Lecture:Air Pressure
advertisement
Air Pressure and Wind Pressure: the amount of force exerted per unit of surface area Pressure can be increased in 2 ways 1. By increasing density or decreasing the volume 2. By Increasing the temperature Air Pressure and Wind Key factor: Unequal heating of the Earthtemperature Changes cause pressure changes! Air Pressure: Exerted in all directions P2 air P1 If P1>P2, then the “box” would Move to your right. Therefore air moves from Higher pressure to lower pressure Standard Sea Level Pressure= 1013.2 mb Wind: H L Measurement of Air Pressure Mercurial Barometer Measurement of Air Pressure Aneroid Barometer The Distribution of Pressure Pressure Gradient & Pressure Gradient Force (PGF) The change in pressure over a distance On a Surface Map: Isobars: Lines of Equal pressure -The spacing indicates the strength of the PGF Where is an area Of high winds? …weak winds??? Pressure Gradient Force (PGF) •Creates the wind •“Blows” from high pressure to low pressure Closely spaced isobars indicate: faster change in pressure Over a distancestrong pressure gradient, and thereforeSTRONG WINDS!! Loosely spaced isobars indicate: slower change in pressure Over a distanceweak pressure gradient, and therefore WEAK WINDS!! Wind direction Vertical Pressure Gradient Force (PGF) WE KNOW: •Pressure decreases with height •PGF moves air from high to low pressure SO……. Why isn’t air always blowing upward????? Answer: Hydrostatic Balance gravity! g VPGF Wind Three Forces: •Pressure Gradient Force (PGF) •Coriolis Effect (CF) •Friction Wind Coriolis Force (CF) p.106 4 Fundamental Characteristics •Occurs because the Earth is rotating! •All objects are deflected •To the right in N. Hem. And to the left in the S. Hem. •Only affects wind direction •Affected by wind speed •Strongest at the poles, weakens equatorward Wind direction Wind Friction (F) •Important to wind in the first 1.5 km at Earth’s surface •Acts to slow air movement!!!! Upper Air: Not affected by friction Upper Air Winds Read pp. 108-110 ………………. 500mbwarm ……………………….. 500mbcold heat m m 1. As the column heats up, it expands 2. It’s mass stays constant, but it stretched over a longer distance a. Therefore you have a lower density 3. The height of 500mb is higher. Upper Air Winds Therefore, in the upper air we plot heights instead of pressure!!!!!! Height contours: lines of equal height Properties of height contours 1. The slope of the height contours is proportional to the horizontal pressure gradient. 2. Heights decrease towards colder air 3. They are label in decameters (units of 10m) 4. They are plotted at an interval of 60m Can you see the Jet Stream? Where are the highest winds?? Upper Air Winds Forces affecting winds in the upper air NO FRICTION!!!! Only the pressure gradient force and coriolis force Fig 4-12 on p. 109 Geostrophic flow: •air is flowing in a straight line •Air has no acceleration •PGF=CF Gradient flow: •air is constantly changing direction •Air has acceleration •The PGF does not balance the CF Highs and Lows Low: Cyclone High: Anticyclone L H What happens when the forces we just described Are applied to pressure centers? Fig.4-14 Upper Air Pressure Systems Why do winds around a low circulate counterclockwise?? Why do winds around a high circulate clockwise?? V L PGF PGF> CF Therefore wind turns CCW CF V= Wind’s original direction PGF=pressure gradient force CF=Coriolis force CCW=counterclockwise CW=clockwise H PGF CF V CF > PGF Therefore wind turns CW Surface Pressure Systems V L PGF Friction slows the wind Weakens the CF CF F=friction CF is outward force in low Therefore winds will circulate CCW and INWARDS!!! CONVERGENCE F H PGF CF V CF is inward force in high Therefore winds will circulate CW and OUTWARDS!!! DIVERGENCE Troughs and Ridges p. 113 fig.4-19 Trough L H Ridge How the Upper Air is Connected to the Surface The upper air drives the weather at the surface •The jet stream is a river of strong winds •Moves from West to East •Exists at 200 (summer) or 300mb (winter) •Brings the weather! Wind slows as it makes this turn and therefore wind Converges on the west side of an upper level trough Chapter 10 p.285 Fig. 10-6 Wind speeds up as it comes Out of the turn and therefore Wind diverges on the east side of an upper level trough How the Upper Air is Connected to the Surface p. 286 fig. 10-7 Below the area of convergence in the jet stream trough, air sinks Causing a build up of pressure at the surface therefore, surface high pressure forms below the west side of an upper Level trough Below the area of divergence in the jet stream trough, air rises Causing a pressure drop at the surface therefore, surface low pressure forms below the east side of an upper level trough H L The upper level trough causes the surface systems to form, strengthen And move!
