11 Air Pressure and Winds

11 Air Pressure and Winds - Air pressure and winds GEOG345...

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Unformatted text preview: Air pressure and winds GEOG345 Oct 12 2006 Textbook: chapter 8 Fig. 8-CO, p. 190 Atmospheric pressure • Mass of air above a given level – weight = mass * gravity -> weight of air above a given level • Air pressure decreases with altitude • The rate of decrease is not constant – consequence of the compression of the air • Air pressure, density and temperature are interrelated -> gas law Gas law (equation of state) Pressure = temperature X density X constant p = T X ρ X C C=287J/kgK Constant temperature: p ~ ρ Constant density: p ~ T Constant pressure: T~ 1/ ρ Fig. 8-1, p. 192 Constant temperature (p ~ ρ ) More air molecules – higher density – higher surface pressure Fig. 8-1, p. 192 Constant temperature (p ~ ρ ) More air molecules – higher density – higher surface pressure Fig. 8-2, p. 193 Constant (initial) pressure (T~ 1/ ρ) Density decreases with increasing temperature (i.e. volume of air column increases) Pressure is lower at the same altitude in the cold air column – pressure gradient force – causes air to move Fig. 8-3, p. 193 Diurnal fluctuation of air pressure density oscillations Pressure measurements • Pressure is force over a given area: Pressure = force / area [Pa (Pascal); N/m 2 ] • 1 millibar (mb) = 1 hectopascal (hPa) = 100 Pa • Use of gas law (p = T X ρ X C) – global mean temperature: 288K – air density at sea level: 1.226 kg/m 3 – surface air pressure: p= 288K X 1.226 kg/m 3 X 287 J/kgK [1J=1Nm] Fig. 8-5, p. 196 Mercury barometer weight of mercury column = weight of air column for standard atmospheric pressure (1013.25 mb): 76 cm mercury has a high density – 13.6 times that of water Fig. 8-4, p. 196 Atmospheric pressure values highest pressure: ~ 70 mb higher than standard lowest pressure: ~ 140 mb lower than standard Fig. 8-6, p. 196 Aneroid barometer air partially removed typical weather conditions often printed an altimeter also shows altitude Fig. 8-7, p. 197 Aneroid barometer Pressure readings - corrections • Temperature correction – consider expansion and contraction with temperature • Gravity correction – non-spherical shape of Earth, altitude of the station (with regard to changing gravity)...
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This note was uploaded on 04/10/2008 for the course GEOG 345 taught by Professor Csiszar during the Fall '06 term at Maryland.

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11 Air Pressure and Winds - Air pressure and winds GEOG345...

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