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and give you an effective change of
wind direction to the left, known as
backing (an increase to the right is
called veering). Air moving round a
low will therefore tend towards the
centre and contribute towards the
lifting characteristic.
Inside a high, though, air movement
(i.e. winds), will tend to increase with
the help of centrifugal force, other
things being equal, but this is offset
by the pressure gradient in a low
being much steeper, creating
stronger winds anyway.
Over the sea, the effect will be less,
giving about 10° difference in
direction, as opposed to the 30° you
can expect over land. If the winds
are high, you could get into a stall on
landing as you encounter windshear,
of which more later.
Wind
The Earth is heated unevenly. Air at
the Equator becomes warmer than it
does at the Poles, so it expands
upwards around the middle of the
Earth and contracts down to the
surface at the Poles. This general
trend gives rise to regular patterns of
air movement, in the shape of winds
that were well known to navigators
on the high seas, such as the trade
winds, caused by the Coriolis effect,
which causes air to accumulate (for
example) in an Easterly direction at
around 30° of latitude in a general
area of high pressure right round the
Earth. Out of that high pressure,
some air flows to the South West,
and some to the North East. Air
Weather 81
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 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 it, resulting
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
draught even when nothing is open -
the air next to the window is cooled,
and flows downwards. An anabatic
wind flows up a hill.
82 JAR Private Pilot Studies
Gusts are rapid changes of speed and
direction that don't last long, whilst
squalls do.
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,
　
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