AIR MASSES AND FRONTS

precipitation accompany it. View A of figure 4-27
shows the height of the inversion in two different parts
of a frontal zone, and view B of figure 4-27 shows a
strong frontal inversion with a consequent dew-point
inversion.
A cold front generally shows a stronger inversion
than a warm front, and the inversion appears at
successively higher levels as the front moves past a
station. The reverse is true of warm fronts. Occluded
fronts generally show a double inversion. However, as
the occlusion process continues, mixing of the air
masses takes place, and the inversions are wiped out or
fuse into one inversion.
It is very important in raob analysis not to confuse
the subsidence inversion of polar and arctic air masses
with frontal inversions. Extremely cold continental
arctic air, for instance, has a strong inversion that
extends to the 700-mb level. Sometimes it is difficult to
find an inversion on a particular sounding, though it is
known that a front intersects the column of air over a
given station. This may be because of adiabatic
A
ATMOSPHERIC SOUNDINGS
IN THE COLD AIR MASS
FRONTAL ZONE
COLD AIR MASS
FRONTAL SURFACE
FRONTAL SLOPE
WARM AIR MASS
SURFACE
POSITION
OF FRONT
WARM AIR MASS
A. HEIGHT AND THICKNESS
OF INVERSION INDICATES SLOPE
OF FRONT AND INTENSITY
1004
1008
HIGH
LOW
HIGH
B
1000
1004
HIGH
LOW
4-28
DEW POINT
Figure 4-27.—Inversions.
HIGH
warming of the descending cold air just under the
frontal surface or excessive local vertical mixing in the
vicinity of the frontal zone. Under conditions of
subsidence of the cold air beneath the frontal surface,
the subsidence inversion within the cold air may be
more marked than the frontal zone itself.
Sometimes fronts on a raob sounding, which might
show a strong inversion, often are accompanied by little
weather activity. This is because of subsidence in the
warm air, which strengthens the inversion. The weather
activity at a front increases only when there is a net
upward vertical motion of the warm air mass.
C
1000
Wind
TEMPERATURE
ACCUMULATION
OF MOISTURE
HERE
B. FRONTAL INVERSION
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Since winds near Earth’s surface flow mainly along
the isobars with a slight drift toward lower pressure, it
follows that the wind direction in the vicinity of a front
must conform with the isobars. The arrows in figure
4-28 indicate the winds that correspond to the pressure
distribution.
1004
HIGH
LOW
HIGH
Figure 4-28.—Types of isobars associated with fronts.
D
HIGH
LOW
996
1000
HIGH
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