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4/7/11
Polar Front Theory
  Polar
front is a semi-continuous
boundary separating cold, polar air from
more moderate mid-latitude air
  Mid-latitude cyclone (wave cyclone)
forms and moves along polar front in
wavelike manner
  Frontal wave, warm sector, mature
cyclone, triple point, secondary low,
family of cyclones
Chapter 12
Concordia University Geog/Sci-381 Chapter 12
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Concordia University Geog/Sci-381 Chapter 12
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Triple Point
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Fig. 10-2, p. 261
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Secondary low sometimes forms new cyclone
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Where do mid-latitude cyclones tend
to form?
  Lee-side
lows (cyclogenesis)
  Nor’easters
  Hatteras
low
Clipper
  Explosive cyclogenesis, bomb
  Alberta
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Where do mid-latitude cyclones tend
to form?
  Topic:
Northeasters
  Mid-latitude cyclones that develop or
intensify off the eastern seaboard of North
America then move NE along coast
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Vertical Structure of Deep Dynamic
Lows
  Dynamic
low = intensify with height
upper-level divergence is stronger
than surface convergence (more air is
taken out of the top than the bottom)
surface pressure drops and low
pressure systems strengthen
  When
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Fig. 1, p. 11
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If lows and highs aloft were always directly above lows and highs at the
surface, the surface systems would quickly dissipate.
• Air flows into the lows, raising pressure.
• Air flows out of the highs, lowering pressure.
Concordia University Geog/Sci-381 Chapter 12
Cyclone formation is driven by
convergence and divergence aloft
• Low pressure aloft is generally west
of the surface low.
• High pressure aloft is generally
southwest of the surface high.
• p. 318
• Divergence above the low removes
air above more quickly than air flows
into the surface low, and the cyclone
builds.
• Convergence above the high adds air
above more quickly than air flows out
of the surface low, and the anticyclone builds.
• Surface pressure systems typically
follow the direction of air at the 500mb level, half the speed of the 500-mb
winds.
• This situation is created by pressure
waves. Why do waves form?
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Upper Level Waves and Mid-latitude
Cyclones
Convergence and Divergence
  Convergence between ridge and trough,
  Longwaves
and shortwaves
  Barotropic vs. baroclinic
divergence between trough and ridge
  Barotropic: Isotherms follow isobars
  Baroclinic: Isotherms cross isobars
○  Occur in shortwaves
○  Drives cyclone formation
  Cold
and warm air advection
  Transport of cold or warm air by wind
Air flows faster in a ridge and
slower in a trough.
Convergence through crowding
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Short waves deepen when they align
with long waves.
Where is the air barotropic?
Where is the air baroclinic?
• Where is cold advection occurring?
• Where is warm advection occurring?
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The Necessary Ingredients for
Development of Mid-latitude Storm
Baroclinic Instability
Cut-Off Low
Baroclinic instability
Upper-Air Support: the overall effect of
differential temperature advection is to
amplify the upper level wave
Role of the Jet Stream: the polar jet
stream can remove air from surface
cyclone and supply air to surface anticyclone
1. 
2. 
3. 
Three factors intensify the cyclone
Storm is cut off from warm
1.  Convergence/Divergence
air and dies.
2.  Cold/Warm advection produces falling and rising air
3.  Rising air causes condensation and increases heat in divergence zone
Concordia University Geog/Sci-381 Chapter 12
 
Jet stream is faster and moves farther south in
winter, so winter cyclones are stronger and
move faster.
Fig. 12-10, p. 19
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Concordia University Geog/Sci-381 Chapter 12
Jet Streaks and Storms
Building
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Dying
  Entrance
and exit regions associated
with divergence and convergence, right
exit allows divergence.
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Concordia University Geog/Sci-381 Chapter 12
Fig. 12-11, p. 22
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The Conveyor Belt Model
  Air
constantly glides through storm;
warm, cold, and dry conveyor belts
  March Storm of 1993
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Fig. 12-14, p. 26
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March, 1993 Snow Storm
The “Storm of the Century”
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Fig. 12-15, p. 27
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Same Day, 500-mb Chart
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Divergence aloft increases vorticity
Vorticity, Divergence and
Development of Cyclones
  Vorticity
is a measure of the spin of
small air parcels
  Positive: cyclonic, negative: anticyclonic
  Divergence aloft is associated with an
increase in the cyclonic vorticity of
surface cyclones: cyclogenesis and
upward air movement
  Positive vorticity indicates divergence aloft.
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Using
Vorticity
Vorticity on a Spinning Planet
Forecasting Weather
  Topic:
  Putting
Vorticity and Longwaves
  Longwaves develop in upper-levels due to
the conservation of absolute vorticity.
Concordia University Geog/Sci-381 Chapter 12
It All Together
  Forecasters review 200mb, 500mb, and
surface maps to examine pressure,
convergence, vorticity, and advection
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The atmospheric conditions for
February 11,1983, at 7 a.m., EST.
The bottom chart is the surface
weather map.
The middle chart is the 500-mb chart
that shows contour lines (solid
lines) in meters above sea level,
isotherms (dashed lines) in oC, and
the position of a shortwave (heavy
dashed line). The upper chart is the
200-mb chart that illustrates
contours, winds, and the position of
the polar jet stream (dark blue
arrow).
Same day,
with vorticity
The letters DIV represent an area of
strong divergence. The region
shaded orange represents the jet
stream core — the jet streak.
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Homework for Chapter 12
 
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Fig. 12-27, p. 38
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Project for Chapter 12
Chapter 12 Questions for Review, p. 336
 
None
  #2, 6, 9, 12, 15, 16, 18
 
Chapter 12 Questions for Thought, p. 336
 
Chapter 12 Problems and Exercises, p. 336
  #4
  None
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