ES 20.2 Weather Fronts
Fronts
ES 20.2 Weather Fronts
Formation of Fronts:




Recall that air masses have different
temperatures and amounts of moisture,
depending on their source regions.
Recall that these properties can change
as an air mass moves over a region. What
in fact happens when two air masses
meet?
When two air masses meet, they form a
front, which is a boundary that
separates two air masses of different
properties.
Fronts can form between any two
contrasting air masses. Fronts are often
associated with some form of
precipitation.
ES 20.2 Weather Fronts




In contrast to the vast size of the air
masses; fronts are very narrow. Most
weather fronts are between 15 and
200 kilometers wide.
Above Earth’s surface, the frontal
surface slopes at a low angle so that
the warmer , less dense air overlies
cooler, denser air.
Occasionally, the air masses on both
sides of a front move in the same
direction and at the same speed.
When this happens, the front acts
simply as a barrier that separates the
two air masses.
ES 20.2 Weather Fronts




In most cases however, the
distribution of pressure across a front
causes one air mass to move faster
than the other.
When this happens, one air mass
advances onto another and some mixing
of air occurs.
Fronts are classified according to the
temperature of the advancing front.
There are four types of fronts: warm
fronts, cold fronts, stationary fronts,
and occluded fronts.
ES 20.2 Weather Fronts
Warm fronts:


A warm front forms when air
moves into an area formerly
covered by a cooler front.
On a weather map, the surface
position of a warm front is
shown by a red line with red
semicircles that point toward
the cooler air.
ES 20.2 Weather Fronts


1.
2.
3.
The slope of a warm front is very
gradual.
As warm air rises, it cools to
produce clouds, and frequently
precipitation. The sequence of
clouds shown at right typically
comes before a warm front.
The first sign of the approaching
warm front is the appearance of
cirrus clouds.
As the front nears, cirrus clouds
change to cirrostratus clouds,
which blend into denser sheets of
altostratus clouds.
About 300 kilometers ahead of the
front, thicker stratus and
nimbostratus clouds appear and rain
or snow develops.
ES 20.2 Weather Fronts



Because of their slow rate of
development and very low slope,
warm fronts usually produce light
to moderate precipitation over a
large area for an extended period
of time.
A gradual increase in temperature
occurs with the passage of a warm
front. The increase is most
apparent when a large temperature
difference exists between adjacent
air masses.
Also a wind shift from the east to
the southwest is associated with a
warm front.
ES 20.2 Weather Fronts
Cold fronts:



A cold front forms when a cold,
dense air moves into a region
occupied by warmer air.
On a weather map, the surface
position of a cold front is shown
by a blue line edged with blue
triangles that point toward the
warmer air mass.
The picture at right shows how
a cold front develops. As the
cold front moves in, pushing the
warm air up over it, it becomes
steeper.
ES 20.2 Weather Fronts



On average, cold fronts are
about twice as steep as warm
fronts and advance more
rapidly than warm fronts do.
These two differences,
steepness of slope and rate of
movement, account for the
more violent weather
associated with cold fronts.
The forceful lifting of air along
a cold front can lead to heavy
downpours and gusty winds.
ES 20.2 Weather Fronts





As a cold front approaches,
towering clouds can often be
seen in the distance.
Once the cold front has passed,
temperatures drop and winds
shift.
The weather behind a cold
front is dominated by a cold air
mass.
So, weather clears soon after a
cold front passes.
When a cold front moves over a
warm area, low cumulus or
altostratus clouds may form
behind the front.
ES 20.2 Weather Fronts
Stationary Fronts:




Occasionally, the flow of air on
either side of a front is parallel
to the line of the front.
In such cases, the surface
position of the front does not
move, and a stationary front
forms.
On a weather map, stationary
fronts are shown by blue
triangles on one side of the front
and red semicircles on the other
side.
Sometimes, gentle to moderate
precipitation occurs along a
stationary front.
ES 20.2 Weather Fronts
Occluded Fronts:




When a cold front overtakes a
warm front, an occluded front
forms.
An occluded front develops as
the advancing cold air wedges
the warm front upward.
The weather associated with an
occluded front is generally
complex.
Most precipitation is associated
with the warm air being forced
upward.
ES 20.2 Weather Fronts




When conditions are suitable,
the newly formed front is
capable of making light
precipitation on it’s own.
It is important to note that the
descriptions of weather
associated with fronts are
general descriptions.
The weather along an individual
front line may or may not
conform to the idealized
conditions we are discussing.
Fronts, like all aspects of
nature, do not always behave as
we would expect.
ES 20.2 Weather Fronts
Mid-latitude cyclones:



Now that we have looked at
air masses and what happens
when they meet, we are
ready to apply the
information to understanding
and predicting weather.
The main weather producers
in the country are middlelatitude cyclones.
On weather maps, these lowpressure areas are shown by
the letter L.
ES 20.2 Weather Fronts




Middle latitude cyclones are
large centers of low pressure
that generally travel from
west to east and cause stormy
weather.
The air in these weather
systems moves in a
counterclockwise direction and
in toward the center of a low.
Most middle-latitude cyclones
have a cold-front, and
frequently a warm front,
extending from a central area.
Forceful lifting causes the
formation of clouds that drop
abundant precipitation.
ES 20.2 Weather Fronts
How do cyclones form?



The first stage is the
development of a stationary
front.
The front forms as two air
masses with different
temperatures move in
opposite directions.
Over time, the front takes a
wave shape. This wave is
usually hundreds of
kilometers long.
ES 20.2 Weather Fronts




As the wave develops, warm
air moves towards Earth’s
poles.
There it invades the area
formerly occupied by colder
air. Meanwhile, cold air
moves toward the equator.
This change in airflow near
the surface is accompanied
by a change in pressure.
The result is a
counterclockwise airflow in
the Northern Hemisphere.
ES 20.2 Weather Fronts



Recall that a cold front
advances faster than a warm
front.
When this occurs in the
development of a mid- latitude
cyclone, the cold front closes in
and eventually lifts the warm
front. This process, which is
known as occlusion, forms the
occluded front we see at right.
As occlusion begins, the storm
often gets stronger. Pressure
at the storms center falls, and
wind speeds increase. In winter,
heavy snowfalls and blizzardlike conditions may develop
from this.
ES 20.2 Weather Fronts
The Role of Airflow Aloft:





Airflow aloft plays an important role in
maintaining cyclonic and anti-cyclonic
circulation.
In fact, these rotating surface wind
systems are actually generated by upperlevel airflow. Cyclones often exist for a
week or longer.
For this to happen, surface convergence
must be offset by outflow somewhere
higher in the atmosphere (divergence).
As long as the spreading out of air high up
is equal to or greater than the surface
inflow, the low-pressure system can be
maintained.
More often than not, air high up in the
atmosphere fuels a mid-latitude cyclone.
ES 20.2 Weather Fronts
The Role of Airflow Aloft:





Because cyclones bring stormy weather,
they have received far more attention
than anti-cyclones.
However a close relationships exists
between these two pressure systems.
The surface air that feeds a cyclone
generally originates as air flowing out of
an anticyclone.
As a result, cyclones and anticyclones
are usually found next to each other.
Like a cyclone, an anticyclone depends
on the flow of air high in the
atmosphere to feed it. In an
anticyclone, air spreading out at the
surface is balanced by air coming
together up high.
cyclone
anticyclone