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- Pressure, Measurement,
Distribution
- Forces Affect Wind
- Geostrophic Balance
- Winds in Upper Atmosphere
- Near-Surface Winds
- Hydrostatic Balance (why the sky
isn’t falling!)
- Thermal Wind Balance
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- Pressure gradient force
- Coriolis force
- Friction
- Centrifugal force
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- PG = (pressure difference) /
distance
- Pressure gradient force force
goes from high pressure to low pressure.
- Closely spaced isobars on a
weather map indicate steep pressure gradient.
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- Coriolis force causes the wind to
deflect to the right of its intent path in the Northern Hemisphere and
to the left in the Southern Hemisphere.
- The magnitude of Coriolis force depends on (1) the rotation of the
Earth, (2) the speed of the moving object, and (3) its latitudinal location.
- The stronger the speed (such as wind speed), the stronger the Coriolis
force.
- The higher the latitude, the stronger the Coriolis force.
- The Corioils force is zero at the equator.
- Coriolis force is one major factor that determine weather pattern.
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- The Coriolis force also causes the east-west wind to deflect to the
right of its intent path in the Northern Hemisphere and to the left in
the Southern Hemisphere.
- The deflections are caused by the centrifugal force associated with the
east-west motion, and , therefore, related to rotation of the Earth, and
are also considered as a kind of Coriolis force.
- Although the description of the deflection effect for north-south and
east-west motions are very different, their mathematical expressions are
the same.
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- Friction Force = c * V
- c = friction coefficient
- V = wind speed
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- The three-way balance of
horizontal pressure gradient, Coriolis force, and the centrifugal force
is call the gradient wind balance.
- The gradient wind is an excellent
approximation to the actual wind observed above the Earth’s surface,
especially at the middle
latitudes.
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- Pressure Gradients
- The pressure gradient force initiates movement of atmospheric mass,
wind, from areas of higher to areas of lower pressure
- Horizontal Pressure Gradients
- Typically only small gradients exist across large spatial scales
(1mb/100km)
- Smaller scale weather features, such as hurricanes and tornadoes,
display larger pressure gradients across small areas (1mb/6km)
- Vertical Pressure Gradients
- Average vertical pressure gradients are usually greater than extreme
examples of horizontal pressure gradients as pressure always decreases
with altitude (1mb/10m)
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- Why didn’t the strong vertical
pressure gradient push the air rise?
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- The hydrostatic equation tells us
how quickly air pressure drops wit height.
- čThe rate at
which air pressure decreases with height (DP/ Dz) is equal to the air density (r) times the
acceleration of gravity (g)
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- An equation of state describes the relationship among pressure,
temperature, and density of any material.
- All gases are found to follow approximately the same equation of state,
which is referred to as the “ideal gas law (equation)”.
- Atmospheric gases, whether considered individually or as a mixture, obey
the following ideal gas equation:
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- Since P= rRT (the
ideal gas law), the hydrostatic equation becomes:
- dP = -P/RT x gdz
- č dP/P = -g/RT x dz
- P = Ps exp(-gz/RT)
- P = Ps exp(-z/H)
- The atmospheric pressure decreases exponentially with height
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- ¶U/¶z µ ¶T/¶y
- The vertical shear of zonal wind
is related to the latitudinal gradient of temperature.
- Jet streams usually are formed
above baroclinic zone (such as the polar front).
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- The hurricane is characterized by a strong thermally direct circulation
with the rising of warm air near the center of the storm and the sinking
of cooler air outside.
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