This document discusses air pressure and winds. It explains that air pressure is caused by the weight of the atmosphere above a given place. Low pressure systems are below 1013.2 mb and high pressure systems are above. Wind is the horizontal movement of air caused by pressure gradient force from high to low pressure areas. The coriolis effect causes winds to deflect right in the Northern Hemisphere and left in the Southern Hemisphere. Friction also affects wind speed near the Earth's surface. Global circulation patterns involve convection between the equator and poles.

1. CH. 18 – AIR PRESSURE & WINDS

2. Air PressureWeight of atmosphere above a given placeStandard barometric pressure = 1013.2 millibars (mb)Low pressure <1013.2 mbHigh pressure >1013.2 mb

3. WindHorizontal movement of air

4. Factors affecting wind speed & direction1) Pressure Gradient Force (PGF)- air at surface always moves from high pressure to low pressure* PGF is only force that generates wind!

5. Pressure Gradient Force Caused by the differential heating of Earth’s surface- causes high & low pressure centers to develop

6. Map w/ isobars

7. Pressure Gradient ForceIsobar = line that connects points of equal air pressureIdeally, air flow would be perpendicular to isobars- shortest distance to low pressure

8. Pressure Gradient ForceSpacing of isobars indicates gradientGradient = change in pressure over certain distance(similar to “slope” of land)

9. Pressure Gradient ForceSteep gradient = isobars spaced closely together = _______ wind speedWeak gradient = widely spaced isobars = _______ winds

10. Factors affecting wind speed & direction2) Coriolis Effect- caused by Earth’s rotation on its axis- free-moving objects are deflected from their path of motionEx: wind, water currents

11. Coriolis Effect

12. Coriolis EffectNorthern Hemisphere – deflected to right of its path of motionSouthern Hemisphere – deflected to left of its path of motion

13. Ocean circulation

14. Coriolis Effecta) Affects only wind direction, not wind speedb) Is affected by wind speedFaster winds = _______ deflectionSlower winds = ______ deflection

15. Coriolis Effectc) Strongest at poles, weakest at equatorHurricanes can’t form between 5oN and 5oS

16. Factors affecting wind speed & direction3) FrictionImportant only w/in a few km of Earth's surface- slows wind speed & reduces Coriolis Effect

17. FrictionStrongest over land, weakest over waterQUESTION:Which diagram represents air flow over land?

18. Surface Circulation PatternsAssociated w/ high & low pressure centersLow pressure center = cycloneAir flow around low pressure = cyclonic air flow

19. Surface Circulation PatternsHigh pressure center = anticycloneAir flow around high pressure = anticyclonic air flow

20. Cyclonic/Anticyclonic air winds

21. Weather GeneralizationsLow pressure center- rising air- cloud formation & possible ppt.- converging surface winds

22. Weather GeneralizationsHigh pressure center- descending air- clear skies- diverging surface winds

23. Surface Circulation PatternsMovement of mid-latitude cyclone w/ changing wind directions- very common occurrence in Midwest

24. Barometric TendencyUsed for short-term weather predictions (several hours ahead)1) Falling – approach of low pressure (clouds)

25. Barometric Tendency2) Rising – approach of high pressure (clear skies)3) Steady – maintain status quoWeather Proverbs (handout)

26. Global Air CirculationGeorge Hadley (1735):On a uniform non-rotating Earth:Heated equatorial air risesUpper-level air cools & sinks at poles

27. Global Air Circulation

28. Global Air CirculationCreates low pressure at equatorCreates high pressure at poles

29. Global Air CirculationHowever:Air flow is deflected by Coriolis EffectBreaks circulation pattern into four global pressure zones

30. Global Pressure ZonesEquatorial Low (0o)- hot humid air rises & cools adiabaticallyproduces clouds & ppt.Ex: Tropical rain forests (Amazon & Congo)

31. Global Pressure ZonesSubtropical High (30oN & 30oS)- air aloft “piles up” from the Coriolis Effectcold air sinks

32. produces prevailing trade winds at surfaceSubtropical HighDescending air heats adiabaticallycreates arid environmentsEx: Sahara Desert (N. Africa), Arabian Desert, Australian deserts

33. July temp’s