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Lecture 8. Pressure Gradient Force• Equal pressures on the left and right side of an air parcel (or any other object) cancel out to...• Unequal pressures produce a horizontal force called the Pressure Gradient Force (PGF).• PGF is stronger if pressure changes more rapidly between the two sides of the parcel• PGF depends on the volume of the box, but not its shape.• Pressure varies with height too, creating vertical PGF. This balances weight of air in volume, ...• Pressure Gradient Force is expressed as force per unit volume, directed from high to low pressure:PGF =On a contour map of pressure at a given altitude above sea level, PGF• points from high to low pressure• is largest where contours are closest.Surface Pressure Maps• Surface pressure maps usually plot sea-level pressure, even though sea- level can be far below ...• By contouring sea-level pressure, we see horizontal pressure variations at sea-level from which...• To draw a contour, e. g. 1008 mb, look for stations with pressure close to 1008 mb. The contour...Upper air pressure maps contour geopotential height, the height of a surface of constant pressure...• At a fixed elevation (e. g. 5640 m), pressure can be found by altitude correction - adding 1 mb...A typical wintertime 500 mb map• Dotted lines show temperature (colder to north).• Solid contours show 500 mb height.• Thick arrows show PGF, pointing from high to low heights, and larger where contours are close.• Thin arrows show wind direction.• Why is wind direction along the contours when PGF is down the contours?Effect of Pressure Gradient Force on WindNewton’s Laws:• 1. An object at rest remains at rest. An object in motion will remain in motion (travelling at ...(e. g. hockey puck)2. Force = Mass ¥ Acceleration(e. g. slapping hockey puck with a stick)• The PGF produces a horizontal force which, acting alone, would rapidly accelerate air from high...• But two other horizontal forces: Coriolis ‘force’ and (near ground) friction.Lecture 8. Pressure Gradient Force• Equal pressures on the left andright side of an air parcel (or anyother object) cancel out to pro-duce no net horizontal force onparcel.• Unequal pressures produce a hor-izontal force called the PressureGradient Force (PGF).• PGF is stronger if pressurechanges more rapidly betweenthe two sides of the parcelMedium MediumPressureNo PGFHigh LowNet PGF from high to low pressure• PGF depends on the volume of the box, but not its shape.High LowNet PGF from high to low pressureHigh LowLess pressure differenceacting over more surface area = same PGFSame volumeDifferent shape• Pressure varies with height too, creating vertical PGF. This balances weightof air in volume, so we normally discuss only PGF due to horizontal pres-sure variation, which is what makes winds.• Pressure Gradient Force is expressed as force per unit volume, directed fromhigh to low pressure:PGF =On a contour map of pressure ata given altitude above sea level,PGF• points from high to low pres-sure• is largest where contours areclosest.Pressure Change (mb)Distance (km)------------------------------------------------------Pressure(at sea level)H (1025 mb)1024102210201018101610141012LSurface Pressure Maps• Surface pressure maps usually plot sea-level pressure, even though sea-level can be far below the ground! This is found by correcting the actual sta-tion pressure for altitude by adding about 1 mb per 10 m of altitude (exactformula depends on station temperature).• By contouring sea-level pressure, we see horizontal pressure variations atsea-level from which we can deduce PGF.• To draw a contour, e. g. 1008 mb, look for stations with pressure close to1008 mb. The contour should be smoothly drawn to divide the stations withpressure less than 1008 mb from those with pressure more than 1008 mb. Tokeep the contour smooth, it is sometimes necessary to leave some stationsslightly on the wrong side of the contourEOM 6.7PGFSea Level Pressure exampleUpper air pressure maps contour geopotential height, the height of a surfaceof constant pressure (e.g. the 500 mb height surface) above sea-level.• At a fixed elevation (e. g. 5640 m), pressure can be found by altitude correc-tion - adding 1 mb for every 15 m the 500 mb height exceeds 5640 m.• High 500 mb heights correspond to high pressure at the 5640 m level.• PGF points from high to low heights, larger where height contours closer.504 mb496 mb500 mbPGFEOM, p. 147A typical wintertime 500 mb map• Dotted lines showtemperature(coldertonorth).• Solid contours show500 mb height.• Thick arrows showPGF, pointing fromhigh to low heights,and larger where con-tours are close.• Thin arrows showwind direction.• Why is wind directionalong the contourswhen PGF is downthe contours?EOM 6.8bEffect of Pressure Gradient Force on WindNewton’s Laws:• 1. An object at rest remains at rest. An object in motion will remain inmotion (travelling at constant velocity in a straight line) as long as no forcesare exerted on the object(e. g. hockey puck)2. Force = Mass × Acceleration(e. g. slapping hockey puck with a stick)• The PGF produces a horizontal force which, acting alone, would rapidlyaccelerate air from high toward low pressure.• But two other horizontal forces: Coriolis ‘force’ and (near ground) friction.HL(1030 mb)(1000 mb)800 kmPGF1 hr 2 hr 3 hr40 m/s = 80


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UW ATMS 101 - Lecture Notes

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