Atmospheric Forces

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Atmospheric Forces

• AOS 101 Discussion Sections 302 and 303

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Why Does the Wind Blow?

• What makes the wind blow?
• We need to think about Newton's Laws
• 1st Law
• An object at rest will remain at rest an object
  in motion will remain in motion as long as no
  force is exerted on the object.
• 2nd Law
• The total force exerted on an object is equal to
  the acceleration of the object times its mass

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Pressure Gradient Force

• Compels fluids to move from high pressure to
  lower pressure

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PGF and Wind
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Isobars and PGF
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PGF
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Coriolis Force

• An apparent force
• Results from the constant rotation of the Earth
• Northern Hemisphere
• Acts at a 90angle to the right of the object in
  motion (such as the wind)
• This means that a wind from the south would have
  a CF acting toward the east

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Imagine Dallas, TX fires a missile at Winnipeg,
Manitoba
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Missile starts at Dallas, which is at a latitude
of 37.28 N, rotates with the Earth at a speed of
465.11 m/s.
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Missile travels toward Winnipeg which, at a
latitude of 52.00 N, rotates with the Earth at a
speed of 286.35 m/s
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Geostrophic Balance
X
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Geostrophic Balance
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Geostrophic Balance
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Geostrophic Balance
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Upper Level Flow
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Geopotential Height
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Geopotential Height
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PGF/ CF/ Centripetal
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Friction Force
Friction
Wind
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Friction Force

• This throws the wind out of geostrophic balance
• There is now a net force acting on the wind in
  the direction opposite its motion

PGF
Wind
FR
CF
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Friction Force

• Upper Level Wind
• Balance PGF/ CF
• Lower Level Wind
• Balance PGF/ CF/ Friction
• Friction causes wind to cross isobars at
  30angle at surface

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Front Collapse Experiment
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Front Collapse Experiment
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Rotating Tank Experiment
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Atmospheric Fronts

• AOS 101 Discussion Sections 302 and 303

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Warm Front
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Warm Front

• Drawn as a red line with red semi-circles
  pointing in the direction of the fronts movement

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Cold Front

• Drawn as a blue line with blue triangles pointing
  in the direction of the fronts movement

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Stationary Front

• Stalled
• No movement of the temperature gradient
• Convergence of wind
• Drawn as alternating segments of red semicircles
  (warm front) and blue triangles (cold front) in
  opposite directions

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Occluded Front

• A region where a faster moving cold front has
  caught up to a slower moving warm front.
• Generally occurs near the end of the life of a
  cyclone
• Drawn with a purple line with alternating
  semicircles and triangles

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Cold Occlusion

• The type most associated with mid-latitude
  cyclones
• Cold front "lifts" the warm front up and over the
  very cold air
• Associated weather is similar to a warm front as
  the occluded front approaches
• Once the front has passed, the associated weather
  is similar to a cold front
• Vertical structure is often difficult to observe

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Warm Occlusion

• Cold air behind cold front is not dense enough to
  lift cold air ahead of warm front
• Cold front rides up and over the warm front
• Upper-level cold front reached station before
  surface warm occlusion

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Fronts
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Identifying Fronts

• We know that we need to look for low pressure and
  a boundary of cold and warm air.
• To pinpoint the parts of our cyclone, look for
  specifics in the observation maps
• Find the center of cyclonic rotation
• Find the large temperature gradients
• Identify regions of wind shifts
• Identify the type of temperature advection
• Look for kinks in the isobars

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