Lecture 8 Pressure Gradient Force Equal pressures on the left and right side of an air parcel or any other object cancel out to produce no net horizontal force on parcel No PGF Medium 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 Pressure Medium High Net PGF from high to low pressure Low PGF depends on the volume of the box but not its shape High Low Net PGF from high to low pressure High Same volume Different shape Low Less pressure difference acting over more surface area same PGF Pressure varies with height too creating vertical PGF This balances weight of air in volume so we normally discuss only PGF due to horizontal pressure variation which is what makes winds Pressure Gradient Force is expressed as force per unit volume directed from high to low pressure Pressure Change mb PGF Distance km 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 Pressure at sea level H 1025 mb 1024 1022 1020 1018 1016 1014 L 1012 Surface Pressure Maps Surface pressure maps usually plot sea level pressure even though sealevel can be far below the ground This is found by correcting the actual station pressure for altitude by adding about 1 mb per 10 m of altitude exact formula depends on station temperature By contouring sea level pressure we see horizontal pressure variations at sea level from which we can deduce PGF To draw a contour e g 1008 mb look for stations with pressure close to 1008 mb The contour should be smoothly drawn to divide the stations with pressure less than 1008 mb from those with pressure more than 1008 mb To keep the contour smooth it is sometimes necessary to leave some stations slightly on the wrong side of the contour Sea Level Pressure example PGF EOM 6 7 Upper air pressure maps contour geopotential height the height of a surface of constant pressure e g the 500 mb height surface above sea level EOM p 147 500 mb 504 mb 496 mb PGF At a fixed elevation e g 5640 m pressure can be found by altitude correction 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 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 EOM 6 8b Effect of Pressure Gradient Force on Wind Newton s Laws 1 An object at rest remains at rest An object in motion will remain in motion travelling at constant velocity in a straight line as long as no forces are 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 rapidly accelerate air from high toward low pressure 800 km PGF H 1030 mb L 1000 mb 1 hr 2 hr 3 hr 40 m s 80 mph But two other horizontal forces Coriolis force and near ground friction
View Full Document
Unlocking...