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CGS Ground School
Meteorology
Fronts
© Crown copyright 2012.
No part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
Fronts
A frontal surface is the boundary between two
adjacent airmasses of different temperatures.
The line where the frontal surface meets the
ground is called a front.
The front which chiefly
affects the UK is the polar
front which marks the
boundary between the polar
and tropical airmasses.
There are different kinds of polar fronts, the
following slides show the main fronts that
affect the UK.
Stationary front
A front making no progress in relation to the
ground.
The frontal surface slopes due to the warm air
being less dense than the cool air.
Synoptic Chart Symbol:
Warm air
Cold air
Warm front
A front at which warm air is replacing cool air.
The frontal gradient is typically between 1:100
and 1:150.
Surface friction reduces the speed of a warm
front to approx ⅔ of the speed of the surrounding
airmass.
Synoptic Chart Symbol:
Frontal movement
Warm air
Cold air
Cold front
A front at which cool air is replacing warm air.
The frontal gradient is typically around 1:50.
Synoptic Chart Symbol:
Surface friction may slow the
progress of the front, however
the cool airmass continues to
move at the speed of the
surrounding airmass.
This causes the formation of a
"nose" approx 2000 ft high.
This nose creates a very
unstable situation, which will
periodically collapse giving a
line squall.
This also explains the erratic
approach of a cold front.
Frontal movement
Cold air
Warm air
Occluded front
A front formed when a cold front catches up with
a warm front and either rides up it (warm
occlusion)
Frontal movement
Warm air
Cool air
Cold air
Occluded front
A front formed when a cold front catches up with
a warm front and either rides up it (warm
occlusion) or pushes beneath it (cold occlusion).
Synoptic Chart Symbol:
Frontal movement
Warm air
Cold air
Cool air
Development of fronts
The polar front separates the cool polar air from
the warmer tropical air.
The warm air tends to flow eastward.
Air in the polar region is at high pressure, the
cool air flowing away from the polar region
therefore tends to flow westward.
To understand the
reason for this deflection
see the presentation
‘wind’.
Development of fronts
In an area where the warm air flows towards
the cold air, it will be forced to rise.
This causes the local pressure to fall
and a wave starts to develop from the
warm air into the cool air.
Pressure within the vicinity
of the wave reduces even
further.
L
Development of fronts
As the wave increases in size the pressure
drops further, becoming lowest at the wave
crest.
The polar front has now become a
warm front to the east and a cold
front to the west.
L
Development of fronts
As the wave increases in size the pressure
drops further, becoming lowest at the wave
crest.
The polar front has now become a
warm front to the east and a cold
front to the west.
As the pressure in the
centre continues to fall, a
system of closed isobars
develops around the
depression.
The polar air will flow in
towards the low pressure
region in an attempt to
equalise the pressure.
L
Development of fronts
The air begins to flow around the depression
in an anti-clockwise direction.
As the depression
continues to deepen, it
moves eastward and the
faster moving cold front
starts to catch up with
the slower moving warm
front.
L
Development of fronts
The air begins to flow around the depression
in an anti-clockwise direction.
As the depression
continues to deepen, it
moves eastward and the
faster moving cold front
starts to catch up with
the slower moving warm
front.
L
Development of fronts
The air begins to flow around the depression
in an anti-clockwise direction.
As the depression
continues to deepen, it
moves eastward and the
faster moving cold front
starts to catch up with
the slower moving warm
front.
Development of fronts
Eventually, in the centre of the depression,
the cold front catches up with the warm front
and an occluded front is formed.
Initially the point of
occlusion remains in
the area of lowest
pressure.
However the pressure
at the point of the
occlusion rises as the
polar air continues to
flow in.
Development of fronts
This causes the point of the occlusion to
move southward, away from the centre of
low pressure.
Development of fronts
This causes the point of the occlusion to
move southward, away from the centre of
low pressure.
As the warm sector
becomes smaller so the
occlusion begins to
degenerate and the
cycle comes to an end.
Wind and pressure changes
As any front passes there will be a change in
wind direction and surface pressure.
The warm front sequence below could
represent any type of front.
Consider an observer at point A.
As the front approaches
the pressure reduces.
As the front passes the
wind veers.
Behind the front
the pressure
964
968
increases.
972
976
980
A
Weather changes
Passage
As
well asofthe
a warm
cold
windfront
direction
front.
and air pressure changing,
the most obvious sign of the passage of a frontal system
Consider an observer at point A
is a change in cloud types and associated precipitation.
At
the
front
theofofcloud
isobserver
almost
at
ground
level.
The
Behind
In the
cloud
approach
passage
cold
thecontinues
sector,
front
the
the
the
the
front
rain
cold
to
lower
continues
isfront
marked
(cirro
is
can
marked
for
stratus
by
see
up
low
clear
by
to&
cloud
50
aalto
skies
build
miles.
and
stratus).
with
up of
The
passage
ofpersistent
the until
frontpossibly
is
marked
byband
the
wind
veering.
Rain
upper
cumulus
heavy
cloud
becomes
cloud
rain.
clouds.
then
(Cirrus
lifts
and
eventually
over
a wide
Cirrostratus)
the
skies
(up
clear
and
to
200
and
medium
nm
In
the (Nimbostratus)
warm
sector
thelevels
cloud
low
(stratus)
and may
ahead
cloud
cumulus
Gradually
Where
of
the
clouds
the
increasing
frontal
front)
begin
nose
and
just
to
the
collapses,
form.
ahead
cloud
ofremains
upper
of
lowers
itthe
will
cloud
front.
also
further
form
be marked
be
accompanied
by light
drizzle.
(nimbostratus
Followed
These
approximately
by
a
line
may
squall.
bycontinue
a&
relatively
400nm
stratus),
toahead
develop
narrow
of the
band
vertically
front.
of rain.
to give
cumulonimbus clouds with associated rain.
Cold sector
40,000ft
30,000ft
20,000ft
Cu
10,000ft
A
Weather changes
Passage of an occluded front.
rain bandofisan
100-200
miles
wide
on a warm
occlusion
The passage
occluded
front
is generally
similar
to a
and 50-100
miles
widecloud
on a cold
occlusion.
warm
front with
upper
gradually
being replaced by
A coldcloud.
occlusion may also have embedded cumulonimbus
lower
clouds at the front.
40,000ft
Cool sector
30,000ft
Cb
Cu
20,000ft
10,000ft
THE END
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