Download Air Masses and Fronts

Air Masses
and Fronts
C. David Whiteman
Atmos 3200/Geog 3280
Mountain Weather and Climate
What is an air mass?
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A regional-scale volume of air with
horizontal layers of uniform temperature
and humidity
They form during high pressure episodes
when weak winds allow air to remain for
several days over a flat area with uniform
surface characteristics
Origin:
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Whiteman (2000)
Tropical ‘T’
Polar ‘P’
Arctic ‘A’
Continental ‘c’
Maritime ‘m’
Examples: mT warm, moist, usually
unstable; cT hot, dry, unstable air at sfc,
stable aloft; mP cool, moist, unstable; cP
cold dry stable; cA extremely cold
Source regions and trajectories for air masses
affecting North America.
Fronts
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The mid-latitudes are not a good
source region for air masses;
they are a region where clashing
air masses meet
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Boundaries between air masses
are called fronts or frontal zones.
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Fronts are associated with
surface low pressure centers
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Fronts move counterclockwise
around a low as the low moves
and evolves
Fronts can be visualized by imagining a
continuous front separating cold air over
the pole with warmer air to the south.
Types of fronts
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Surface front – A front that intersects
the earth’s surface
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Cold front – A front where cold air is
advancing
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Warm front – A front where cold air
is retreating
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Occluded front – the surface where
cold and warm fronts meet (or
zones of warm and cold advection
meet)
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Back-door cold and warm fronts –
fronts that move into an area from
an unusual direction (e.g., a cold
front from the east)
Upper-level front – A front that is
located aloft, but does not extend to
the surface
Whiteman 2000
Typical wintertime weather map with a front
Define: warm sector
Buys-Ballot Rule
Development of a polar front
More about fronts
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Additional characteristics
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Locally high stability (cold air near surface, warm aloft)
Wind directions shift across front
Change in wind with height (vertical wind shear)
Pressure troughing
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Frontogenesis – Process of increasing the magnitude of the horizontal
temperature gradient (creating a front)
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Frontolysis – Process of decreasing the magnitude of the horizontal
temperature gradient (destroying a front)
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Frontal slopes
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Warm front 1:150 to 1:300, speed 3-7 m/s
Cold front 1:50 to 1:150, speed 7-13 m/s
Front-cloud associations
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Frontal analysis in complex terrain of
western U.S.
Challenges
Errors arise from the reduction of surface pressure to sea
level
Difficult to determine intensity or even existence of
horizontal temperature gradients in regions due to
variability in surface station elevation
Conventional observations (NWS/FAA/DoD) over western
U.S. are of low density and are located primarily in valleys
Diabatic effects and boundary layer processes can obscure
large-scale airmass changes
Surface-based inversions mask temperature changes
Terrain-induced flows (thermally or dynamically driven)
mask wind changes
There can be contrasts in frontal intensity and position
between low and high elevation stations
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J. Steenburgh