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words, the air in a high is descending
(and diverging), and that in a low is
ascending (and converging), so the
weather is settled in the former and
tends to instability in the latter, since
upwards movement implies clouds
forming. You might often see a
secondary low, which is a smaller one
inside a larger system, in which the
weather is more intense as it feeds
on its bigger brother, although the
winds will be lighter between them.
However, just because cloud is
mostly absent in a high, don’t expect
clear skies, as the descending air
might trap haze or smoke, leading to
a phenomenon called anticyclonic gloom
near industrial areas. Sometimes,
areas of layer cloud may also get
caught if they are below an inversion
(see below).
As an example of the influence a
high pressure area can have, a strong
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. There
are permanent ones over the poles,
as well.
Other patterns are the trough, which
is a longish area of low pressure, like
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
Weather 117
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
　
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