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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 they pass
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.
They do freeze, however, when
they hit something, like a wing,
where the liquid runs back and
forms clear ice (see below).
· 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
Weather 95
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
from the mixing of air currents. It
comes from various sources, such as
convective, orographic, windshear
and mechanical, and is reported as:
· Light, with small changes in
height or attitude, near
stratocumulus.
· Moderate, more severe, but you
are still in control. A good
indicator is Cumulus-type
clouds, which may also warn
you about....
· Severe, with abrupt changes, and
being temporarily out of
control, indicated by
Cumulonimbus and lenticular
clouds, if there are many
stacked on top of each other.
Expect the latter when winds
across mountain ranges are
more than 40 kts.
· Extreme, which is impossible to
control.
If turbulence is likely, use the
turbulence speed in the flight
manual, which will be rather less
than normal. Mention it to the cabin
crew and advise the passengers to
return to, and/or remain in their
seats, ensuring their seat
belts/harnesses are securely
fastened. Catering and other loose
equipment should be stowed and
secured until the risk has passed.
Thunderstorms
The airflow is greatly disturbed
anywhere near a thunderstorm,
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