Download Fronts

Fronts
Fronts are boundary zones between warmer and colder air masses:
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Fronts are usually associated with low-pressure systems (see next
page)
- In the classic setup, the warm front extends generally
eastward from the low center
- The cold front extends either westward or southward (or
somewhere in between) depending on the stage of
development
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As time evolves, the fronts move counter-clockwise around the
low center (following the wind)
- And as we'll see, the cold front typically moves faster
than the warm front
classic arrangement of the fronts and low center in
an evolving cyclone
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The type of front is defined by which air mass is advancing and
which retreating
- If colder air is advancing into a region with warmer air, it's
a cold front
- If warmer air advances into a region with colder air, it's a
warm front
- And if the boundary between the warmer and colder air
isn't moving, then it's a stationary front
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warm air
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Roughly speaking, fronts are often found below the jet stream
- Remember that the jet is determined by regions of
temperature contrast
Lsfc
Lsfc
300 mb heights and winds
surface temperature
Properties of Fronts
Fronts are defined by a contrast in
temperature. But they're also usually
associated with
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A kink in the isobar pattern, with
lower pressure at the front
A shift in the wind direction
A change in dew point temperature
(usually)
Widespread clouds and
precipitation (usually)
an example cold front
Cold front: colder air advancing into a warmer region
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As the colder air advances, it undercuts the warmer air and
causes lifting and associated clouds (often thunderstorms)
Cold front: colder air advancing into a warmer region
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As the colder air advances, it undercuts the warmer air and
causes lifting and associated clouds (often thunderstorms)
radar image showing
thunderstorms along a
cold front (storms show
up in red)
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The boundary between the warmer and colder air masses slopes
with height, in the direction towards the colder air
- Think of the colder air 'spilling' along the ground
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The slope is relatively steep, by atmospheric standards (roughly
1 km depth for every 50 km distance)
Warm front: warmer air advancing into colder region
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As the warmer air advances, it rides up over the colder air,
producing widespread clouds and rain (often nimbostratus)
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As with the cold front, the boundary between the warm and
cold air slopes with height, in the direction of the colder air
- But the slope is much less steep (roughly 1 km depth
for every 150 km distance)
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Because the slope is less steep, the vertical motion is more
gentle---so widespread rain instead of thunderstorms
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Often there's a gradual change in cloud types across a warm
front: starting with cirrus, to cirrostratus, to alto-stratus, to
nimbostratus, to stratus, and finally fog
An observer at the ground sees the full progression as the front
passes by
Schematic of wintertime warm
front showing progression of
precipitation types as warm air
advances
snow
sleet / freezing rain
rain and fog
Cold fronts move faster than warm fronts. Why?
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Generally, cold air tends to spill along the ground, spreading
into regions of warmer air
motion of frontal system
around the low
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For a cold front, this spreading is in the same direction as the
system motion, which speeds things up
motion of frontal system
around the low
cold air spilling
adds to motion
of the system
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For a cold front, this spreading is in the same direction as the
frontal propagation, which speeds things up
For the warm front, the spilling is opposite the system motion,
which slows things down
motion of frontal system
around the low
cold air spilling
adds to motion
of the system
cold air spilling
reduces motion
of the system
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The end result is that the cold front will eventually catch up to
the warm front!
motion of frontal system
around the low
cold air spilling
adds to motion
of the system
cold air spilling
reduces motion
of the system
Occluded front: when the cold front catches the warm front
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As the fronts circle the low, the cold front generally moves faster
If the cold front catches the warm front, we get an occluded
front (or occlusion) where the two cold air masses meet
schematic
occluded front
(purple)
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In an occluded front, the
warm air mass is lifted
entirely off the ground
Often this leads to a
widespread area of
clouds around the low
pressure system
Stationary front: the boundary between the air masses stays fixed
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If the boundary between the warm and cold air doesn't move
appreciably, we have a stationary front
Often a stationary front is a precursor to cyclone development
Example surface map showing cold, warm, stationary
and occluded fronts