# 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.

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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 Earth  temperature Changes cause pressure changes! Air Pressure: air Exerted in all directions Standard Sea Level Pressure= 1013.2 mb Wind: H L P1 P2 If P1>P2, then the “box” would Move to your right. Therefore air moves from Higher pressure to lower pressure

Measurement of Air Pressure Mercurial Barometer

Measurement of Air Pressure Aneroid Barometer

The Distribution of Pressure Isobars: Lines of Equal pressure Pressure Gradient & Pressure Gradient Force (PGF) On a Surface Map: -The spacing indicates the strength of the PGF The change in pressure over a distance

Where is an area Of high winds? …weak winds???

Pressure Gradient Force (PGF) Closely spaced isobars indicate: faster change in pressure Over a distance  strong pressure gradient, and therefore  STRONG WINDS!! Loosely spaced isobars indicate: slower change in pressure Over a distance  weak pressure gradient, and therefore  WEAK WINDS!! Wind direction Creates the wind “Blows” from high pressure to low pressure

Vertical Pressure Gradient Force (PGF) Pressure decreases with height PGF moves air from high to low pressure WE KNOW: 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) 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 p.106 4 Fundamental Characteristics

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 m heat 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. ……………………….. ………………. 500mb warm 500mb cold Read pp. 108-110

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 What happens when the forces we just described Are applied to pressure centers?  Fig.4-14 L H

Upper Air Pressure Systems Why do winds around a low circulate counterclockwise?? Why do winds around a high circulate clockwise?? V= Wind’s original direction PGF=pressure gradient force CF=Coriolis force CCW=counterclockwise CW=clockwise L V CF PGF PGF> CF Therefore wind turns CCW CF > PGF Therefore wind turns CW H V PGF CF

Surface Pressure Systems L V CF PGF H V CF F=friction F Friction slows the wind Weakens the CF CF is outward force in low Therefore winds will circulate CCW and INWARDS!!! CF is inward force in high Therefore winds will circulate CW and OUTWARDS!!! CONVERGENCE DIVERGENCE

Troughs and Ridges p. 113 fig.4-19 Trough Ridge H L

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 Wind speeds up as it comes Out of the turn and therefore Wind diverges on the east side of an upper level trough Chapter 10 p.285 Fig. 10-6

p. 286 fig. 10-7 How the Upper Air is Connected to the Surface 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!

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