Chapter 8

Atmospheric Pressure

What causes air pressure to change in the horizontal?

Why does the air pressure change at the surface?

Atmospheric Pressure

Horizontal Pressure VariationsIt takes a shorter column of dense, cold air

to exert the same pressure as a taller column of less dense, warm air

Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure

At surface, horizontal difference in temperature = horizontal pressure in pressure = wind

Atmospheric Pressure

Special Topic: Gas Law

P is proportional to T x ρ

P = pressure

T = temperature

ρ = density

Atmospheric Pressure

Daily Pressure VariationsThermal tides in the tropicsMid-latitude pressure variation driven by

transitory pressure cells

Pressure MeasurementsBarometer, barometric pressure

○ Standard atmospheric pressure 1013.25mbAneroid barometers

○ Altimeter, barograph

Atmospheric Pressure

Pressure ReadingsInstrument error: temperature, surface

tensionAltitude corrections: high altitude add

pressure, 10mb/100m above sea level

Surface and Upper Level Charts Sea-level pressure chart: constant

height Upper level or isobaric chart: constant

pressure surface (i.e. 500mb)High heights correspond to higher than

normal pressures at a given latitude and vice versa

Surface and Upper Level Charts Observation: Constant Pressure Surface

Pressure altimeter in an airplane causes path along constant pressure not elevation

May cause sudden drop in elevationRadio altimeter offers constant elevation

Newton’s Law of Motion

AN object at rest will remain at rest and an object in motion will remain in motion as long as no force is executed on the object.

The force exerted on an object equals its mass times the acceleration produced.Acceleration: speeding up, slowing down,

change of direction of an object.

Forces that Influence Winds Pressure Gradient Force: difference in

pressure over distanceDirected perpendicular to isobars from high

to low.Large change in pressure over s short

distance is a strong pressure gradient and vice versa.

The force that causes the wind to blow.

Forces that Influence Winds Coriolis Force

Apparent deflection due to rotation of the Earth (the rotation rate of Venus is so slow that the Coriolis force is extremely small on Venus)

Right in northern hemisphere and left in southern hemisphere

Stronger wind = greater deflectionNo Coriolis effect at the equator greatest at poles.Only influence direction, not speedOnly has significant impact over long distances

Forces that Influence Winds Geostrophic Winds

Earth turning windsTravel parallel to isobarsSpacing of isobars indicates speed; close =

fast, spread out = slow

Topic: Math & Geostrophic Winds

Vg = 1 x Δp

fρ d

Forces that Influence Winds Gradient Winds Aloft

Cyclonic: counterclockwiseAnticyclonic: clockwiseGradient wind parallel to curved isobarsCyclostrophic near Equator

Observation: Estimates AloftClouds indicate direction of winds, place

pressure in location consistent with cloud location.

Stepped Art

Fig. 8-29, p. 214

Forces that Influence Winds Winds on Upper-level Charts

Winds parallel to contour lines and flow west to east

Heights decrease from north to south

Surface WindsFriction reduces the wind speed which in

turn decrease the Coriolis effect.Winds cross the isobars at about 30° into

low pressure and out of high pressureBuys-Ballots Law

Winds and Vertical Motion Replacement of lateral spreading of air

results in the rise of air over a low pressure and subsidence over high pressure

Hydrostatic equilibrium and equation Topic: Hydrostatic equation

Δp = -ρg

Δz