The Formation of Fronts and the Weather Changes Associated with Them
A front is a zone of transition formed between converging air masses.
The front forms because the air masses cannot mix intimately, in order
to form a uniform parcel of air. This inability to do so is caused by
differences in the physical conditions of the air masses involved.
are usually formed when one of the air masses is warm, while
the other is cold or when one of the air masses is moist and the other
is dry. Also, when air parcels have different densities, it is not
possible for them to combine.
There are four main types of fronts: cold fronts, warm fronts,
stationary fronts and occluded fronts. These fronts are formed by
different processes and so differ in properties/ characteristics.
A cold front is formed when a cold air mass moves towards a warm air
mass, converging with it. The cold air mass undercuts the warm parcel
of air, due to its higher density, and forces/pushes the warm air
upwards, higher into the atmosphere. In simpler terms, at a cold
front, cold air replaces warm air.
During the passage of a cold front, atmospheric/ weather
change. As a cold front nears temperatures begin to decrease. All the
same, atmospheric pressure also steadily decreases as the cold front
approaches. An approaching cold front causes moderate precipitation
usually in the form of showers of rain.
As a cold front passes there is a sudden drop in temperature. These
temperatures continue to fall after the front passes. A minimum
pressure value is also reached, but following this pressure slowly
begins to rise again and continues to rise. Due to this low pressure
found at cold fronts, they are associated
With the passing of a cold front, the showers intensify to form heavy
rains with thunderstorms often present. Wind is also affected by the
passing of a cold front. Wind direction changes and wind speed and
wind strength/intensity also increase, due to steep pressure
gradients. The warm air that rises forms cumulonimbus and cumulus
clouds. As a result, visibility is very poor.
There is quite a large difference in temperature either side of a cold
front; sometimes a difference as large as 20 degrees Fahrenheit.
On a map, cold fronts are represented by a solid blue line with
projecting triangles, pointing in the direction in which the cold air
Warm fronts are formed when a mass of warm air advances on a mass of
cold air. As the warm air is less dense and therefore lighter, it
rises over the cold air surface along a gentle slope. This process is
called overrunning. The passage of a warm front also brings on weather
As a warm front approaches a particular location, the pressure falls
slightly and high cirrus clouds form ahead of the front. Thicker and
lower clouds including cirrostratus, altostratus and nimbostratus soon
replace these clouds. Due to the presence of these clouds, visibility
is very poor. Light precipitation is present in the form of rain,
snow, sleet and drizzle. As a warm front approaches there is also a
slight rise in temperature. Just before the warm front passes, stratus
clouds and fog form.
During the passage of a warm front, there is a steady rise in
temperature. Also, the pressure stops falling and precipitation
essentially ceases. During a warm front, the wind direction shifts,
while the visibility improves due to dissipation of fog.
On weather maps, solid red lines with semicircles pointing towards the
direction in which the warm air moves represent warm fronts.
The weather changes brought on by the passage of a warm front are less
profound than those caused by cold fronts, but the area upon which a
warm front acts is much more extensive.
A stationary front is the name given to a front in which neither the
warm air or the cold air masses or advancing to or retreating from
each other. This occurs because both air masses possess the same
pressure. Owing to this, winds at a stationary front blow parallel to
it. The weather conditions caused by a stationary front are comparable
to those caused by a warm front, although less intense and extensive.
Stationary waves can either, weaken and dissipate, or they can become
a warm or cold front and begin to move. There are temperature
differences and wind direction differences between the two sides of a
stationary front. A stationary front is represented by a symbol that
comprises the warm front symbol and cold front symbol, drawn back to
In a depression, there is usually a warm front and a cold front. As
the depression intensifies, the cold front travels faster than the
warm and often catches up with it. The boundary line where two fronts
meet is termed an occluded front. There are two types of occluded
fronts: the cold occlusion and the warm occlusion.
The cold occlusion behaves similarly to a cold front. When the air
mass behind the occluded front is colder than the air mass in front,
the cold mass undercuts the cooler mass forcing it to rise.
occlude3.jpg (6461 bytes)
The warm occlusion is similar to a warm front. This occurs when the
air mass behind the occluded front is cool and lighter than the cold
and heavier air mass in front of it. The cool air mass overruns the
cold air mass.
occlude4.jpg (6423 bytes)
With the passing of an occluded front, wind direction changes. The
pressure falls until it reaches a minimum point. At this point it
begins to rise again. Cloud types present are cirrus, cumulus,
altostratus, nimbostratus sometimes followed by cumulonimbus and then
finally cumulus clouds. During the passing of an occluded front,
precipitation may be light, moderate or continuous depending on the
amount of rising air and the moisture content of it. Due to cloud
presence and precipitation, visibility is usually poor, but improves
once the occlusion has passed.
Cold and warm occlusions both cause temperature changes. The passage
of a warm occlusion causes a rise in temperature, whereas the passage
of a cold occlusion causes a drop in temperature.
In conclusion, fronts are simply zones of transition, where air masses
of different physical conditions, converge. Depending on the
conditions of each air mass (temperature and humidity mainly), a
certain type of front is formed. Different types of fronts bring about
different weather and atmospheric conditions.