WHAT IS A FRONT?
Fronts = the boundaries between air masses
A front is defined as the transition zone
between two air masses! As an example in
the diagram below there is a front that exists
between the advancing cold air mass and the
retreating warm air mass. A cold front is
the LEADING edge of the cold air mass. A
warm front is the LEADING edge of a warm
air mass.
One thing to always remember:
Cold air is denser than warm air.
Colder air is always on the bottom
pushing around the warmer air up
and out of the way.
THERE ARE FOUR MAIN TYPES OF FRONTS: (Cold, Warm, Occluded, Stationary)
COLD FRONTS:
A cold front shows the advancement of cold air into
a region of warmer air. Interactions along the edge
of this front often results in showers and storms
developing, as the warmer air is pushed upward by
the advancing colder air. Usually the precipitation
will last only a few hours as the front passes
The air behind a cold front is noticeably colder and
drier than the air ahead of it. When a cold front
passes through, temperatures can drop more than 15
degrees within the first hour!
There is typically a noticeable
temperature change from one side of
a cold front to the other. In the map
of surface temperatures to the right,
the station east of the front reported a
temperature of 55 degrees Fahrenheit
while a short distance behind the
front, the temperature decreased to
38 degrees. An abrupt temperature
change over a short distance is a
good indicator that a front is located
somewhere in between.
Common characteristics associated with cold fronts have been listed in the table below.
Temperature
Pressure
Precipitation
Weather Change
From warm humid air to colder less
humid air
The air pressure steadily increases
Short Period of Showers while the
front is passing which could last about
2 to 3 hours
WARM FRONTS:
Warm Fronts are regions of warm air that
are advancing on colder air. Warm fronts
usually move more slowly than cold fronts.
Warm front usually have continual and
prolonged precipitation lasting several hours
or more. Warm fronts on a map are red and
have scalloped edges pointing towards the
cool air it's attempting to override.
When a warm front passes through, the air
becomes noticeably warmer and more
humid than it was before.
Weather Change
From cool temps to warmer temps
Temperature
Pressure
Precipitation
The air pressure steadily drops
Long Period of Showers while the
front is passing which could last up to
24 hours
STATIONARY FRONTS:
Stationary Fronts are pretty much just that:
Stationary. The two air masses just don’t
move. The weather along these fronts is
similar to warm fronts, less turbulent, but
often longer lasting.
Occluded Fronts are usually only seen in mature
cyclones, and they develop right around the same
time a storm system has peaked in strength and
will begin to weaken. Here onrushing cold air,
which is most dense and thus digs under all air
with warmer temperature, has at last elevated all
warm air ahead of it and has become separated
from a warm influx and is left with cool air in its
place. Warm air may still be fed into the system for energy, but it will be at points farther
and farther away from the beginning of the occlusion.
HOW ARE FRONTS DIAGRAMMED?
In order to understand the weather, you have to understand the “language” of weather
maps. Pay close attention and you will be able to understand the weather report on the
nightly news and explain it to you parents.
The COLD FRONT:
The leading edge (the moving portion) of a cold front is
always indicated by a blue line with forward pointing
blue triangles.
Blue line with blue triangles = cold front (ahead of the
cold air mass)
The map below shows a typical weather map of a cold
front. Notice that the cold air is moving to the east (to
the left). Notice that the temperatures behind the cold front are much cooler then the
warm air ahead of it. In the next few hours, the warm air ahead of the cold front will be
replace by that cold air!
The WARM FRONT:
The leading edge (the moving portion)
of a warm front is always indicated by a
red line with forward pointing red semicircles.
Red line with red semicircles = warm
front (ahead of the warm air mass)
So, in the picture to the right, the warm
air mass is moving up, and the cold air
mass is moving down and to the left –
trying to fill in the area behind the warm
air mass.
The STATIONARY FRONT:
When a warm or cold front stops moving, it becomes a stationary front. A stationary front
is represented by alternating blue and red lines with blue triangles pointing towards the
warmer air and red semicircles pointing towards the colder air.
HOW DOES PRECIPITATION FROM?
Precipitation Along a Cold Front
(precipitation when a cold air mass overtakes a warmer air mass)
The animation below depicts the development of precipitation ahead of and along a cold
front. The surging blue mass represents colder air behind the cold front (solid blue line)
while the yellow shading indicates the warm moist air mass ahead of the front which is
being moved out of the way.
Animation by: Hall
As the front advances, the colder air lifts the warmer air ahead of it (red arrows). As the
warm air rises, it condenses to produce clouds and precipitation.
If condensation of water vapor persists, precipitation may develop, typically in a narrow
band just ahead of the cold front.
Due to the steep slope of a cold front, vigorous rising motion is often produced, leading
to the development of showers and occasionally severe thunderstorms.
Precipitation Along a Warm Front
(precipitation when a warm air mass overtakes a colder air mass)
The animation below depicts the development of precipitation ahead of and along a warm
front. The region shaded in blue represents a colder air mass while the yellow shading
indicates the warm moist air behind the warm front (solid red line).
Animation by: Hall
Since warm air is less dense then cold air, the leading edge of the warm front has a
tendency to move up and over the colder air mass ahead of it. As the warm air rides up
and over the cold air mass it produces clouds and rain. The clouds and rain actually
occur many hours before warm air actually arrives to the area. Because the lifting of the
warm air is very gradual and steady, generally widespread and light intensity
precipitation develops ahead of a warm front.
A Closer Examination of the Animation:
Initially, a warm air mass (in yellow) nudges against a colder air mass (in blue) ahead of
it, (separated from each other by the warm front). The lighter warm moist air behind the
front is lifted upward and "overrides" the colder air.
A layer of thin clouds is occasionally observed more than a 400 miles in advance of a
surface warm front. As the front gets closer, the clouds thicken and eventually light
precipitation begins to fall.
Adapted from the works of WW2010. Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign. Credits and Acknowledgments