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from the Poles, settling down and
flowing South, creates weather
fronts (see below) when it meets the
warmer air.
Navigators also had to deal with the
doldrums, which are areas of complete
calm either side of the equator,
where the only movement of air is
up (it slides North and South once it
hits the tropopause). Air gets sucked
in from just outside, causing the
wind. The doldrums move with the
Sun according to season.
Wind is expressed as a velocity, so it
needs direction and speed to fit the
definition. It always comes from
somewhere, expressed as a true
bearing in weather reports (magnetic
from the Tower), so a Southerly
wind is from 180°. The speed is
mostly in knots, or nautical miles per
hour, as if you didn’t know already.
Wind speed is measured with an
anemometer, while direction is
measured with a wind vane.
Obstacles interfere with the wind in
different ways. A forest is like large
brush, slowing it down and mixing it
up. It will tend to build up before an
obstacle and create turbulent eddies
behind it. This will result in gusts and
lulls as the 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
118 Canadian Professional Pilot Studies
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.
Diurnal Effect
The surface wind increases speed
and during the day in the Northern
Hemisphere, and vice versa by night.
In the Southern Hemisphere, it
increases and backs by day,
decreasing and veering by night.
Mountain Ranges
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.
Weather 119
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
　
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