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with a rotor cloud downwind from
each one. Rotors are always in
circular motion, constantly forming
and dissipating as water vapour is
added and taken away. They are
dangerous, and the most turbulence
will be found in them, or between
them and the ground. Lenticular
clouds tend to remain stationary and
will produce airframe icing.
An aircraft affected by mountain
waves can expect severe turbulence
below any rotors, downdraughts that
may be stronger than the rate of
climb and greater than normal icing
in associated clouds.
Although the effects, such as
turbulence and up and down
draughts reduce with height, at
normal cruise altitudes, mountain
waves are usually free from clear air
turbulence, unless associated with
jetstreams or thunderstorms.
Watch out for long-term variations
in speed and pitch attitude in level
cruise (the variations may be large).
Use the autopilot height-lock to
maintain altitude, but change power
as well - bear in mind that at cruise
height the margin between low and
high speed limits can be relatively
small. Near the ground in a
mountain wave area, severe
turbulence and windshear (see
below) may be encountered. The
quickest way out of turbulence is up,
with the next best directly away from
the range. Flying parallel to the range
in an updraught, avoiding peaks,
gives the most comfort.
Low level windshear is found under the
anvil of a cumulonimbus.
Windshear
This is the name for airspeed
changes over about 10 kts resulting
from sudden horizontal or vertical
changes in wind velocity—more
severe examples will change not only
airspeed, but vertical speed and
aircraft attitude as well. Officially, it
becomes dangerous when the
variations cause enough
displacement from your flight path
for substantial corrective action;
severe windshear causes airspeed
changes greater than 15 kts, or
vertical speed changes over 500 feet
per minute. Expect it to occur
mostly inside 1,000 feet AGL, where
it is most critical, because you can't
quickly build up airspeed—
Weather 83
remember the old saying; altitude is
money in the bank, but speed is
money in the pocket.
Although mostly associated with
thunderstorms (see below), where
you have the unpredictability of
microbursts to contend with, it's also
present with wake vortices,
temperature inversions, mountain
waves and the passage of fronts, not
forgetting obstructions near the
runway, and can occur over any size
of area. You can even get it where
rain is falling from a cumulus cloud,
as the air is getting dense from the
cooling, and will therefore fall
quicker. It's not restricted to
aeroplanes, either—helicopters can
suffer from it above and below tree
top level in forest clearings, when a
backlash effect can convert
headwind to tailwind.
All fronts are zones of windshear—
the greater the temperature
difference across them (over 10°C),
the greater the changes will be.
Warm fronts tend to have less than
cold ones, but as they're slower
moving, you catch it for longer. In
general, the faster the front moves
(say, over 30 kts), the more vigorous
the weather associated with it; if it
goes slower, the visibility will be
worse, but you can still get
windshear even then and always for
up to an hour after its passage.
Warm air moving horizontally above
cold air can produce turbulence
where they join, as would be typical
with an inversion. In a valley, in
particular, when moving warm air
hits a mountainside, it will be forced
downwards, but will not be able to
penetrate the cold air, so it is forced
to move over the top of the cold air
in the valley bottom, so watch out
on those cold, clear mornings.
The most significant effect of
windshear is, of course, loss of
airspeed at a critical moment, similar
to an effect in mountain flying,
where a wind reversal could result in
none at all! You would typically get
this with a downburst from a
convective type cloud, where,
initially, you get an increase in
airspeed from the extra headwind,
but if you don’t anticipate the
reverse to happen as you get to the
other side, you will not be in a
position to cope with the resulting
loss. This has led to the
classifications of performance increasing
or performance decreasing. With the
former, you get more airspeed and
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