air movement

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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 small
• 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/d
emosd5/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 earths 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/oct96/30
0mb_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.h
tml
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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/2008-02-0
6/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/2008-02-0
6/weather/12z_500mb.jpg
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surface winds or friction layer winds

• friction between atmpsphere and earths 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 all of you who still havent enough
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Global Atmospheric Circulation

• ENVS 110 - 10-03-2008

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http//en.wikipedia.org/wiki/ImageIntertropicalCo
nvergenceZone-EO.jpg
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Vertical air movement in the atmosphere
http//en.wikipedia.org/wiki/ImageOmega-500-july-
era40-1979.png
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Seasonal Changes

• link

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Polar Front

• Planetary frontal zone
• Mixing 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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