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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
90 JAR Private Pilot Studies
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.
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
against what it could hold at that
temperature or, in other words, the
percentage saturation, which will decrease
if the air gets warmer, as when
subsiding in a high pressure area,
because temperature is raised by
compression, and can absorb more
moisture (exam question). Relative
humidity could change as a result of
the air absorbing more moisture, say
when moving over the sea, but it is
more likely to change quickly
through temperature changes, at
least for our purposes. Water added to
air makes it moister, and less dense, and
therefore more likely to rise.
Mostly, air is made to reach its
saturation point by force, such as
being moved up the sides of
mountains or over large areas of
slower moving air (large scale ascent)
and, if the conditions are right, cloud
will form. In fact, the ways of
cooling air are many. Where
horizontal movement of air over a
cooler surface is involved, it is called
advective, as found over the praires
when air is moving from East to
West, and a frequent cause of fog.
The convection currents we have
already met with cause adiabatic
cooling by lifting air so it expands
and cools, and expansion cooling is
Weather 91
really the same thing caused by
upslope movement or mechanical
turbulence, another name for low level
air being mixed and moved upward.
Radiation cooling tends to happen
overnight with clear skies, when the
Earth’s heat radiates out into space.
　
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