Unformatted text preview: Chapter 6: Air Pressure and Winds Atmospheric pressure Measuring air pressure Surface and upperair charts Why the wind blows Surface winds Measuring and determining winds Atmospheric Pressure air pressure definition Air pressure is, quite literally the weight of the atmosphere above us. air pressure and temperature P = RT at constant P, cold parcel is denser; at constant T, higher surface P means denser air; at constant density, higher surface P means higher air T pressure gradient force Same density Stepped Art Fig. 6-2, p. 143 Measuring air Pressure
Standard atmospheric pressure: 1013.25 mb = 1013.25 hPa = 29.92 in.Hg Barometers mercury barometer aneroid barometer altimeter barograph Pressure Readings after corrections of station pressure temperature, gravity, and instrument error (surface tension of mercury) after corrections of sealevel pressure isobars altitude (1mb/10m) Surface and Upper Air Charts isobaric maps contour lines ridges troughs Color-filled contour maps are the same as ordinary contour maps, except that the area between adjacent lines is filled in with color. Figure 2, p. 150 Why the Wind Blows
Newton's Laws of Motion An object at rest (or in motion) will remain at rest (or in motion) as long as no force is exerted on the object Newton's first law F = ma (force = mass times the acceleration) Newton's second law acceleration could be change of speed or direction Four forces include pressure gradient force, Coriolis force, centripetal force (or its opposite, centrifugal force), and friction Forces that Influence the Wind net force and fluid movement Wind is the result of a balance of several forces. Pressure Gradient Force pressure gradient (pressure difference/distance) pressure gradient force (from high to low pressure) strength and direction of the pressure gradient force The horizontal (rather than the vertical) pressure gradient force is responsible for causing air to move. Coriolis Force
real and apparent forces Coriolis force strength and direction of the Coriolis force factors that affect the Coriolis force object's speed, earth rotation, latitude It is sometimes claimed that "water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres". This is not true. Fig. 6-14, p. 153 Straightline Flow Aloft combination of the pressure gradient and Coriolis forces geostrophic wind Geostrophic winds can be observed by watching the movement of clouds. Curved Winds Around Lows and Highs Aloft cyclonic and anticyclonic flow centripetal force (opposite to centrifugal force) gradient wind Winds on Upperlevel Charts gradients in contour lines meridional and zonal winds Height contours on upper-level charts are interpreted in the same way as isobars on surface charts. Figure 4, p. 157 Surface Winds planetary boundary layer friction frictional effects on the wind Most people rarely venture out of the planetary boundary layer. Fig. 6-21, p. 160 Winds and Vertical Motions divergence and convergence hydrostatic equilibrium (vertical PGF = gravity) Measuring and Determining Winds characterization of wind direction prevailing winds wind rose The Influence of Prevailing Winds Wind direction is defined in the opposite way as ocean currents: a southerly current means water is moving towards the south. Wind Instruments wind vane anemometer aerovane radiosonde wind profiler By observing flags and smoke plumes, our eyes are also effective wind instruments. Wind Power Fig. 6-29, p. 163 ...
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- Fall '07
- Force, pressure gradient force, coriolis force