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usually noticeable by strong up and
down draughts, together with heavy
rain and lightning, or even
tornadoes. Because of the inflow of
warm air and the outflow of cold air,
the gust front can extend up to 15-
20 miles ahead of a moving storm.
Avoid them even at the cost of
diversion or an intermediate landing,
but should this be impossible, there
are certain things you can do.
It can be at least as dangerous up
high as way down low—you can
expect anything from lightning and
turbulence to icing and hail, each
96 JAR Private Pilot Studies
with hazards of their own—
lightning, for instance, could explode
a fuel tank, and strikes can occur up
to 20 nm from a storm cell. Not only
that, even over baby ones near to
larger storms, you will need at least
5000 feet clearance. Similarly, try not
to fly underneath, either, or make
steep turns. The currents inside a
thunderstorm will easily be enough
to suck in the average light aircraft,
or spit it out.
A squall line is a series of storms,
which Murphy's Law dictates will be
right across your flight path (they
will be too wide for a detour and too
severe to penetrate):
They can appear anywhere the air is
moist, but often ahead of cold fronts
in late afternoon or early evening.
They are the product of severe cold
frontal conditions, in advance of the
front, where it is nudging under the
warm sector (watch for an acute
bend in the isobars at the front, or
low roll cloud across the advance).
Pressure usually falls rapidly as a
thunderstorm approaches, then rises
rapidly with the first gust. It returns
to normal after it passes.
An embedded thunderstorm will
have penetrated overlying bands of
stratiform cloud on its way up.
To start a thunderstorm, you need
moisture, a steep (unstable) lapse
rate and a lifting, or trigger, agent,
which could be orographic, convective,
frontal or nocturnal, as occurs in the
midwest plains after night time
radiation from unusually moist air at
height (of course, you could get two
trigger actions, as when a front hits
the Rockies). Here is a picture of one
in the early stages of development:
A thunderstorm is actually a
collection of several cloud cells in
varying stages of development, with
varying diameters.
During the development stage (see
picture, above), several cumulus
clouds will begin to merge, where
the system consists mainly of
updraughts, and will grow to around
4 miles wide at the base and 20,000
feet in height. Water droplets are
merging as well to form larger
raindrops, which get to be a hazard
once they get above the freezing
level and become supercooled (see
Icing, below). When they are big
enough, they will fall, and pull cold
air down with them, which is where
the downdraughts come from. So,
rain at the surface is a good
indication of the transition to ….
The mature stage, which is
distinguished by the rainfall.
Weather 97
Updraughts won't get any faster, and
the top of the cloud reaches the
tropopause, where an inversion
stops the ascent and strong winds
produce the anvil shape. When
downdraughts hit the ground, they
spread out into a gust front, ahead of
which are more, called downbursts,
which may themselves contain
microbursts. Expect lightning as well.
The characteristics of dissipation are
downdraughts and disappearing
cloud.
The different cells in a thunderstorm
may be developing, maturing or
dissipating at rates of their own,
which could form their own trigger
actions and make the storm selfperpetuating.
Approaching the area
Seat belts should be tightened, and
loose articles stowed. One pilot
should control the aircraft and the
other should monitor the flight
instruments. Select an altitude for
penetration that will keep you clear
of obstacles, and use the weather
radar to select the safest track. Set
the power for the recommended
turbulence speed, adjust the trim and
note its position, so any excessive
changes from autopilot or mach trim
can be quickly assessed. Height,
mach, rate of climb or descent and
airspeed locks should be disengaged
but the yaw damper(s) should be on.
Switch on the pitot heaters, deicing,
and continuous ignition system,
where fitted. Disregard any beacons
subject to interference, such as ADF
(although tuning it to its lowest
frequency will give you a primitive
　
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