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AIR PRESSURE FORCES AND WIND. PRESSURE GRADIENT FORCE. Definition: The difference in atmospheric pressure per unit distance PGF acts at right angles to isobars of equal pressure. H. L. 102.2. 99.8. 101.4. 100.6. 600 km. Pressure Gradient Force = 2.4 kPa / 600 km

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## AIR PRESSURE FORCES AND WIND

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**AIR PRESSURE**FORCES AND WIND**PRESSURE GRADIENT FORCE**Definition: The difference in atmospheric pressure per unit distance PGF acts at right angles to isobars of equal pressure H L 102.2 99.8 101.4 100.6 600 km Pressure Gradient Force = 2.4 kPa / 600 km = 0.4 kPa / 100 km**Where are winds strongest ?**Oct 18, 2004 Solution: HUDSON BAY Check the spacing of the isobars of equal surface pressure**CORIOLIS FORCE**Source: NASA**FRICTION FORCE**Surface roughness decreases wind speed Reduces impact of Inertial Coriolis Force Winds cross isobars, spiralling out of ANTICYCLONES (H), and into CYCLONES (L) H L**Can you infer wind direction & speed?**weather.unisys.com**WINDS ALOFT ARE DISTINCT**H L Air tends to be unstable in low pressure (tendency to rise) Air tends to be stable in high pressure (tendency to fall)**GLOBAL**ATMOSPHERIC CIRCULATION**MAIN PRESSURE ZONES**• Equatorial Low Pressure Trough • Subtropical High Pressure Cells • Subpolar Low Pressure Cells • 4. Weak Polar High Pressure Cells**Intertropical**Convergence Zone**Geostrophic wind**(above strong influence of friction)**WIND**SMALL-SCALE & LOCAL SYSTEMS CHAPTER 10**SONIC**ANEMOMETER ULTRASONIC SOUND WAVES DETERMINEWIND SPEED, DIRECTION & TEMPERATURE Sonic transducers**SNOWDRIFT**http://www.wrds.uwyo.edu/wrds/rmfres/fendrift.html**Effect of**Surface Roughness and Instability on Windspeed**Depth of mixing depends on:**Surface Heating WIND SPEED TOPOGRAPHY SURFACE ELEMENTS**Effect of Eddies on Wind Inclination**UNSTABLE STABLE

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