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Topic 07A_colour

Topic 07A_colour - Topic 07a Global Circula>on Winds...

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Unformatted text preview: Topic 07a Global Circula>on: Winds Introduc>on 1. Atmospheric pressure 2. Wind, Coriolis Effect and Fric>on (i) Pressure Gradient Force Geography 211 (Winter 2009) 9/30/09 1 (ii) Coriolis Effect To earth observer, it is apparent curving of wind flow Analogy Outside rota>ng reference: ball in straight line, children rota>ng around ball Within reference: ball being pushed off path by some unseen force www.windpower.org/en/tour/wres/coriolis.htm If weapon fired toward equator from North Pole, projec>le would land to right of its true path. If weapon fired toward equator from South Pole, projec>le would land to le: of its true path. Target will move Path it follows Target moves Path it follows Target you hoped it would hit Someone watching air parcel from space would not see parcel deflect but would rather see parcel moving straight and earth rota>ng out from under air parcel Instead of wind blowing directly from high to low pressure, rota>on of Earth causes wind to appear deflected Geography 211 (Winter 2009) 9/30/09 2 (iii) Fric8on Force Close to Earth's surface fric>on reduces wind speed, which in turn reduces effect of Coriolis Force Net result of All Forces Northern Hemisphere Low Southern Hemisphere Low High High 3. Cyclones and An>cyclones 4. Convec>on Loop a) b) Uniform atmosphere Air at Y expands in response to hea>ng and pushes isobars upward pressure gradient induced Air moves away from Y' now less air above Y, so pressure at ground decreases (opposite at X,Z) In response air begins to move and convec>on loop As air alo` leaves warm column, weight of air column decreases and surface pressure drops Accumula>on of air in cold column causes surface pressure to rise c) d) Geography 211 (Winter 2009) 9/30/09 3 5. Land and Sea Breezes Land Breeze 6. Global Winds and Pressure Paherns heat and moisture redistribu>on upload.wikimedia.org/wikipedia/commons/1/12/ IntertropicalConvergenceZoneEO.jpg Thunderstorms of Intertropical Convergence Zone Geography 211 (Winter 2009) 9/30/09 4 At 30 N and S: convergence of air alo` subtropical high pressure belt at surface air descends and becomes warm, dry Cooled air sinks Some air flows northwards towards Poles HIGH SURFACE PRESSURE Some air flows back towards Equator At 5060 N and S: Warm air of midla>tudes moving north/south meets cold polar air mass and rises Air reaches polar areas, cools and descends Dry, high surface pressure Warmer air rises Geography 211 (Winter 2009) 9/30/09 5 Focus on Surface Winds 7. Upper Atmosphere Geostrophic Winds For winds alo: (al>tudes > 1km above ground), ignore fric>on Air parcel ini>ally at rest moves from high pressure to low pressure As air parcel begins to move, it is deflected by Coriolis force As wind gains speed, deflec>on increases un>l Coriolis force equals PGF Geography 211 (Winter 2009) 9/30/09 6 Jet Streams hhp://www.weatherques>ons.com/What_causes_the_jet_stream.htm Jet Stream Zones of pressure contrast Very steep pressure gradient, which intensifies wind speed causing jet stream Forms along polar front and above Hadley cell in subtropics Rossby Waves large undula>on that develops in jet stream's axis of flow separates cold, polar air from warm, tropical air waves may pinch off forms due to oceanland temp contrasts and topographic forcing (winds flowing over mountains) Geography 211 (Winter 2009) 9/30/09 7 ...
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