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air movement ENVS 09-26-08 what makes air move ? • air moves from areas of high atmospheric pressure to areas of low atmospheric pressure • low pressure areas – troughs • high pressure areas - ridges http://www.dnr.sc.gov/climate/sco/Education/wxmap/wxmap.gif • isobars = equal lines of pressure • distance between isobars → change in pressure over distance = pressure gradient • isobars close together → pressure varies rapidly → air flows rapidly form high to low pressure → strong winds pressure gradient force • describes how rapidly air pressure changes between two points • steep gradient (isobars close together) → air moves rapidly → pressure gradient force is large • shallow gradient → pressure gradient force is small but… • most atmospheric patterns are swirls of clouds and winds … • very few straight patterns The Rotating Earth → Coriolis “Force” • Earth is a rotating system • On earth things that move straight appear to move on curved path as earth rotates beneath • swirly cloud patterns on satellite images, zone of westerlies etc. Coriolis Effect does NOT determine the way your sink drains ! Coriolis Effect with (almost) no Math a) movement to and from the poles N-S moving air: • air has larger tangential velocity at equator than at pole (equator = max, pole = 0) • as air moves polewards it maintains its tangential velocity • since it is encountering slower moving air it “overtakes” these higher latitude air masses • apparent deflection towards the right (in N – hemisphere) • air moving towards the equator seems to lag lower latitude air masses → again, deflection to the right from: www.bom.gov.au/info/ftweather/ page_14.shtml Coriolis Effect Summary: • earth is a rotational system • standing on earth, objects that move on straight line seem to be following curved paths (because we are rotating) • Coriolis effect deflects air masses – towards right on northern hemisphere – towards left on southern hemisphere Coriolis Effect - Magnitude • effect is very small • only affects large scale movements (Hurricanes, ocean currents etc.) • it will not affect small, household scale currents air currents II influences on air currents • pressure gradient force: air flows from areas of high pressure to areas of low pressure – the more rapid the change in pressure the stronger the flow • Coriolis effect: due to rotation of the earth air masses are deflected to the right on northern hemisphere and deflected to the left on southern hemisphere • resulting flow: geostrophic flow Coriolis effect • rotating earth → air masses are moving on curved paths across earth • on northern hemisphere: deflection to the right • on southern hemisphere: deflection to the left coriolis effect http://www.physics.umd.edu/lecdem/services/demos/demosd5/d5-11.htm Coriolis effect demonstrations stationary disk rotating disk – stationary observer rotating disk, rotating observer one last time: • on northern hemisphere – air is deflected to the right • on southern hemisphere – air is deflected to the left due to the rotation of the earth close to ground: • friction between atmosphere and surface slows air down • winds are stronger at high elevations three influences on air movement • pressure gradient force • Coriolis effect • friction conservation of angular momentum • combination of pressure gradient force and Coriolis effect cause air to spin • as air is sucked into low-pressure area it spins faster and faster due to the conservation of angular momentum at high altitudes • friction between atmosphere and earth’s surface is negligible • air currents are influenced by balance between pressure gradient force and Coriolis effect → geostrophic winds development of geostrophic flow • pressure gradient force gets air moving • Coriolis effect deflects to right and keeps deflecting until… • effects of pressure gradient and Coriolis effect cancel out → now wind flows almost parallel to isobars ! airflow symbols http://www.atmos.washington.edu/~mcmurdie/oct96/300mb_geownd_17oct.gif two ways to depict air pressure • lines of equal pressure (isobars) • equal pressure surfaces http://www.physicalgeography.net/fundamentals/7d.html constant pressure levels • are at higher elevation for high pressure areas • are at lower elevation for low pressure areas http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg Effect on Hurricanes: • balance between Coriolis force and pressure gradient • causes rotation of storm system • counterclockwise in N-hemisphere • clockwise in S-hemisphere surface winds http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg surface winds or friction layer winds • friction between atmpsphere and earth’s surface prevents air masses to reach equilibrium between coriolis effect and pressure gradient force • friction layer winds blow at an angle to the isobars local wind systems • land and sea breezes • Föhn winds (Chinook) land and sea breezes • differential heating between sea and land • land heats up faster and warm air rises (low pressure area over land) • process reverses at night Föhn winds • orographic rain – air masses lose most of their moisture when they cross a mountain range • on the other side the air is warm and very clear for all of you who still haven’t enough… Global Atmospheric Circulation ENVS 110 - 10-03-2008 http://en.wikipedia.org/wiki/Image:IntertropicalConvergenceZone-EO.jpg Vertical air movement in the atmosphere http://en.wikipedia.org/wiki/Image:Omega-500-july-era40-1979.png Seasonal Changes link Polar Front Planetary frontal zone Mixing of warm and cold air exchange of heat from subtropics to higher latitudes Local effects • Asian monsoon • North American monsoon http://img.timeinc.net/time/photoessays/2007/india_monsoon/india_monsoon_01.jpg Differential warming between continents and oceans • Summer – Indian sub continents heats up • Develops low-pressure zone over continent • Sucks in moist air from surrounding oceans • India: – Warm subtropical oceans extremely moist oceanic air – Orographic rain on southern edge of