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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.
92 Canadian Private Pilot Studies
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
continue further.
If the lapse rate lies to the left of the
DALR (that is, it is steeper – see the
diagram below), air is unstable.
If it is between the DALR and
SALR, it is conditionally unstable,
meaning stable when dry, but not if
saturated. To the right of the SALR
(shallower), limited convection is
possible, and to the right of the
isothermal (i.e. vertical), you get total
stability from an inversion.
If the lapse rate above the
condensation level is greater than the
SALR, the rising air actually gets
warmer than that around it, which
will give intense ascents and be
instrumental in forming
thunderstorms. Once this condition
exists, all you need is a trigger action to
cause condensation.
In summary, stable air can produce
poor visibility at low levels, constant
drizzle, light or calm winds with
layer cloud, and no turbulence.
Unstable air, on the hand, tends to
be associated with good visibility,
with heavy precipitation, heap type
clouds, strong winds, turbulence and
stormy weather.
Clouds
Cloud names were actually coined by
an amateur meteorologist, Luke
Howard, in 1803, who based them
on the Latin words for hair, heap,
layer and rain-bearing (cirrus, cumulus,
stratus and nimbus) not to mention
middle and broken (alto and fracto).
Clouds form in the first place
because air contains water vapour,
and because the air is cooled, causing
the vapour to condense out at the
saturation point. Air holds more
vapour when it is warm, and a given
amount can become saturated in two
ways – you can either add more
water to it, or reduce its temperature.
The excess vapour changes from gas
to liquid, with the droplets coalescing
to form clouds. Most clouds arise
from cooling – addition of water
tends to happen when a dry air mass
moves over a moist surface. Cooling
occurs when air expands as it is
forced upwards in various ways,
already described under Stability. To
remind you, though, they include:
· Uplift over a land mass
· Uplift over an air mass
· Convection currents
· Eddying (around the surface, or
at the boundaries of two layers
of air at different speeds)
Weather 93
· Wave action in the lee of
mountains
· Uplift from a depression
Clouds affect surface heating by
shielding the Earth’s surface and
absorbing the Sun’s Rays, or acting
like a blanket to keep the heat in at
night.
Those above the freezing level are
largely ice crystals. Otherwise, there
are two main types, layer, or heap,
associated with stable and unstable
conditions (see below), which might
also be called stratiform or cumuliform,
meaning horizontally or vertically
developed, respectively. There are a
further three classifications based on
the height of the cloud base, namely:
Low (Strato)
From sea level to about 6,500 feet,
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