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the system. At 100 miles, it will be
2,640 feet, which comes from
1/200*100, making half a mile,
multiplied by 5280 (feet).
Clouds will appear in this order as
you fly towards a warm front –
cirrus, cirrostratus, altostratus,
nimbostratus and stratus.
The shallow slope ensures that
whatever is coming will last some
time, and you can expect the
pressure to fall, the cloud to get
lower, the wind to back and increase
in speed, rising humidity, bad
visibility, drizzle and rain, though
not necessarily in that order.
The freezing level will be lower in
front than behind, and the slope
means that freezing rain will be
falling on anything underneath (see
diagram overleaf). Once you see ice
pellets, expect freezing rain next
(exam question).
As the front passes, the rain will
stop, then become drizzle under an
overcast sky, and the wind will veer.
As humidity rises to saturation point,
visibility will be poor. You will then
be in the warm sector, where
conditions will be more settled for a
few hours. For the exam, when
asked to predict the future position
of a warm front (and the type of
weather), use the direction of the
isobars in the warm sector, since the
front moves parallel to them. Since
warm air finds it hard to displace
cold air, a warm front will move at
about half the speed of a cold front
in the same conditions (see below).
After the warm sector comes......
The Cold front
This has a much steeper slope (1:50)
and brisker activity, with more of a
likelihood of thunderstorms. The
rain becomes more showery and the
wind veers more, to the West or
Northwest. Pressure gets higher, and
temperature and humidity decrease.
In temperate climates, large amounts
of Cu-nim are unusual at this point,
but they are not over continental
land areas.
The rain belt is relatively small
compared to the warm front, and
visibility will improve markedly.
Expect questions on weather at a
cold front, and after its passage
(good vis, some turbulence – see
Weather 141
below). Wind shifts will be usually
more pronounced. Look for this
icicle-like symbol:
A cold front moves at about the
speed of the wind perpendicular to it
just above the friction level (i.e.
about 2,000 feet), but they are faster
in Winter because the air is colder
and exerts greater pressure. It's
generally colder after its passage, and
with less cloud, because pressure is
greater to the West and less to the
East, limiting the inflow of air.
The Occlusion
This occurs because cold fronts
move faster than warm fronts. When
one catches up with the other, the
warm sector (the bit between a warm
and a cold front) is lifted from the
ground, leaving only one front on
the surface (the rate depends on the
temperature difference between the
air masses). Here is its symbol:
The same naming convention
applies, and you get more rain with a
warm occlusion. You also get a
quicker transition from warm to cold
front weather. A cold occlusion is
more or less the same as...
Trowal
A Trough of Warm Air Aloft, i.e. a cold
front catching up with a warm one.
Line Squalls
The product of severe cold frontal
conditions, in advance of the front,
where the cold front is nudging
under the warm sector (watch for an
acute bend in the isobars at the
front, or low roll cloud across the
advance).
Secondary Front
You get one of these when air just
behind a cold front flows down and
undercuts the warm air ahead. As it
compresses it gets warmer and
forms another front.
Upper Front
Arises from cold air trapped on the
surface forcing other air to go over
it. No change of air mass is recorded
as stations detect no change.
Jetstreams
These occur just under the
tropopause, where it has been
lowered after the polar front has
moved towards the Equator. Air is
channelled into a tubular ribbon of
high-speed air because of horizontal
temperature gradients, or large
thermal variations over short
distances (upper level winds are
caused by thermal effects).
The tropopause does not have the
same consistency around the Earth.
It is higher, and therefore colder, at
the equator, moving along with an
air mass. As it does so, the air mass
may "fold" the tropopause and
create a temporary temperature
gradient where it overlaps, and with
　
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