.Modifications of the GeneralCirculation• Minor monsoonsystems32Figure 5-33
  • 33. © 2011 Pearson Education, Inc.Localized Wind Systems• Sea breezes– Water heats more slowlythan land during the day– Thermal low over land,thermal high over sea– Wind blows from sea to land• Land breezes– At night, land cools faster– Thermal high over land,thermal low over sea– Wind blows from land to sea33Figure 5-34
  • 34. © 2011 Pearson Education, Inc.Localized Wind Systems• Valley breeze– Mountain top during the dayheats faster than valley, creatinga thermal low at mountain top– Upslope winds out of valley• Mountain breeze– Mountain top cools faster atnight, creating thermal high atmountain top– Winds blow from mountain tovalley, downslope34Figure 5-35
  • 35. © 2011 Pearson Education, Inc.Localized Wind Systems• Katabatic winds– Cold winds that originate fromcold upland areas, bora winds– Winds descend quickly downmountain, can be destructive• Foehn/Chinook winds– High pressure on windwardside of mountain, lowpressure on leeward side– Warm downslope winds– Santa Ana winds35Figure 5-36
  • 36. © 2011 Pearson Education, Inc.El Niño-Southern Oscillation• Warming of waters in theeastern equatorial Pacific• Associated withnumerous changes inweather patternsworldwide• Typically occurs on timescales of 3 to 7 years forabout 18 months36Figure 5-37
  • 37. © 2011 Pearson Education, Inc.El Niño-Southern Oscillation• Circulation patterns—Walker circulation37Figure 5-38
  • 38. © 2011 Pearson Education, Inc.El Niño-Southern Oscillation• Patterns associated withEl Niño• ENSO—Southernoscillation• La Niña—opposite of ElNiño• Causes of El Niño– Atmosphere changes firstor ocean changes first?– Weather effects of El Niño38Figure 5-40
  • 39. © 2011 Pearson Education, Inc.Other Multiyear Atmospheric andOceanic Cycles• Pacific decadal oscillation(PDO)• North Atlantic Oscillation(NAO) and ArcticOscillation (AO)39Figure 5-41
  • 40. © 2011 Pearson Education, Inc.Summary• Atmospheric pressure and wind affect the geographiclandscape in several ways• Atmospheric pressure is the force exerted by airmolecules on all objects the air is in contact with• Pressure is influenced by temperature, density, anddynamic• Isobars show areas of high pressure and low pressure• Vertical and horizontal atmospheric motions are calledwind• Wind is affected by many forces40
  • 41. © 2011 Pearson Education, Inc.Summary• Geostrophic balance represents a balance between theCoriolis force and the pressure gradient force• Friction slows the wind and turns it towards lowerpressure• Wind patterns around high and low pressure systemsare anticyclonic and cyclonic, respectively• Areas of divergence at the surface are associated withsinking motion, convergence at the surface with risingmotion• Close isobar spacing indicates faster winds41
  • 42. © 2011 Pearson Education, Inc.Summary• Winds increase rapidly with height, pressure decreasesrapidly with height• The global atmospheric circulation is called the generalcirculation• There are seven components to the general circulation• Each component has associated weather conditions• Seasonal modifications to the general circulation exist,including monsoons• Localized wind systems affect wind direction locally ondiurnal time scales42
  • 43. © 2011 Pearson Education, Inc.Summary• El Niño is a warming of eastern equatorial Pacific waterand subsequent switching of the high and low airpressure patterns• El Niño is associated with varied weather patterns indifferent locations globally• Other examples of teleconnections include the PDO andthe NAO/AO.43