Class #5: Air pressure and winds Chapter 8 1Class #5 Tuesday, July 13, 2010.
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Transcript of Class #5: Air pressure and winds Chapter 8 1Class #5 Tuesday, July 13, 2010.
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Class #5 Tuesday, July 13, 2010 1
Class #5: Air pressure and winds
Chapter 8
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Class #5 Tuesday, July 13, 2010 2
Air pressure and winds
Chapter 8
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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 = pressureT = temperatureρ = 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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Class #5 Tuesday, July 13, 2010 23Table 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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Class #5 Tuesday, July 13, 2010 26Fig. 2, p. 204
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Class #5 Tuesday, July 13, 2010 27Fig. 3, p. 204
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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 Δpfρ d
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Class #5 Tuesday, July 13, 2010 42Fig. 4, p. 211
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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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Class #5 Tuesday, July 13, 2010 44Fig. 5, p. 212
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Class #5 Tuesday, July 13, 2010 49Stepped Art
Fig. 8-29, p. 214
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Class #5 Tuesday, July 13, 2010 50Fig. 6, p. 215
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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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Class #5 Tuesday, July 13, 2010 54Fig. 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 = -ρgΔz
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Class #5 Tuesday, July 13, 2010 58Fig. 7, p. 218
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Class #5 Tuesday, July 13, 2010 59Fig. 8-35, p. 220
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Class #5 Tuesday, July 13, 2010 60Fig. 8-36, p. 221
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Class #5 Tuesday, July 13, 2010 61Fig. 8-CO, p. 192