the approach of a warm front, and then level
Stationary Front
off once the front has passed. When a warm
Sometimes, weather systems stall and a
front precedes a deep depression (an area of
front will stop moving forward or will oscil-
particularly low pressure indicating a severe
late back and forth. A stalled front is called a
storm), the atmospheric pressure may fall as
stationary front. Near a stationary front, the
rapidly as 3 Mb per hour.
weather is often gray and dull with precipita-
In the Northern Hemisphere, the wind will
tion. Rain and snow may persist for days and
shift from east-northeast, to east-southeast,
changes in barometric pressure, wind, cloud,
and to south-southwest as a warm front ap-
temperature, and dew point are usually
proaches and passes. Clouds appear in the
slight. There will be little change until new
following order before a warm front: cir-
developments in the upper-level airflow get
rus, cirrostratus, altostratus, nimbostratus,
the weather system moving again.
and stratus. These clouds will usually break
up and may even clear out as the warm air
Occluded Front
moves in with its greater capacity to hold
An occluded front forms where a faster
moisture. The temperature will rise, often
moving cold front overtakes and combines
dramatically, with the approach of a warm
with a warm front. The barometric pressure
front, then level off after it passes.
normally falls with the approach of an oc-
As a warm front approaches, the dew point
clusion and rises with its passage. However,
will rise and the separation between the
pressure may become steady for periods
temperature and dew point will diminish. At
depending on whether the associated low-
the passage of the warm front, humidity will
pressure system is deepening.
reach nearly 100 percent near the surface.
If you are in the Northern Hemisphere
Precipitation usually precedes the passage
and located west of the occlusion, the wind
of a warm front and is produced by middle
will back from east-southeast, to east-
and low clouds: altostratus, nimbostra-
northeast, and finally to north-northwest.
tus, and stratus. Precipitation is normally
If you are east of the occlusion, it will veer
widespread and heavy. During winter in the
from east-southeast, to south-southeast, to
r
mid-latitudes, precipitation starts as snow
south-southwest, and finally to west-south-
and often changes to sleet, freezing rain, and
west or west-northwest.
finally rain. The precipitation usually ends
e
athe
You’ll usually find precipitation on both
with the warm front passage.
W
sides of an occluded front. The clouds as-
Figure 3a
r
e
athe
r
standing W
Figures 3a-d When a warm and cold front merge around a deepening system of low pressure,
nde
mariners can expect a significant and volatile change in the weather. In the four stages of
U
frontal activity depicted here, note the closely spaced isobars around the developing low, and
the corresponding increase in the wind strength as evidenced by the wind arrows. Ahead of
and along the warm front, expect rain and inclement weather. The high pressure filling in
from behind the fronts will bring clear skies and north- and north-westerly winds.
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