air movement

air movement ENVS 09-26-08

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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

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http://www.dnr.sc.gov/climate/sco/Education/wxmap/wxmap.gif

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• 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

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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

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but… • most atmospheric patterns are swirls of clouds and winds … • very few straight patterns

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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.

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Coriolis Effect does NOT determine the way your sink drains !

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Coriolis Effect with (almost) no Math a) movement to and from the poles

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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

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from: www.bom.gov.au/info/ftweather/ page_14.shtml

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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

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Coriolis Effect – Magnitude • effect is very smal• only affects large scale movements (Hurricanes, ocean currents etc.) • it will not affect small, household scale currents

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air currents II

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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

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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

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coriolis effect http://www.physics.umd.edu/lecdem/services/demos/demosd5/d5-11.htm

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Coriolis effect demonstrations stationary disk rotating disk – stationary observer rotating disk, rotating observer

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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

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close to ground: • friction between atmosphere and surface slows air down • winds are stronger at high elevations

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three influences on air movement • pressure gradient force• Coriolis effect• friction

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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

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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

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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 !

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airflow symbols

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http://www.atmos.washington.edu/~mcmurdie/oct96/300mb_geownd_17oct.gif

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two ways to depict air pressure • lines of equal pressure (isobars) • equal pressure surfaces http://www.physicalgeography.net/fundamentals/7d.html

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constant pressure levels • are at higher elevation for high pressure areas • are at lower elevation for low pressure areas

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http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg

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Effect on Hurricanes: • balance between Coriolis force and pressure gradient • causes rotation of storm system• counterclockwise in N-hemisphere• clockwise in S-hemisphere

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surfacewinds http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg

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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

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local wind systems • land and sea breezes• Föhn winds (Chinook)

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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

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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

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for al of you who stil haven’t enough…

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Global Atmospheric Circulation ENVS 110 – 10-03-2008

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http://en.wikipedia.org/wiki/Image:IntertropicalConvergenceZone-EO.jpg

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Vertical air movement in the atmosphere http://en.wikipedia.org/wiki/Image:Omega-500-july-era40-1979.png

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Seasonal Changes link

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Polar Front Planetary frontal zoneMixing of warm and cold air → exchange of heat from subtropics to higher latitudes

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Local effects • Asian monsoon• North American monsoon http://img.timeinc.net/time/photoessays/2007/india_monsoon/india_monsoon_01.jpg

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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 Himalayas

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