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Forces that Influence Winds |
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An object at rest will remain at rest and an
object in motion will remain in motion as long as no force is exerted on
the object. |
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The force exerted on an object equals its mass
times the acceleration.
(Acceleration is speeding up, slowing down, or changing direction.) |
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Pressure gradient force |
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Coriolis force |
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Centripetal force |
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Friction |
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Pressure is force per unit area. |
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The high pressure region exerts more force so
the net force is from high (H) to low (L). |
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The pressure gradient is the change in pressure
divided by the distance over which that change occurs. |
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A large pressure gradient will create a strong
wind. (O’Dome example) |
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The Coriolis force describes the apparent force
due to the rotation of the earth. (demonstration) |
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The Coriolis force causes the wind to deflect to
the right in the Northern Hemisphere. |
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Except at the equator a free-moving object
heading east or west will appear to deviate from its path. |
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There is more deviation at higher latitudes. |
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The apparent deflection of the Coriolis force
increases with speed. |
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With only the pressure gradient force the wind
would blow directly from high pressure to low pressure. |
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500mb chart (~3 miles up): Aloft the winds blow
more or less parallel to isobars (constant pressure contours). |
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These are called geostrophic winds
(earth-turning). |
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The pressure gradient force (PGF) balances the
Coriolis force (CF) so the wind flows parallel to isobars. |
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Closer contours imply a larger pressure gradient
force. |
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This is balanced by a larger Coriolis force,
which means higher wind speeds. |
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The orientation and spacing of isobars allows
one to determine the wind direction and speed. |
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In the northern hemisphere air flows
counterclockwise around lows (cyclones) and clockwise around highs
(anticyclones). |
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Circular motion means the direction is
constantly changing, which is a form of acceleration. |
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Centripital acceleration = velocity2/radius. |
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To create this acceleration, the pressure grad.
force is larger than the Coriolis force. |
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Air leaving a high curves to the right in the
northern hemisphere. |
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The rotation about an anticyclone (H) is thus
clockwise in the northern hemisphere. |
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For cyclones (L) the rotation is
counterclockwise. |
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The zonal wind flow (west to east) across the US
is the reason it is faster to fly from west to east. |
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Meridonal wind flow goes in a north-south
trajectory. |
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Near the surface, wind direction is not parallel
to the isobars, but crosses them. |
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Friction near the surface slows down the wind
and hence reduces the Coriolis force. |
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The pressure gradient force dominates. |
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The wind crosses isobars. |
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If we stand with our back to the wind, then turn
clockwise about 30o, lower pressure will be to our left. |
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The Coriolis force is reduced and the wind
crosses isobars.
(N. Hemisphere shown) |
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Where does the air go? |
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To keep the pressure the same at a low, the
converging air at the surface must be balanced diverging air above. |
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If the divergence slows, the pressure in the low
will rise. |
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If the divergence increases, the pressure in the
low will decrease. |
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Since the pressure is higher on the ground than
further up in the atmosphere, why doesn’t the pressure gradient force cause
the air to flow uniformly up? |
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Gravity balances the pressure gradient force to
create hydrostatic equilibrium. |
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The wind is influenced by four forces: |
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Pressure gradient force |
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Coriolis force |
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Centripetal force |
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Friction. |
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From these forces we are able to understand the
speed and direction of the wind both near the ground and further aloft. |
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