a rather broad zone of altostratus and nimbostratus
clouds accompany the front and extend several hundred
miles behind the front. If the warm air is unstable (or
conditionally
unstable),
thunderstorms
and
cumulonimbus clouds may develop within the cloud
bank and may stretch for some 50 miles behind the
surface front. These cumulonimbus clouds form within
the warm air mass. In the cold air there may be some
stratus or nimbostratus clouds formed by the
evaporation of falling rain; but, generally, outside of the
rain areas, there are relatively few low clouds. This is
because of the descending motion of the cold air that
sometimes produces a subsidence inversion some
distance behind the front.
The ceiling is generally low with the frontal
passage, and gradual lifting is observed after passage.
Visibility is poor in precipitation and may continue to
be reduced for many hours after frontal passage as long
as the precipitation occurs. When the cold air behind
the front is moist and stable, a deck of stratus clouds
and/or fog may persist for a number of hours after
frontal passage. The type of precipitation observed is
also dependent upon the stability and moisture
conditions of the air masses.
Upper Air Characteristics
Upper air contours show a cyclonic flow and are
usually parallel to the front as are the isotherms. The
weather usually extends as far in back of the front as
these features are parallel to it. When the orientation
changes, this usually indicates the position of the
associated upper air trough. (A trough is an elongated
area of relatively low pressure.)
The temperature inversion on this type of front is
usually well marked. In the precipitation area the
relative humidity is high in both air masses. Farther
behind the front, subsidence may occur, giving a
second inversion closer to the ground.
The wind usually backs rapidly with height (on the
order of some 60 to 70 degrees between 950 and 400
mb), and at 500 mb the wind direction is inclined at
about 15 degrees to the front. The wind component
normal to the front decreases slightly with height, and
the component parallel to the front increases rapidly.
On upper air charts, slow-moving cold fronts are
characterized
by
a
packing
(concentration)
of
isotherms behind them. The more closely packed the
isotherms and the more nearly they parallel the fronts,
the stronger the front.
FAST-MOVING COLD FRONTS (INACTIVE
COLD FRONT)
The fast-moving cold front is a very steep front that
has warm air near the surface being forced vigorously
upward. At high levels, the warm air is descending
downward along the frontal surface. This front has a
slope of 1:40 to 1:80 miles and usually moves rapidly;
25 to 30 knots may be considered an average speed of
movement. They move with 80 to 90 percent of the
wind component normal to the front. As a result of
these factors, there is a relatively narrow but often
violent band of weather.
Figure 4-32 shows a vertical cross section of a
fast-moving cold front with resultant weather. Also
indicated in the lower half of the diagram is the surface
weather in advance of the front and the upper airflow
above the front.
If the warm air is moist and unstable, a line of
thunderstorms frequently develops along this front.
Sometimes, under these conditions, a line of strong
convective activity is projected 50 to 200 miles ahead of
the front and parallel to it. This may develop into a line
of thunderstorms called a squall line. On the other hand,
when the warm air is stable, an overcast layer of
altostratus clouds and rain may extend over a large area
ahead of the front. If the warm air is very dry, little or no
cloudiness is associated with the front. The front
depicted is a typical front with typical characteristics.
The fast-moving cold front is considered an
inactive front because lifting occurs only at and ahead
of the front. The lifting is caused by descending air
ahead of the front and only in part by the frontal
surface.
Surface Characteristics
Pressure tendencies fall ahead of the front with
sudden and strong rises after frontal passage. If a squall
line lies some distance ahead of the front, there may be
a strong rise associated with its passage and a shift in
the wind. However, after the influence of the squall line
has passed, winds back to southerly and pressures level
off. The temperature falls in the warm air just ahead of
the front. This is caused by the evaporation of falling
precipitation. Rapid clearing and adiabatic warming
just behind the front tend to keep the cold air
temperature near that of the warm air. An abrupt
temperature change usually occurs far behind the front
near the center of the high-pressure center associated
with the cold air mass. The dew point and wind
4-33