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one commonly sits over Eastern
Canada in late Spring because
Hudson Bay is still frozen. It is very
good at stopping the movement of
other systems and weather.
Other patterns are the trough, which
is a longish area of low pressure, like
80 Canadian Private Pilot Studies
a valley, with V-shaped isobars,
which will be found between highs,
and its opposite number, the ridge,
found between two lows. A col is a
neutral area between highs and lows.
A complex low is one with several
fronts and air masses overlapping
each other. When asked questions,
try to create a 3D image that will
show you which fronts and air
masses are on top or below to get
the sort of weather on the ground.
Air moves clockwise round a high
and anticlockwise round a low,
because the Earth is spinning, and
air is deflected because of it. The
Earth moves faster at the equator
than it does at the Poles - if you
threw something from the North
Pole to the Equator, progressively
more of the Earth’s surface would
pass under its track, giving the
illusion of curving to the right (West)
as it lags behind. If you threw
whatever it was the other way, it
would be advancing on the track and
"moving East". This is called in
some places the Coriolis effect, but
technically is Geostrophic Force.
Thus, in the Northern Hemisphere,
air coming from the South is
deflected East, and West if coming
from the North, which accounts for
the anticlockwise movement, so,
according to Professor Buys Ballot’s
law, if you stand with your back to
the wind in the Northern
hemisphere, the low pressure will be
on your left. The implication of this
is that, if you fly towards lower
pressure, you will be drifting to
starboard as the wind is from the left
(common exam question, but
worded differently). It's the opposite
way round in the anticyclone, and in
the Southern Hemisphere.
At about 2,000 feet, air movement is
parallel to the isobars, but, as you
descend, friction with trees, rocks,
etc will slow it down by about 10 kts,
which will lessen the coriolis effect
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
Weather 81
Earth. Out of that high pressure,
some air flows to the South West,
and some to the North East. Air
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.
　
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