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If the winds are light, just enough to
stir things up (3-5 kts), fog will form,
predictably enough called radiation
fog. Night-time cooling is less over
water than land, as water traps heat –
this is called the maritime effect. It
could also cause a nocturnal inversion.
What’s known as the cloud effect
reduces the results, because they
absorb some radiation themselves.
The topographical effect results from
cold air getting trapped in valleys
and lower areas.
The process of sublimation occurs
when water vapour goes directly to
the solid state (i.e. ice) without going
through a liquid stage, or vice versa.
Anyhow, the lapse rate changes
when we add moisture to the mix.
The Dry Adiabatic Lapse Rate (DALR)
is 3°C per 1,000 feet, and you use it
to find the cloud base, after which
you switch to the Saturated Adiabatic
Lapse Rate (SALR) of 1.5°C per
1,000 feet (dry, in these
circumstances, just means a relative
humidity of less than 100%).
For example, your ground
temperature is 10°C, and the
dewpoint 7°C. To find the cloudbase
and freezing level, take the DALR
and divide it into the difference
between the temperature and
dewpoint, which in this case is 3,
then divide that by the lapse rate (3),
so your cloudbase would be at 1,000
feet. Then use the SALR from the
cloudbase to count down to zero, so
divide 7°C (the dewpoint) by 1.5 and
add the converted number in
thousands of feet to the cloudbase,
to get 4,660 (from 4.66), so the
freezing level would be at 5660 feet.
The reason for the difference in
lapse rates is latent heat. The word
means undeveloped, implying that an
amount of heat is lurking in the
background waiting to do
something. Converting water from
one state to the other requires
energy, which originally comes from
the Sun’s rays as water is evaporated
in the first place, and is stored with
the vapour. While there, it is known
as latent, and released when the
water condenses, hence the lesser
rate that ascending saturated air
cools at, since the air becomes
warmer as a result. Latent heat
becomes involved when you change
the form of a substance without
changing its temperature. There can
be so much heat released that flight
in normally stable layer cloud can be
bumpy, due to internal eddying.
Latent heat is the reason for the
Chinook, a warm wind common in
the lee of the Rockies around
Calgary, which can raise the air
temperature to 20° in the middle of
Winter. Saturated air made to rise by
the mountains cools at SALR, and
when it descends on the other side,
having dropped its moisture, it
warms at twice that, i.e. DALR, so
you get a dry, warm wind with clear
skies. In France, it is called the
Mistral, and is accepted as a defence
in court for weird actions, as the air
is also ionised. In the Alps, it is
known as the Fohn Effect.
92 JAR Private Pilot Studies
Stability
Remembering the previous mention
of convection currents, you can see
that air has vertical movement as
well as horizontal. The less there is
the more stable it is, and the less
bumpy, because it tends to resist
vertical motion.
A cold air mass moving over a
warmer surface will be unstable
because the lower layers will pick up
moisture and temperature, and start
rising (the moisture makes the air
less dense). This will carry on into
the night over the sea, as the water
will keep its heat better than the land
will. On the other hand, a warm air
mass over a cold one will have its
lower layers reduced in temperature,
possibly as far as an inversion, which
is about as stable as you can get.
Instability arises when air warmer
than that surrounding it begins to
rise, as it is bound to do, because it is
less dense. It may have been lifted in
the first place by convection, convergence,
mechanical turbulence, orographic (over a
geographic barrier, such as a
mountain range) or frontal means.
The warmer it is when it starts, the
more energy the bubble has to keep
going, but it’s really the lapse rate
that determines when it stops (well,
OK, humidity counts as well). As it
rises, air expands, and therefore
cools, beginning to match the air
around it, until it eventually cools off
quicker than the surrounding air, and
stops. Once it becomes saturated,
though (and cloud forms), cooling
slows down and allows the ascent to
　
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