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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, any
thickness up to 10-12,000'.
Weather 131
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,
looking like a mackerel sky.
· Cirrostratus (Cs) translucent high
cloud, very delicate, made of ice
crystals. When in front of the
Sun, you may see a halo.
· 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
has alto in front and high clouds have
cirro. Nimbo means rain-bearing.
A corona or halo around the Sun or
Moon comes from light being
refracted by moisture particles in
medium-level cloud.
Other
Heap clouds, vertically developed,
etc.:
· Cumulus (Cu), small amounts of
heap cloud at low and medium
levels, looking a bit like small
balls of cotton wool:
It’s actually convection cloud,
which gives you a clue as to
how it is made. Its outlines are
much sharper in Winter than in
Summer, but are ragged and
woolly when decaying. In strong
winds, you might see them in
long lines, and you will get
showers from larger ones. Socalled
“fair weather cumulus”,
typically seen on a nice
Summer’s day, are usually
leftovers from dead Alto-Cu.
· Cumulonimbus is towering
stormcloud, which may appear
spontaneously over praires or
be part of an advancing front
associated with a depression:
“Towering” means up to 25,000
feet, and the anvil shape at the
top is due to an inversion,
which stops the cloud’s ascent.
They are mostly found around
late afternoon. When passing
overhead, you may notice gusty
winds and the altimeter
overreading by 6-700 feet.
There is a sleet centre around
the freezing level, and severe
up- and downdraughts, with
cells of severe activity between
them. Expect vast quantities of
water in large drops, frozen or
otherwise, but there are
supercooled water droplets in the
lower layers, drops that do not
become solid when they reach
zero degrees, but remain liquid.
132 Canadian Professional Pilot Studies
They do freeze, however, when
they hit something, like a wing,
where the liquid runs back and
forms clear ice.
· Lenticular, found at the crest of
standing waves formed in the
lee of mountain waves.
Precipitation
This comes from anything with
nimbo in its name. It will be
continuous from stratiform clouds,
and intermittent from cumuliform
clouds. Virga evaporates before
reaching the ground - it comes from
cumuliform cloud, and looks like
streamers just below the cloud base.
Precipitation is the end result of a
chain of events that starts with the
cooling through ascent of a parcel of
dirty moist air ("dirty" meaning that
it contains microscopic particles that
water can bind on to). Once the
saturation point is reached,
condensation occurs and droplets
coalesce to fall out as rain, snow, or
whatever, according to temperature.
Nitrogen oxides that come from
lightning discharges can attract water
droplets that can grow.
Hail & Sleet
Hail forms from large water droplets
forced above the freezing level.
Snowflakes are combined ice crystals
which come from the freezing of
water vapour without going through
a liquid stage. Sleet is half-melted
snow, that begins to unfreeze during
descent below freezing level when it
is quite high above the surface.
Ice usually forms on aircraft during
flight in cloud, but it can happen in
the clear.
Turbulence
This is found in cloud and clear air
(that is Clear Air Turbulence, or CAT),
and usually is a result of the friction
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