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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,
Weather 93
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)
· 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,
consisting mainly of water:
· Stratus (St), thin, uniform, low.
· Stratocumulus (Sc). Like stratus,
but with small globules popping
up here and there. Often
formed in eddy currents at the
boundary of air masses at
different speeds.
· Nimbostratus (Ns for short),
which is thick, dark, low rain
cloud, typical in warm fronts.
Middle (Alto)
From 6,500-20,000 feet, made of
water, ice, or super cooled water
droplets, depending on temperature:
· Altocumulus (Ac) is similar to Sc
(above), but higher.
· Altostratus (As), medium sheet
greyish or bluish cloud of any
thickness up to 10-12,000 feet.
High (Cirro)
From 20,000', made of ice crystals,
so they have some transparency:
· Cirrocumulus (Cc) is high sheet
cloud, made of small cloudlets
(for want of a better word)
which do not cast shadows. It
looks like a mackerel sky.
· Cirrostratus (Cs) translucent high
cloud, very delicate, made up of
ice crystals. When in front of
the Sun, you may see a halo
round it.
94 JAR Private Pilot Studies
· Cirrus (CIF) is a high and
fibrous filament. It indicates
that a warm front is around 200
nm away. Otherwise known as
Horse tails, or Mares' tails.
The limits of each classification vary
with latitude (with the troposphere).
Low clouds do not have a prefix
added to their name, medium cloud
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