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- Newton’s second law of motion states that the rate of change of momentum
(i.e., the acceleration) of an object, as measured relative to
coordinates fixed in space, equals the sum of all the forces acting.
- For atmospheric motions of meteorological interest, the forces that are
of primary concern are the pressure gradient force, the gravitational
force, and friction. These are the
fundamental forces.
- For a coordinate system rotating with the earth, Newton’s second law may
still be applied provided that certain apparent forces, the centrifugal
force and the Coriolis force, are included among the forces acting.
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- PG = (pressure difference) /
distance
- Pressure gradient force goes from
high pressure to low pressure.
- Closely spaced isobars on a
weather map indicate steep pressure gradient.
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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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- Frictional force (drag) is strongest near the Earth’s surface and
decreases rapidly with height.
- The atmospheric layer in which frictional force is important is call
thed boundary layer, whose depth can vary from a few hundred meters to a
few thousand meters.
- There are three sources to generate turbulence eddies to give rise to
the frictional force: (1) mechanical turbulence (airs encounter surface
roughness), (2) thermal turbulence (air near Earth’s surface get heated,
and (3) wind-shear induced turbulence.
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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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- Friction Force = c * V
- c = friction coefficient
- V = wind speed
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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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