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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
Weather 89
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
subtracting 30 from Fahrenheit and
dividing the remainder in half to get
Celsius is probably good enough!
The Fahrenheit scale assumes that
water freezes at 32°, and boils at
212°. Celsius starts at 0° and finishes
at 100°, which is more logical, but
the scale is coarser.
The freezing level is that where the
temperature is 0°C.
If you apply an equal amount of heat
to various substances, some will heat
up quicker than others – the
standard for comparison is that
applied to water, which has a specific
heat value of 1. If the same heat were
applied to different amounts of the
same substance, smaller quantities
increase temperature more rapidly.
We have already seen that the
standard reduction of temperature
with height is 1.98°C per thousand
feet. Where it remains constant,
there is an isothermal layer. Where it
increases (typical in anticyclonic
conditions), you have an inversion, but
the lapse process stops at the
tropopause anyway. You may get
slight turbulence flying through one.
Performance is affected by variations
in temperature, and inversions will
do so adversely. Large ones
encountered shortly after take-off
can seriously degrade climb
performance, particularly when
you're heavy. Even a small one in the
upper levels can prevent you
reaching a preferred cruising altitude.
At lower levels, expect deteriorating
visibility, as an inversion can prevent
fog clearance for prolonged periods
(to improve your chances of seeing
the surface, fly higher above a mist
layer). Another good reason for
avoiding the top of an inversion is
that all the industrial pollutants
collect there, especially in the stubble
burning season which may include
incinerated pesticides.
However, the problem is that air is
rarely dry, and cloud or water vapour
will change the figures anyway.
90 Canadian Private Pilot Studies
Moisture
A given parcel of air can hold a
certain amount of moisture at a
certain temperature. This ability is
increased as it gets warmer, and
decreased as it gets colder. The
dewpoint is the temperature at which
it reaches 100% saturation, or the
point at which water vapour begins
the process of condensation into visible
water droplets (the condensation level),
so if the temperature and dewpoint
at an airfield are the same, it will take
very little incentive for clouds to
form - the further apart they are, the
less likely you are to get cloud, and
therefore icing if the temperature is
low enough. The warmer the wet air
is, the more likely you are to meet
bad weather.
The hygrometer is one instrument used
to measure how wet the air is, and
it's very simple in the way it works.
A human hair, which gets longer the
moister it gets, is laid out against a
calibrated scale of known humidities.
A suitable linkage transmits its
movements to show the relative
humidity, which is how much
moisture an air parcel is holding
　
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