Class #5: Air pressure and winds
Chapter 8
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Chapter 8
Air pressure and winds
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Atmospheric Pressure
• What causes air pressure to change in the
horizontal?
• Why does the air pressure change at the
surface?
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Atmospheric Pressure
• Horizontal Pressure Variations
– It 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
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Atmospheric Pressure
• Special Topic: Gas Law
P is proportional to T x ρ
P = pressure
T = temperature
ρ = density
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Atmospheric Pressure
• Daily Pressure Variations
– Thermal tides in the tropics
– Mid-latitude pressure variation driven by
transitory pressure cells
• Pressure Measurements
– Barometer, barometric pressure
• Standard atmospheric pressure 1013.25mb
– Aneroid barometers
• Altimeter, barograph
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Atmospheric Pressure
• Pressure Readings
– Instrument error: temperature, surface tension
– Altitude corrections: high altitude add pressure,
10mb/100m above sea level
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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
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Table 8-1, p. 203
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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 elevation
– Radio altimeter offers constant elevation
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Fig. 2, p. 204
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Fig. 3, p. 204
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.
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Forces that Influence Winds
• Pressure Gradient Force: difference in
pressure over distance
– Directed 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.
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Forces that Influence Winds
• Coriolis Force
– Apparent deflection due to rotation of the Earth
– Right in northern hemisphere and left in southern
hemisphere
– Stronger wind = greater deflection
– No Coriolis effect at the equator greatest at poles.
– Only influence direction, not speed
– Only has significant impact over long distances
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Forces that Influence Winds
• Geostrophic Winds
– Earth turning winds
– Travel parallel to isobars
– Spacing of isobars indicates speed; close = fast,
spread out = slow
• Topic: Math & Geostrophic Winds
Vg = 1 x Δp
fρ d
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Fig. 4, p. 211
Forces that Influence Winds
• Gradient Winds Aloft
– Cyclonic: counterclockwise
– Anticyclonic: clockwise
– Gradient wind parallel to curved isobars
– Cyclostrophic near Equator
• Observation: Estimates Aloft
– Clouds indicate direction of winds, place pressure
in location consistent with cloud location.
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Fig. 5, p. 212
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Fig. 8-29, p. 214
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Fig. 6, p. 215
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 Winds
– Friction 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 pressure
– Buys-Ballots Law
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Fig. 8-32, p. 217
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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
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Fig. 7, p. 218
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Fig. 8-35, p. 220
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Fig. 8-36, p. 221
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Fig. 8-CO, p. 192