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1Chapter 6: Air Pressure Measuring air pressureVariations due to temperature and water vaporDevelopment of pressure systemsGeneration of windsUnderstanding Air Pressure: -pressure exerted by the weight of air above-force exerted against a surface by the continuous collision of gas molecules -sea level pressure (slp) = 14.7 lbs/in2, 1kg/cm2Measuring Air Pressure: Newtons: unit of force (F = ma, kg m/s2)slp: 101,325 Newtons100 Newtons = 1 milibar (slp = 1013.25 mb)Inches of Mercury: mercury barometerslp = 29.92 in of mercuryMeasuring Air Pressure: aneroid barometer: metal chamber changes shape based on pressure, changes lever locationsMeasuring Air Pressure: Rising pressure: fair and dryFalling pressure: rainy, wet, stormy2Measuring Air Pressure: barograph: continuous log of aneroid barometer measurementsDigital barographsPressure Changes With Altitude: Density decreases w/ altitudeWeight of overlying column of air decreases with altitudePressure decreases w/ altitudehalves ~ every 5 km Pressure Altimeter: aneroid barometer marked in metersHigh flying commercial jets tend to fly along lines of constant pressureAdjusting to Sea-Level PressureCorrection depends on temperatureHorizontal Variations in Air Pressure: Horizontal pressure variations rarely exceed:30 mb > sea level pressure60 mb < sea level pressureIsobarsHorizontal Variations in Air Pressure:3Pressure Changes with TemperatureCold air mass temperaturesLow kinetic energySlow moving moleculesCloser togetherHigher PressurePressure Changes with TemperatureWarm air mass temperaturesHigh kinetic energyFast moving moleculesFurther apartLow PressurePressure Changes with Temperature Pressure Changes with TemperaturePressure Changes with Water VaporWater molecule lighter than N2and O2Adding water vapor displaces heavier moleculesAir pressure decreases1 atomic mass unit (u) ≈ 1.66053886 × 10−27kgAirflow and Pressure: Keep in mind atmosphere is 3DimensionalRegions of Convergence = pressure increasesRegions of Divergence = pressure decreasesSummarize: Cool, dry = high pressureWarm, moist = low pressure4Wind: Horizontal movement (advection) of airResults from horizontal differences in air pressureFactors Affecting Wind: 1. The pressure gradient force2. The Coriolis force3. frictionPressure Gradient: Pressure change occurring over a given distanceClosely spaced isobars = steep pressure gradient, stronger windsPGF = 1 change in pressureair density distanceHorizontal Pressure Gradient and WindHorizontal Pressure Gradient and Wind Vertical Pressure Gradientbalanced by gravitational forcehydrostatic balance or equilibriumlarge scale vertical movements are slow5Coriolis Force: Coriolis Force: Coriolis Force: Ex. 1hr move from pole to equatorEquatorial Circumference 40,076 kmCoriolis Force: Fc = 2νΩ sin (φ)ν = wind speedΩ = angular velocityφ = latitudeF = m(kg)a(m/s2)F/m = a (m/s2)Effect of FrictionGreatest near surfaceNegligible a few km above the surface6Winds Aloft and Geostrophic Flowpressure gradient force = (balanced by) coriolis forcegeostrophic balance, reach equilibrium, no accelerationgeostrophic winds flow in a straight line parallel to isobarsUpper Level Geostrophic WindsRidge = high pressureTrough = low pressureGeostrophic WindsHigh = anticyclones, clockwise flow (NH)Low = cyclones, counter clockwise flowGradient WindsNear the surface friction slows windspressure gradient force exceeds corriolis forceUnbalance force turns winds, centripetal accelerationSurface WindsFriction = reduces speed, weakens coriolis forceocean = 2/3 geostrophic speed (10-20oisobars)rugged terrain = 1/2 geostrophic speed (45oisobars)Surface H & L Pressure SystemsHigh = divergence, net flow of air away from the centerLow = convergence, net flow of air toward the center7Low Pressure System: vertical motionsurface convergencerising column of air (inc pres)divergence aloft to maintain systemHigh Pressure System: vertical motionsurface divergencedescending column of air (adiabatic warming, clear skies)convergence aloft to maintain systemWind MeasurementWind VanePrevailing WindCup AnemometerAerovaneWind


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CUNY PGEOG 130 - Air Pressure

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