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we said above, air density decreases
as it gets warmer, a point in your
imaginary column of air above a
station would be higher on a warm
day than otherwise (see picture).
If, therefore, as is typical near the
Rockies in Winter, the air is very
much colder than standard, you will
be lower than you should be
(actually, the phrase above is still
valid, in that going from HIGH
temperature to LOW, your
instruments will be HIGH). This is
serious because, in low temperatures,
combined with other effects caused
by movement of wind over ridges,
you could be as much as 3000 feet
below your projected altitude, which
could really spoil your day. Another
factor that arises from the above
diagram is the creation of a wind
purely from the temperature
difference. The cooler column will
have a lower pressure at altitude, and
the warmer one will have a higher
pressure, causing air movement to
occur, from left to right, in this case.
Applying Buys Ballot's law, low
temperature is on the left in the
Northern Hemisphere if you stand
with your back to the wind.
When the surface temperature is well
below ISA, correct your altitudes by:
Surface Temp (ISA) Correction
–16°C to –30°C + 10%
–31°C to –50°C + 20%
–51°C or below + 25%
See also the Instruments chapter.
The altimeter setting is the station
pressure reduced to mean sea level
under ISA temperatures and
standard lapse rates, that is, taking
the elevation of the aerodrome into
88 Canadian Private Pilot Studies
account, expressed in inches of
mercury (Hg) in North America. It is
what your altimeter must be set to
when flying near aerodromes or
other places that may issue it,
because otherwise it will only tell you
your height above the point you
started from.
Its other importance is its use on
weather maps to create isobars. To
adjust station pressure for sea level,
take the elevation and get its
equivalent in inches. 900 feet would
therefore be .9", which is added to
whatever the reading is, because the
sea is lower and would give lower
figures anyway. MSL pressure is
station pressure corrected to sea
level using the average temperature
over the last 12 hours.
Pressure altitude is the height within
the standard atmosphere that you
may find a given pressure (usually
29.92"), and is a favourite starting
point for any calculations you may
wish to make, certainly for
performance, TAS, etc. To find it,
get the local altimeter setting, find
the difference between it and 29.92,
convert it to feet (1"=1,000), then
apply it the opposite side of 29.92 (you
could get it from the altimeter itself,
by placing 29.92 in the setting
window, and reading the figures
directly). For example, on an
aerodrome 400 feet above sea level,
with an altimeter setting of 29.72,
your PA would be 600 feet, and
where you would enter your chart
(the altimeter setting is below, so your
answer should go above). Again, you
are adding because the sea is lower,
and the figures ought to be higher.
In the exam, always write down
29.92 first, then subtract the
altimeter setting and multiply by
1,000. If the answer is negative, take
it away from the elevation. If it's
positive, then add.
Density altitude is different altogether,
and is your real altitude resulting
from the effects of height,
temperature and humidity, and is
more to do with performance, as it is
a figure that expresses where your
machine thinks it is, as opposed to
where it actually is – see under
Performance in Flight Planning
Temperature
Believe it or not, most, if not all, of
the Earth’s heat comes from the
Earth itself, that is, from below. The
Sun’s rays do not produce heat (or
light) until they hit something (that's
why it’s cold and dark in space), so
the air will get warmed by conduction
from the ground which has been
heated up by them, known as
insolation. Glider pilots and anyone
who has done basic physics will
know that lighter areas radiate heat
better than dark areas do, so
different parts of the earth will
produce different amounts of heat,
and thermals, which is what keeps
gliders up in the air (for exam
purposes, thermals are called
convection currents, and they are just
rising parcels of warm air). The
ocean is always slower to warm up
　
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