曝光台 注意防骗
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It is green, and needs care to provide
uniform cover, especially over Type
I fluid already there.
Type I fluids have good de-icing
properties, but may refreeze - they
are for de-icing, not anti-icing. Union
Carbide Ultra fluid (i.e. Type IV)
appears to increase holdover times
by 1.5 over Type II and way more
for Type I. The holdover time is
how long the effects of the fluid
should last – it can be affected by
high winds or jet blasts damaging the
fluid film, and temperature,
humidity, etc.
General Precautions
Deposits must be swept from
hinges and system intakes, and
the sprays themselves should
not be directed to them, since
the fluid may be further diluted
by the melting ice it is designed
to remove, and may refreeze. It
may also cause smearing on
cockpit windows and loss of
vision during take-off.
Afterwards, confirm that flying
and control surfaces are clear
and move over their full range,
and intake and drain holes are
free of obstructions. Jet engine
compressors should be rotated
by hand to ensure they are not
frozen. Check propeller
spinners for trapped snow or
moisture, which could
subsequently refreeze and cause
an imbalance. Don't forget the
undercarriage.
Air Masses
A large body of air has the
characteristics of its origin,
particularly with regard to moisture
and temperature. To acquire the
uniform characteristics required to
meet the classification, a mass of air
has to stay in one place for several
days in a more or less uniform place.
In Canada, the main ones are:
· Continental Arctic (cA), which is
dry and cold from polar land
regions.
· Maritime Arctic (mA), moist and
cold, from polar oceans.
Weather 139
· Maritime Polar (mP) comes from
temperate oceans, so it will be
moist and slightly less cold.
· Maritime Tropical (mT), moist
and warm, from tropical oceans.
All can be modified if they move
over different areas. Maritime
Tropical, for example, will become
Maritime Polar if it moves North for
long enough.
Frontology
A front is a line of discontinuity, or a
narrow transition zone between air
masses where they are forced to mix,
even though they don’t want to. The
difference is usually in temperature,
but may be purely due to moisture
content. Fronts are always associated
with depressions, which are
sometimes referred to as a frontal
wave. Fronts will rotate
(anticlockwise) around the low.
Warm tropical air could be forced
over colder arctic air, for example,
because it is less dense and, if moist,
will form a typical cloud structure
that we on the ground can use to tell
when a front is coming. The name
of a front, that is, warm, or cold,
comes from whichever air mass is
overtaking the other, whereas the
type of weather you get is
determined by the stability and
moisture content of the warm air
mass (exam question). The actual
temperature is less important than its
relationship to that of the surface it
is passing over.
The Polar Front is an area where
south- and north-westerly airstreams
meet to form long series of
depressions, starting off the Atlantic
Coast of North America.
Frontogenesis is the term for the
forming of a front, and frontolysis the
one for its dissipation. The cold air
mass does not move at a stationary
front, and you get an upper front when
very cold air is caught on the surface
with the weather higher up.
The Warm Front
This exists where warm air overtakes
a colder air mass and is forced
upwards, meaning clouds. Its symbol
on a weather map, resembling beads
of sweat, is:
The frontal slope has a gradient of
somewhere between 1:150 and
1:200, although the clouds
themselves will be about 5 miles
high, starting with Nimbostratus at
more or less ground level, through
alto-stratus to cirrostratus (when
flying towards it, you would see the
clouds the other way round, of
course). Once you start seeing cirrus
clouds, you know that a warm front
is somewhere ahead, anywhere
140 Canadian Professional Pilot Studies
between 300-600 miles away, or
nearly 24 hours at a typical speed of
about 25 kts, so have an overnight
kit if you have to wait it out (rain will
typically be 200 miles ahead). You
can use the typical slope figure to
figure out the cloud base in front of
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