Air Masses and Fronts

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Chapter 9 Air Masses and Fronts
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The areas where air masses form are called source
regions. Based on moisture content, air masses
can be considered either continental (dry) or
maritime (moist). According to their temperature,
they are either tropical (warm), polar (cold),
or arctic (extremely cold). Meteorologists use a
two-letter shorthand scheme for categorizing air
masses. A small letter c or m indicates the
moisture conditions, followed by a capital letter
T, P, or A to represent temperature.
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Continental polar (cP) air masses form over
large, high-latitude land masses. In addition to
having very low temperatures, winter cP air
masses are extremely dry. Continental arctic (cA)
air is colder than continental polar and
separated by a transition zone similar to the
polar front called the arctic front.
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Maritime polar (mP) air masses are similar to
continental polar air masses but are more
moderate in both temperature and dryness.
Maritime polar air forms over the North Pacific
as cP air moves out from the interior of Asia.
Maritime polar air also affects much of the East
Coast with the circulation of air around
mid-latitude cyclones after they pass over a
region. The resultant winds are the famous
northeasters or noreasters (above) that can
bring cold winds and heavy snowfall.
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Continental tropical (cT) air forms during the
summer over hot, low-latitude areas. These air
masses are extremely hot and dry, and often
cloud-free. Maritime tropical (mT) air masses
develop over warm tropical waters. They are warm
(though not as hot as cT), moist, and unstable
near the surface, which are ideal conditions for
the development of clouds and precipitation.
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A cold front occurs when a wedge of cold air
advances toward the warm air ahead of it. A warm
front represents the boundary of a warm air mass
moving toward a cold one. A stationary front
differs in that neither air mass has recently
undergone substantial movement. Occluded fronts
appear at the surface as the boundary between two
polar air masses, with a colder polar air mass
usually advancing on a slightly warmer air mass.
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In a typical mid-latitude cyclone, cold and warm
fronts separated by a wedge of warm air meet at
the center of low pressure. Cold air dominates
the larger segment on the north side of the
system.
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Cold fronts typically move more rapidly and in a
slightly different direction from the warm air
ahead of them. This causes convergence ahead of
the front and the uplift of the warm air that can
lead to cumuliform cloud development
and precipitation. In this example, the cold air
(in blue) advances from west to east (notice that
the wind speed depicted by the thin arrows
increases with height). The warm air (in red) is
blowing toward the northeast. The cold air wedges
beneath the warm air and lifts it upward.
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Warm fronts have gentler sloping surfaces and do
not have the convex-upward profile of cold
fronts. Surface friction decreases with distance
from the ground, as indicated by the longer wind
vectors away from the surface (a). This causes
the surface of the front to become less steep
through time (b).
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Warm fronts separate advancing masses of warm air
from the colder air ahead. As is the case with
cold fronts, the differing densities of the two
air masses discourage mixing, so the warm air
flows upward along the boundary. This process is
called overrunning, which leads to extensive
cloud cover along the gently sloping surface of
cold air.
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Nonmoving boundaries are called stationary
fronts. Although they do not move as rapidly as
cold or warm fronts, they are identical to them
in terms of the relationship between their air
masses. As always, the frontal surface is
inclined, sloping over the cold air.
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The most complex type of front is an occluded
front or an occlusion, which refers to closure
such as the cutting off of a warm air mass from
the surface by the meeting of two fronts. When
the cold front meets the warm front ahead of it,
that segment becomes occluded, as shown above.
The warm air does not disappear, but gets lifted
upward, away from the surface. The occluded
front becomes longer as more of the cold front
converges with the warm front.
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Eventually, the cold front completely overtakes
the warm front, as shown above, and the entire
system is occluded. In this occlusion, the air
behind the original cold front was colder than
that ahead of the warm front. This is an example
of a cold-type occlusion.
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Occlusions sometimes occur when the circular core
of low pressure near the junction of the cold and
warm fronts changes shape and stretches backward,
away from its original position. In (a), the cold
and warm fronts are joined at the dashed line. At
some later time (b), the cold and warm fronts
have the same orientation with respect to each
other as they did in (a), but both have been
pulled back beyond the dashed line. The circular
isobar pattern of (a) becomes elongated to form a
trough over the occluded region.
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The boundaries separating humid air from dry air
are called drylines and are favored locations for
thunderstorm development. The dryline above (the
dashed line) separates low humidity to the west
while to the east humidity is higher as indicated
by the dew point temperatures.
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The next chapter examines mid-latitude cyclones.