Pressure and Winds

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Pressure and Winds

• General Circulation of the Atmosphere
• Geog 210 - Physical
  Geography
• Geog 311 - Climatology

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Global balance local imbalances

• Is the earth system in radiative equilibrium? The
  diagram below shows the annual mean, averaged
  around latitude circles, of the balance between
  the solar radiation absorbed at the ground (in
  blue) and the outgoing infrared radiation from
  Earth into space (in red). The two curves
  generally balance over the entire globe, but not
  at every single latitude. In the tropics, there
  is an excess of radiation (solar radiation
  absorbed exceeds outgoing terrestrial radiation)
  in middle and high latitudes all the way to the
  poles, there is a deficit (Earth is radiating
  into space more than it receives from the sun).
  The atmosphere and ocean systems are forced to
  move about by this imbalance, and bring heat by
  convection and advection from equator to the
  poles. 

Source Columbia University
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The imbalance of radiative budget between the
tropics and higher latitudes is the main cause
that drives weather and climate! The ultimate
energy source is, of course, the sun.
Global weather phenomena as observed by NOAAs
geostationary satellite GOES-12 on Oct 7, 2003.
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Whither the Wind?

• Pressure Gradient
• Coriolis Effect
• Friction

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• Pressure unevenness is the main cause of winds.
  This unevenness causes a pressure gradient from
  areas of high pressure areas of low pressure. The
  pressure gradient force causes winds to blow.
• In the absence of other forces, winds will blow
  from high pressure to low pressure.
• The unit of pressure is millibars (mb). The
  mean sea-level pressure is 1013.25 mb, or
  roughly 1000 mb.

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Coriolis Force

• But there are other forces that may influence
  winds. One of the most important is the Coriolis
  force in honor of the French physicist Coriolis.
• Coriolis force is not a real force but an
  apparent force. It appears only because the
  observer (i.e., people like you and I) is on a
  rotating reference frame.
• Zero at equator maximum at high latitudes

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Coriolis Force

• The same phenomenon occurs for any motion on
  earth due to the earth rotation.
• When we observe the movement of an air parcel on
  earth, we find that the moment the air parcel
  starts to move, it will be subject to this force
  immediately. N
• ET in
• The deflection of the movement is such that the
  parcel will be deflected to the right in the
  Northern Hemisphere and to the left in the
  Southern Hemisphere.
• Thus, the wind patterns are rotating in opposite
  sense near high and low pressure centers in
  Northern and Southern Hemispheres, as shown in
  the figure to the right.

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Geostrophic Balance and Geostrophic WindsBalance
between the pressure gradient force and Coriolis
force

• Will the wind direction forever turn to right
  (left) in Northern (Southern) Hemisphere? The
  answer is no. It will keep turning until a
  balance between the pressure gradient force and
  Coriolis force is achieved.
• This balance happens when the wind direction
  becomes parallel to the isobars. This is called
  the geostrophic balance.
• The wind satisfying the geostrophic balance
  condition is called the geostrophic wind .
• Geostrophic winds are very close to (but not
  exactly the same as) the actually observed winds.
• Unisys Weather
• http//weather.unisys.com/
• Datastreme Project
• http//www.ametsoc.org/amsedu/dstreme/in
  dex.html

Northern Hemisphere
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Friction

• Upper Atmosphere

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Convergence and Divergence

• In the real world, there are more forces (such as
  friction and curvature effect) involved in
  shaping wind direction and magnitude aside from
  the pressure gradient and Coriolis forces. The
  final results are that winds tend to converge
  toward the low pressure center and diverge
  outward from the high pressure center.

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Scale - swirls within swirls
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Sea Breeze
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Land Breeze
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Single-Cell Circulation Model
The basis for average air flow around the earth
can be examined using a non-rotating, non-tilted,
ocean covered earth. Heating is more intense at
the equator, which triggers Hadley cells to
redistribute rising heat from the tropical low to
the polar highs.
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3-Cell Model simple!

• A simplified 3-cell model of the general
  meridional circulation of the atmosphere.
• In general, descending air has a drying effect
  while ascending air causes condensation.

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The wind system on the earth surfaceThe earth is
rotating, and rotation makes the wind system a
bit more complicated than implied by the 3-cell
structure.

• In the northern hemisphere, the tropical ( 0
  30ºN) prevailing winds are easterlies (NE
  winds)-called trade winds. In the middle
  latitudes (roughly 30 60ºN) the prevailing
  winds are westerlies (SW winds). The polar
  regions prevailing winds are easterlies.
• The wind system in the southern hemisphere is
  like the mirror image of the northern hemisphere.

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Hadley Cell
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Seasonal shifts
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ITCZ Limits
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Winter Season
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Summer Season
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Winter
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Coastal Summer Weather
The semi-permanent Pacific high blocks moist
maritime winds and rain from the California
coast, while the Bermuda high pushes moist
tropical air and humidity over the eastern states.
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Coastal Winter Weather
During winter months, the Pacific high migrates
southward and allows for maritime winds with
moisture and rains to reach California. On the
east coast, precipitation is rather even
throughout the year, and moisture is always from
the Gulf of Mexico and the tropical Atlantic.
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January Winds Aloft
Land-sea temperature differences trigger ridges
and troughs in the isobaric surface.
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Whither the Westerlies?
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Thermal wind is the most fundamental and
significant dynamical balance controlling
thelarge-scale circulation of the atmosphere and
ocean. It is a consequence of hydrostatic and
geostrophic balance, and relates horizontal
buoyancy gradients to changes in the horizontal
wind with height.
What goes up must come down, spinning wheel got
to go round. Blood, Sweat and
Tears http//www.youtube.com/watch?v8T97f2kBzOQ
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Surface 500 mb Maps
Surface maps chart pressure contours, highs and
lows, and wind direction. Winds blow clockwise
around highs, called anticyclones. 500 mb maps
reveal patterns that on average are 5600 m above
the surface, where westerly winds rise and fall
across ridges and troughs.
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Climagraphs of U.S. Citieshttp//drought.unl.edu/
whatis/climographs.htm
World Climagraphs http//cwx.prenhall.com/bookbind
/pubbooks/lutgens3/medialib/abcontrol/pages/questi
on.html
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World Climates (Ritter)Please go to the website
below and look at the climagraphs for locations
all over the world. Study the controlling
factors.
http//southhill.vsb.bc.ca/Departments/Humanities/
Geogpraphy/Kyle/Notes/2_Atmosphere/Climographs/Wor
ldClimates.html