• Equrighothducpar

• UnizoGra

• PGchathe

Medium

Low

w pressure

Lecture 8. Pressure Gradient Force

al pressures on the left andt side of an air parcel (or any

er object) cancel out to pro-e no net horizontal force oncel.

equal pressures produce a hor-ntal force called the Pressuredient Force (PGF).

F is stronger if pressurenges more rapidly between two sides of the parcel

Medium Pressure

No PGF

High

Net PGF from high to lo

• PG

F depends on the volume of the box, but not its shape.High Low

Net PGF from high to low pressure

High Low

Less pressure differenceacting over more surface area = same PGF

Same volumeDifferent shape

• Pre balances weightof a orizontal pres-sur

• Pre e, directed fromhig

On aa givPGF• poi

sur• is l

clo

mb)

ssure varies with height too, creating vertical PGF. Thisir in volume, so we normally discuss only PGF due to h

e variation, which is what makes winds.ssure Gradient Force is expressed as force per unit volumh to low pressure:

PGF =

contour map of pressure aten altitude above sea level,

nts from high to low pres-eargest where contours aresest.

Pressure Change (mb)Distance (km)

------------------------------------------------------

Pressure(at sea level)

H (1025

1024

102210201018

1016

1014

1012L

• Sur though sea-lev g the actual sta-tion altitude (exactfor

• By e variations atsea

• To ssure close to100 he stations withpre an 1008 mb. Tokee some stationsslig

Surface Pressure Maps

face pressure maps usually plot sea-level pressure, evenel can be far below the ground! This is found by correctin pressure for altitude by adding about 1 mb per 10 m of

mula depends on station temperature). contouring sea-level pressure, we see horizontal pressur-level from which we can deduce PGF.draw a contour, e. g. 1008 mb, look for stations with pre8 mb. The contour should be smoothly drawn to divide tssure less than 1008 mb from those with pressure more thp the contour smooth, it is sometimes necessary to leavehtly on the wrong side of the contour

EOM 6.7

PGF

Sea Level Pressure example

Uppe ight of a surfaceof co a-level.

• At altitude correc-tion s 5640 m.

• Hig 0 m level.• PG ontours closer.

7

r air pressure maps contour geopotential height, the henstant pressure (e.g. the 500 mb height surface) above se

a fixed elevation (e. g. 5640 m), pressure can be found by - adding 1 mb for every 15 m the 500 mb height exceedh 500 mb heights correspond to high pressure at the 564F points from high to low heights, larger where height c

504 mb

496 mb500 mb

PGF

EOM, p. 14

• Dotemnor

• Sol500

• ThiPGhigandtou

• Thiwin

• Whalowhthe

EOM 6.8b

A typical wintertime 500 mb map

tted lines showperature (colder toth).id contours show mb height.ck arrows showF, pointing fromh to low heights, larger where con-rs are close.n arrows showd direction.y is wind directionng the contoursen PGF is down contours?

Newt• 1. ill remain in

mo ong as no forcesare

(e

2.

(e

• The ould rapidlyacc

• Bu round) friction.

Effect of Pressure Gradient Force on Wind

on’s Laws:An object at rest remains at rest. An object in motion wtion (travelling at constant velocity in a straight line) as l exerted on the object

. g. hockey puck)

Force = Mass × Acceleration

. g. slapping hockey puck with a stick)

PGF produces a horizontal force which, acting alone, welerate air from high toward low pressure.

t two other horizontal forces: Coriolis ‘force’ and (near g

H L(1030 mb) (1000 mb)

800 km

PGF

1 hr 2 hr 3 hr40 m/s = 80 mph!