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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.
Jetstreams (see below) over 110 kts
give turbulence in the tropopause
above the core, in the front below it
and on the low pressure side, more
severe in the lee of mountain ranges.
Watch for 30 kt isotachs that are
closer than 90 nm to each other,
usually on the low pressure side of
the axis. CAT is also likely when the
vertical shear is over 5 kts/1,000 feet
(watch isotherm spacing here,
especially if 5° ones are closer than
120 nm). Also watch for curving
jetstreams around a pressure trough.
If turbulence is likely, use the
turbulence speed in the flight
Weather 133
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 (below) 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
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 (you could get two triggers, as
when a front hits the Rockies).
Above is a picture of one in the early
stages of development.
During the development stage (see the
picture below), several cumulus
clouds will begin to merge, where
the system consists mainly of
134 Canadian Professional Pilot Studies
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. The
updraughts won't get any faster, and
the top of the cloud reaches the
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