southeasterly direction to a southwesterly direction
behind the front. This shift is not as pronounced as with
the cold front.
Temperature generally is constant or slowly rising
in advance of the front until the surface front passes, at
which time there is a marked rise. This rise is dependent
upon the contrast between the air masses. Dew point
usually increases slowly with the approach of the front
with a rapid increase in precipitation and fog areas. If
the warm sector air is maritime tropical, the dew point
shows a further increase.
WEATHER
A characteristic phenomenon of a typical warm
front is the sequence of cloud formations (fig. 4-36).
They are noticeable in the following order: cirrus,
cirrostratus, altostratus, nimbostratus, and stratus. The
cirrus clouds may appear 700 to 1,000 miles or more
ahead of the surface front, followed by cirrostratus
clouds about 600 miles ahead of the surface front and
altostratus about 500 miles ahead of the surface front.
Precipitation
in
the
form
of
continuous
or
intermittent rain, snow, or drizzle is frequent as far as
300 miles in advance of the surface front. Surface
precipitation is associated with the nimbostratus in the
warm air above the frontal surface and with stratus in
the cold air. However, when the warm air is
convectively unstable, showers and thunderstorms may
occur in addition to the steady precipitation. This is
especially true with a cyclonic flow aloft over the warm
front. Fog is common in the cold air ahead of a warm
front.
4-38
AG5f0436
STABLE
WARM
AIR
30,000
20,000
10,000
SFC
0 C
O
S C
N S
A S
C S
C I
A S
S T
F O G , R A I N A N D L O W N I M B O S T R A T U S
200
100
0
100
200
300
400
500
600
SC
SC
0 C
O
C I
C S
A S
N S
C B
UNSTABLE
WARM
AIR
30,000
20,000
10,000
SFC
0 C
O
200
100
0
100
200
300
400
500
600
L I G H T R A I N A N D S H O W E R S
N S
S T
S H O W E R S
A C
A S
SC
0 C
O
Figure 4-36.—Vertical cross section of a warm front with stable and unstable air.
