Transcript Class #25: Friday, March 7

Class #20: Friday, October 15
Air Masses
Fronts
Class #20: October 15, 2010
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Air Masses
• An air mass is an extremely large body of
air whose properties of temperature and
moisture content (humidity) are similar in
any horizontal direction.
• In a typical year, air mass weather kills
more people in the U.S. than all other
weather phenomena combined.
– Heat waves, most dangerous weather type
– Cold air outbreaks are also dangerous
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Air mass types by temperature
• Polar (P): formed poleward of 60º
– Cold or cool
• Arctic (A): formed over the arctic
– Very cold
• Tropical (T): formed within 30º of the
equator
– Hot or warm
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Air mass types by moisture amount
• Continental (c): formed over large land
masses
– Dry
• Maritime (m): formed over the oceans
– Moist
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Fronts
• Air masses are important in themselves,
and at their boundaries, fronts occur.
• A front is the transition zone between two
different air masses.
• Fronts were named around the time of
World War I (1910s) because they had
disruptive weather and looked like the
boundaries on military maps separating
armies.
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The generic front
• Is the boundary between 2 (3 for the
occluded front) air masses of differing
temperature.
• Slopes in the vertical up from the surface
toward the colder air mass.
• Always has the warmer air mass above
the colder air mass (never the reverse).
• Is the scene of frontal lifting if winds blow
in part across the front.
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The generic front (continued)
• Always has a temperature contrast at the
surface between the two air masses.
• Is of synoptic scale along the front and
mesoscale across the front.
• Has a cyclonic (counterclockwise in NH)
wind shift, a minimum (trough) in surface
pressure, and usually a change in humidity
across the front.
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The generic front (continued)
• Looks like a line on a surface weather
map.
• Is called a frontal zone where it meets the
ground on the surface weather map.
• Is an area where weather conditions
change rapidly over short distances
(maybe even a few miles) from one air
mass to another.
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Different types of fronts
• Stationary front:
– Remains in roughly the same location
– Surface winds in both air masses blow along
the front
– Precedes the development of an extratropical
cyclone
– Common in the location of the polar front
– Separates T and P
– More on stationary fronts later
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Cold and warm fronts
• Form together when a stationary front
starts to move
• Form when the surface winds along a
stationary front start to blow across the
front
• Form when a stationary front deforms into
a comma or wavelike shape
• Form when a surface low center develops
on the stationary front
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Cold and warm fronts
• Are named by the temperature changes
that result after an air mass passes
• Are enhanced by convergence that
intensifies contrasts in temperature,
pressure, wind, and humidity
• Air is colder after a cold front passes
• Air is warmer after a warm front passes
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Cold fronts
• Have a slope up from the surface that is
closer to vertical than warm fronts.
• Have the colder air mass replacing the
warmer air mass at the surface.
• Have some of the most dramatic frontal
passages at the surface—greatest
weather changes in the shortest amount of
time.
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Cold fronts
• Move fairly rapidly
• May have thunderstorms in the warm
moist unstable air ahead of the front (mT)
or along the front
• Usually have fairly narrow rainbands along
and across the front
• Frequently lines of thunderstorms called
squall lines form ahead of and parallel to
cold fronts.
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Real cold fronts
• Don’t always look exactly like the idealized
fronts in the textbook
• The meteogram shows a frontal passage
at about 2200 UTC
• May be dry, with no clouds or precipitation
• May have blowing dust
• Can cause precipitation even at night
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Warm fronts
• Have a slope upward from the ground
inclined more towards the horizontal than
cold fronts
• Have weaker vertical motions than warm
fronts
• Have a special name for the upglide of
horizontal and vertical motion called
overrunning, warmer air over colder air
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