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the scale is coarser.
The freezing level is that where the
temperature is 0°C.
-273°C is equal to 0°K (Kelvin), or
Absolute, which is the point at which
vibration of atoms is at a minimum.
Alternatively, you could say that 0°C
is equal to 273° K (or A), from
which you can deduce that the 1°
steps in both scales are the same.
If you apply an equal amount of heat
to various substances, some will
warm up quicker than others – the
standard for comparison is that
applied to water, which has a specific
heat value of 1. 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
Weather 127
that all the industrial pollutants
collect there, especially in the stubble
burning season which may include
incinerated pesticides.
However, the problem is that the air
is rarely dry, and cloud or water
vapour changes the figures anyway.
ISA Conversions
Many exam questions involve
comparing the ambient temperature
against what it should be under ISA
conditions. Although the reduction
is technically 1.98°C per thousand
feet, 2° is often used for
convenience, as is done with the jet
standard atmosphere.
For example, you might be asked
what the temperature deviation is at
FL 290 with an OAT of -47°C?
First of all, find out what the
temperature difference from sea level
should be, so, using 2°C per
thousand feet, we find it should be
58° lower (29 x 2). Given that the
temperature at sea level is always
15°C in ISA, subtract one from the
other to get -43°C (-58+15). As the
OAT is -47°C, the temperature
deviation is ISA - 4°C (that is, 4°
colder than it should be).
Working the other way round, you
could be asked to find the
temperature, given a deviation at a
flight level. If it's ISA -7°C at FL
250, +15 (ISA) – 50 (25 x 2) gets us -
35°C, the normal ISA temperature.
Applying the deviation, which is
colder, we get -42°C.
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
　
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