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under ISA temperatures and
standard lapse rates, that is, taking
the elevation of the aerodrome into
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
Weather 89
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
than the land, as are any marshy
areas and forests. By comparison,
rocks, roadways and pavements are
very quick. As a general rule, the
drier the better.
Since the Earth does not get hotter
and hotter as the Sun shines on it, it
follows that heat must be radiated
away from the Earth. This explains
the difference in temperatures
between day and night, known as
diurnal variation. The temperature
begins to rise shortly after sunrise,
and starts to fall mid-afternoon,
carrying on through the night until
the process starts again. This is less
marked over water, which reacts
more slowly. Changes over the sea
will not be much more than 1°C.
Clouds will also absorb and reflect
some energy from the Sun during
the day, and act as a blanket
overnight to stop heat being radiated
from the Earth, further reducing
diurnal differences.
There are two ways of measuring
temperature, Fahrenheit or Centigrade
(Celsius), and it’s a real pain to
convert between the two. The quick
and easy way is to use a flight
computer, but here are the
calculations for people who want to
show off:
F - C Tc = (5/9)*(Tf-32)
C - F Tf = ((9/5)*Tc)+32
16°C is equal to 61°F, 20°C is 68°F
and 30°C is 86°F, for quick
conversions.
Given the standard of performance
charts in the average flight manual,
doubling the Celsius amount and
adding 30 to get Fahrenheit, or
　
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