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Obstacles interfere with the wind in
different ways. A forest acts as a
large brush, slowing it down and
mixing it up. It will tend to build up
before an obstacle and create
turbulent eddies behind, to result in
gusts and lulls as speed varies.
Land and sea breezes arise out of a
temperature difference between land
and sea areas. When the land is
warmer than the sea, the space left
by the rising air over it is filled with
more coming from over the water,
producing a sea breeze (in fact, a
relatively high pressure is created at
about 1000 feet over land. With
lower pressure at the same height
over the water, there will be air
movement towards the sea, which
will subside to come back towards
the land). At night, the process is
reversed to get a land breeze.
Cool air on a slope will flow down,
because it is more dense, and
therefore more subject to gravity,
causing a katabatic wind. It’s the
same effect you get in a closed room
on a cold day, where there is a
draught even when nothing is open -
the air next to the window is cooled,
and flows downwards. An anabatic
wind flows up a hill.
Gusts are rapid changes of speed and
direction that don't last long, whilst
squalls do.
82 Canadian Private Pilot Studies
When a mountain range has an
airflow greater than 20 knots
blowing broadside on (within about
30°) and over it in stable conditions,
standing waves may exist downwind,
noticeable by turbulence and strong
persistent up and down draughts:
The airflow follows the general
shape of the surface and, flying into
wind, you will experience a strong
downdraught just before the ridge
(the most dangerous bit, as it may be
stronger than your climb capability)
and an updraught just afterwards.
There are several miles between
peaks and troughs of the waves,
extending 10 or 20,000 feet above
the range and up to 200 or 300 miles
downwind.
You will see a cap cloud over the top
of the range, creeping down the lee
side (i.e. downwind), as a result of a
downdraught. At the crest of each
wave, there will be a lenticular cloud,
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—
remember the old saying; altitude is
money in the bank, but speed is
money in the pocket.
Weather 83
Although mostly associated with
thunderstorms (see below), where
you have the unpredictability of
　
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