Transcript air movement - Trinity College

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