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Chapter 9
Air Masses and Fronts
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.
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.
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 nor’easters
(above) that can bring cold winds and heavy snowfall.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
The next chapter examines
mid-latitude cyclones.