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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 2/3 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
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 marks the final stages of a
frontal depression, and 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 altogether,
leaving only one front on the surface
(the rate depends on the temperature
difference between the air masses).
Here is its symbol:
Weather 103
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, from a
cold front catching up with a warm
one.
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.
Visibility
Defined as the greatest horizontal
distance a dark object (of known
dimensions) can be seen and
recognised against a light
background. This, of course poses a
problem at night, so night visibility
really refers to how far you would be
able to see in daylight.
Visibility may be reduced by fog,
mist, cloud, precipitation, sea spray,
smoke, sand, dust and industrial
haze (you may think of others). The
best visibility in haze is obtained
when down-sun and up-moon.
Fog
This is essentially cloud at ground
level, when you can't see more than
1,000 metres because of water
droplets suspended in the air.
Radiation fog forms over land,
preferably low-lying, when
temperatures approach the dewpoint
with very slight winds (2-8 kts), and
where moisture is present. It doesn't
form over the sea, because the
diurnal temperature variation is less,
and it is often found in the early
morning after a clear night, since its
formation is most favoured by high
relative humidity, light winds and
clear skies. It usually clears quickly,
once the Sun’s heat gets to work,
and will often get worse before it
gets better. If the windspeed is too
low, water cannot be held in
suspension, so it falls out as dew. If
it is too high, you will get low
stratus. It will disperse with wind,
heat, or a drier air mass.
Upslope fog forms from the cooling of
rising air up slopes.
Advection fog arises from warm air
flowing over a cold surface. It is not
the same as radiation fog because air
movement is involved, and the
coolness does not arise from diurnal
variations, but longer periods, as
with the sea, where this type of fog
is commonly found. Over land, it
could arise from a warm, moist air
mass flowing over cold ground.
Hill fog is low cloud covering high
ground, which may or may not have
contributed to its formation.
Frontal fog may be low cloud over
high ground, or come from rain
through unsaturated air beneath.
In shallow fog, you may be able to see
the whole of the approach and/or
runway lights from a considerable
distance, even though reports from
the aerodrome indicate fog. On
　
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