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it a pressure gradient, and very high
winds, strongest in the core. In other
words, the jetstream is found at a
vertical part of the tropopause where
tropical and polar air masses meet,
142 Canadian Professional Pilot Studies
with low pressure air rising from the
equator meeting high pressure air
from the poles, to create a spinning
vortex of fast moving air. It
therefore has a circular rotation,
when looked from behind or in
front, with the rising air being
tropical, so you might see cirrus
clouds on the equatorial side,
although clouds are not usually
associated with jetstreams, making
them more difficult to detect.
Jetstreams lie to the North of frontal
systems where the gradients are most
intense, between the warm air
tropopause on one side, and the cold
air tropopause on the other (taking
the name of the system causing it).
They are stronger in Winter, are at
least 60 kts, and may only be a few
hundred miles wide, but they will be
thousands of miles long with
extreme turbulence, which can
extend as much as 15,000 feet below
the tropopause, usually on the polar
side—also, head wind components
will naturally increase your fuel
consumption for the trip. Bear in
mind that the tropopause lowers in
Winter, which will move the
unstable air beneath the jetstream
further downwards as well (it’s
unstable because the jetstream is
sucking it up like a vacuum, which is
why low pressure areas and fronts
typically follow a jetstream's path –
the jetstream will be where the
relevant air masses meet at
tropopause level, generally to the
North of a front's surface position).
A jetstream doesn't need fronts to
exist, but you can tell where it is
from their ground position. A high
level one is about 100 miles behind a
cold front, parallel and blowing to
the low pressure center. It's also
parallel to a warm front, but away
from the low, 300-400 miles ahead,
always just inside the warm air.
One problem with jetstreams arises
out of flight from East to West,
against the flow. Across the North
Atlantic, tracks are planned with
reference to minimum time, rather
than minimum distance, which
avoids the jetstream. However, the
Polar front changes position quickly,
so you shouldn't expect a jetstream
to be where it was forecast to be,
meaning that extra fuel reserves may
be needed.
Jetstreams are also associated with
Clear Air Turbulence (CAT), which
comes from the mixing of air masses
moving at different speeds – it will
be found on the polar side, level
with or above the core. Doppler or
INS are both useful for detecting
rapidly changing windspeeds, which
should be read in conjunction with
rapidly changing OAT to help you
deduce its presence. They also help
to produce tornadoes, as they will
suck air out of a thunderstorm to
pull more in – strong updraughts
force the air into a tube.
The Japanese used jetstreams to
bomb the USA and Canada from
balloons, although only 6 people
were ever killed.
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.
Weather 143
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, which officially exists when
you can't see more than 1,000 metres
due to water droplets 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,
　
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