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is quite high above the surface.
Ice usually forms on aircraft during
flight in cloud, but it can happen in
the clear.
Weather 95
Turbulence
This is found in cloud and clear air
(that is Clear Air Turbulence, or CAT),
and usually is a result of the friction
from the mixing of air currents. It
comes from various sources, such as
convective, orographic, windshear
and mechanical, and is reported as:
·  Light, with small changes in
height or attitude, near
stratocumulus.
·  Moderate, more severe, but you
are still in control. A good
indicator is Cumulus-type
clouds, which may also warn
you about....
·  Severe, with abrupt changes, and
being temporarily out of
control, indicated by
Cumulonimbus and lenticular
clouds, if there are many
stacked on top of each other.
Expect the latter when winds
across mountain ranges are
more than 40 kts.
·  Extreme, which is impossible to
control.
If turbulence is likely, use the
turbulence speed in the flight
manual, which will be rather less
than normal. Mention it to the cabin
crew and advise the passengers to
return to, and/or remain in their
seats, ensuring their seat
belts/harnesses are securely
fastened. Catering and other loose
equipment should be stowed and
secured until the risk has passed.
Thunderstorms
The airflow is greatly disturbed
anywhere near a thunderstorm,
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
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.
Pressure usually falls rapidly as a
thunderstorm approaches, then rises
96 Canadian Private Pilot Studies
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.
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
　
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