Forces that affect the wind

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Forces that affect the wind

• Newtons Laws
• An object at rest will remain at rest. An object
  in motion will remain in motion, traveling at a
  constant speed and in a straight line as long as
  no forces are exerted on the object.
• An object (including an air mass) will always
  accelerate in the direction of the total force
  acting on it where,
• F Mass ? Acceleration
• Forces on an air mass affecting wind
• Pressure gradient force
• Centripetal force
• Coriolis force
• Friction

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Forces that affect the wind
Pressure gradient force (PGF) - review The PGF
is a horizontal force which, acting alone, would
accelerate air from high towards low pressure
PGF
H (1030 mb)
L (1000 mb)
1 hr
2 hr
3 hr
40 m/s 80 mph
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Forces that affect the wind
Pressure gradient force (PGF) - review On a
weather map, the PGF is strongest where lines of
constant pressure are closest together.
Fig. 6-11, p.151
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Formation of Direct Thermal Circulation
Air flow from higher toward lower pressures occurs
a. at lower altitudes only b. at higher altitudes
only c. at both higher and lower altitudes
X
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Forces that affect the wind
Centripetal force (CenF) The centrifugal force is
an apparent force felt by an object rotating
around a circle. The actual force is the
centripetal force keeping the object on a
circular path.
Apparent centrifugal force
Centripetal force needed to keep object going in
a circle
Direction of motion of rotating object
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Forces that affect the wind
Coriolis force (CF) The Coriolis force, like the
centrifugal force, is an apparent force due to
the earths rotation.
Apparent force deflects ball to the right as the
merry-go-round turns counter-clockwise or
cyclonically.
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Forces that affect the wind
Coriolis force (CF) Airplanes path

• Earth rotates counter-clockwise when viewed from
  the NH
• Airplanes path is deflected to the right
  (southward for a plane headed from west to east)
  in the NH, to the left in the SH.
• Coriolis force increases from zero at the equator
  to a maximum value at the poles (north or south
  pole).

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Forces that affect the wind
An airplane flying south in the northern
hemisphere
a. is deflected to the east b. is deflected to
the west c. could be deflected in either direction
X
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Forces that affect the wind

• Coriolis force (CF)
• The Coriolis force only affects an objects
  direction, never an objects speed.
• The amount of deflection depends on,
• The rotation speed of the earth
• The latitude of the object
• The objects speed
• The Coriolis force acts at right angles to the
  wind, only influencing wind direction.
• The Coriolis force is negligible over short
  distances, even for strong winds.

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Forces that affect the wind
Straight-line flow aloft PGF vs. CF
1. Wind is set in motion by PGF
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Forces that affect the wind
Winds around a low aloft PGF vs. CF vs. CenF
1. Wind set in motion by PGF toward low pressure
region
CenF
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Forces that affect the wind
Winds around a high aloft PGF vs. CF vs. CenF
1. Wind set in motion by PGF away from high
pressure region
2
1
3
CenF
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Forces that affect the wind
Review of an upper-level 500 mb chart
zonal winds
fast
L
slow
H
H
meridional winds
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Forces that affect the wind
Surface winds

• The frictional force is present up to 1000 m and
  reduces wind speed in this layer (boundary
  layer). Effect is greater at 0 m than at 1000 m.
• A reduction in wind speed also reduces the CF.
• Thus, the winds in the boundary layer point
  slightly towards a L (away from a H), since the
  CF is not exactly balancing the PGF.