Transcript Class #25: Friday, March 7

Class #20: Friday, October 16
Air Masses
Fronts
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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 warmer 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 air masses
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More on stationary fronts
• Surface winds blow along the front, so that
it remains stationary
• Above the surface, winds may blow across
the front and cause frontal lifting,
cloudiness, and precipitation with the
stationary front
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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
warm 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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Real warm fronts
• The meteogram shows a warm frontal
passage at 1000 UTC
• Move at about half the speed of cold fronts
• Have a sequence of layer clouds
• Have the highest clouds well ahead of the
front at the surface
• Are very 3-dimensional
• Can stall, for example in mountains
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Real warm fronts
• Can stall when the cold dense air is hard
to replace
• Can have broad bands of moderate
precipitation
• Can produce long periods of precipitation
when they stall
• Can produce frontal fog with evaporation
• Are associated with freezing rain and sleet
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Stationary fronts (continued)
• Weather along a stationary front can
resemble a warm front
• Although the front is stationary at the
surface, strong winds aloft may blow
across the front aloft, causing overrunning
• Can have extended periods of cloudiness
and precipitation on the cold side of the
front.
• Can have a jet stream aloft
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Occluded fronts
• Involve 3 air masses
– 2 polar air masses at the surface, usually mP
and cP
– 1 tropical air mass, mT that has been lifted
entirely off the surface, and is occluded or
hidden from the surface weather map
• Have weather like warm fronts where mT
and mP air masses meet, and weather like
cold fronts where mT and mP air masses
meet
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Occluded fronts (continued)
• Are of 2 types, warm occlusions and cold
occlusions, named for the change in
temperature behind the front.
• Warm occlusions form on the west coast of the
U.S. and Europe, when the air mass behind the
front is usually from the ocean, mP.
• Cold occlusions frequently form in the eastern
half of the U.S., with mP ahead and cP behind
the front.
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Drylines
• Are not true fronts because there is no
temperature contrast across the front.
• Resemble fronts because there is a
boundary between air masses, cT and mT.
• Resemble fronts because there is a wind
shift, pressure trough, convergence, and
often convective clouds along the dryline.
• Occur in West Texas in spring and early
summer, and severe weather can occur.
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Drylines (continued)
• Have a strong contrast in humidity and
wind direction across the front
• Can persist for several days.
• Can move westward at night (called the
dew-point front)
• Move from west to east during the day
• Are a powerful source of convergence
• The moister air is lighter, and rises
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