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The Pressure is On!

The Pressure is On!. Definition Pressure = Force / unit area weight of the air above us Pressure always decreases with height Measurements Barometers mercury, water, alcohol units: inches, millibars, hecta-pascals (Hpa). Ideal Gas Law PV=nRT at constant V:

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The Pressure is On!

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  1. The Pressure is On! • Definition • Pressure = Force / unit area • weight of the air above us • Pressure always decreases with height • Measurements • Barometers • mercury, water, alcohol • units: inches, millibars, hecta-pascals (Hpa)

  2. Ideal Gas Law PV=nRT at constant V: P is proportional to T but inverse to density V is not constant adiabatic - no energy change expands as rises -cools contracts as descends and warms Pressure Gradients Wind is driven by PGF Always from Hi to Lo Horizontal PGF 980 to 1030 = 50/1000 surface winds: 5-25m/s Vertical PGF 500 to 1000 = 1/2 vertical winds: 5-25cm/sec The Pressure is On!

  3. Pressure change in Warm Air High Temperature, lower density (Tropics) Pressure decreases more slowly with height Therefore; higher pressure aloft at the same elevation Pressure change in Cold Air Low Temperature, higher density (Polar) Pressure decreases more quickly with height Therefore; lower pressure aloft at the same elevation The Pressure is On!

  4. The Pressure is On!

  5. The Pressure is On! • High pressure aloft over Tropics • Low pressure aloft over the Polar Regions • PGF goes from Hi to Lo pressure, so the wind blows from South to North (NH) • Include the spinning Earth Force (Coriolis) and the wind turns to the right - Westerlies!

  6. The Pressure is On!

  7. The Pressure is On! • Coriolis Force - an apparent force on a rotating sphere • air at equator is moving at 22,000 miles/day • or nearly 1,000 miles an hour • air at the poles is not spinning at all • or has no velocity • when air travels from the equator toward the poles it is bringing with it this ‘apparent’ speed • causes air to turn to ‘right’ in NH, ‘left’ in SH

  8. The Pressure is On!

  9. The Force of Friction always acts against the velocity proportional to speed primarily in the boundary layer of the atmosphere Friction turns the wind turns the wind toward low pressure away from high pressure usually at 20-40 degree angle The Pressure is On!

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