Cold%20Front

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Air mass source regions and their paths.
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
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Cold Front
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Cold Front Cross-Section
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Possible Conditions Near a Cold Front
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Warm Front
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Warm Frontal Cross-Section
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Possible Conditions Near a Warm Front
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Quasi-Stationary Fronts
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Occluded Fronts

• As a surface cyclone evolves, its associated cold
  front will move faster than its warm front and
  the two will eventually collide to form an
  occluded front
• Two Types of Occlusion
• Warm
• Cold

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Warm Occlusion

• Warm is a relative term meaning cool air moves
  over colder air

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Cold Occlusion

• Very cold air overtakes the cold air already in
  place

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Cyclogenesis

• Cyclogenesis Development or strengthening of a
  mid-latitude cyclone
• Cyclones form where there is a contrast between
  air masses (warm vs. cold)

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Polar Front Theory
(b)
(c)
(a)
(d)
(e)
(f)
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Step One
cP and mT air meet at a stationary front. There
is a trough of low pressure with higher pressure
on both sides, cold air to the north, and warm
air to the south. Wind flows parallel to the
front.
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
15
Step Two
Under the right conditions, a wavelike kink forms
on the front. The wave that forms is known as a
frontal wave. The circulation of winds around the
cyclone tends to produce a wavelike deformation
on the front.
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
16
Step Three
Steered by the winds aloft, the system typically
moves east or northeastward and gradually becomes
a fully developed open wave in 12 to 24 hours.
Open wave the stage of development of a wave
cyclone where a cold front and a warm front
exist, but no occluded front. The center of
lowest pressure in the wave is located at the
junction of the two fronts.
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
17
Step Four
The central pressure is now lower and several
isobars encircle the wave. The more tightly
packed isobars create a stronger cyclonic flow
and winds swirl counterclockwise and inward
toward the lows center. Energy for the storm is
derived from rising warm air and sinking cold air
transforming potential energy to kinetic energy
(energy of motion). Condensation supplies energy
through latent heat release. Converging surface
winds produce an increase of kinetic energy. The
cold front advances on the warm front.
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
18
As the open wave moves eastward, central
pressures continue to decrease, and the winds
blow more vigorously. The faster-moving cold
front constantly inches closer to the warm front,
squeezing the warm sector into a smaller area.
Eventually the cold front overtakes the warm
front and the system becomes occluded. The storm
is usually most intense at this time, with clouds
and precipitation covering a large area.
Step Five
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
19
Step Six

• The intense storm gradually dissipates because
  cold air now lies on both sides of the cyclone.
  Without the supply of energy provided by the
  rising warm, moist air, the old storm system dies
  out and gradually disappears. Occasionally, a
  new wave will form on the westward end of the
  trailing cold front.
• The entire life cycle of a wave cyclone can last
  from a few days to over a week.

From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
20
A series of wave cyclones (a "family" of
cyclones) forming along the polar front
From C. Donald Ahrens Essentials of Meteorology
An Invitation to the Atmosphere and D. Miller at
UNCA
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