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Chapter 8. Air pressure and winds. Atmospheric Pressure. What causes air pressure to change in the horizontal? Why does the air pressure change at the surface?. Atmospheric Pressure. Horizontal Pressure Variations - PowerPoint PPT Presentation
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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