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breathing, but this is covered
elsewhere. Thus, the actual pressure
at a given place depends on its
height, and the temperature and
density at that point (see Density
Altitude, below).
Station pressure is the atmospheric
pressure at a particular place. Several
of these are taken, converted to sea
level pressure and marked on a map,
with the ones that are equal
connected up. The lines that join the
dots are called isobars (iso is Greek for
same), and will be 4 mb apart.
The closer the isobars are together,
the more the millibars drop per mile
and the more severe the pressure
gradient will be, so you get stronger
winds (air moves from high to low
pressure). Isobars are like contours
on a map, and make common
patterns, two of which are the low or
high, the common names for cyclone
and anti-cyclone, respectively (this has
nothing to do with the cyclones that
always seem to do severe damage to
trailer parks. Another name for a low
is a depression).
Three arrows (see below) are the
traditional way of displaying the
wind direction. The numbers in the
middle of each figure are typical
pressures (the exact position of a
system will be marked by an X).
The arrows across the isobars are
wind directions after Coriolis Force
has taken effect (see below).
The patterns are round, being circles
of isobars, and difference between
them is simple; a high pressure area
has most pressure in the middle,
whilst a low has the least. In other
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.
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.
Other patterns are the trough, which
is a longish area of low pressure, like
a valley, with V-shaped isobars,
80 JAR Private Pilot Studies
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
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