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heat, or a drier air mass.
Upslope fog forms from the cooling of
rising air up slopes.
Advection fog arises from warm air
flowing over a cold surface. It is not
the same as radiation fog because air
movement is involved, and the
coolness does not arise from diurnal
variations, but longer periods, as
with the sea, where this type of fog
is commonly found. Over land, it
could arise from a warm, moist air
mass flowing over cold ground.
Hill fog is low cloud covering high
ground, which may or may not have
contributed to its formation.
Frontal fog may simply be low cloud
touching high ground, or come from
rain falling through unsaturated air.
In shallow fog, you may be able to see
the whole of the approach and/or
runway lights from a considerable
distance, even though reports from
the aerodrome indicate fog. On
descending into the fog layer, your
visual reference is likely to drop
rapidly, in extreme cases from the
full length of the runway and
approach lights to a very small
segment. This may give the
impression that you're pitching nose
up, making you more likely to hit the
ground after corrective movements.
You should be prepared for a missed
approach whenever you have the
slightest doubt about forward
visibility. The minimum RVR to land
from a visual circuit is 1200m.
Mist
Essentially, thin fog, and the same
definition applies, except that the
visibility is over 1,000m.
Whiteout
Defined by the American
Meteorological Society as:
“An atmospheric optical phenomenon of
the polar regions in which the observer
appears to be engulfed in a uniformly
white glow”.
That is, you can only see dark nearby
objects – no shadows, horizon or
clouds, and you lose depth
perception. It occurs over unbroken
snow cover beneath a uniformly
overcast sky, when the light from
both is about the same. Blowing
144 Canadian Professional Pilot Studies
snow doesn’t help. It’s particularly a
problem if the ground is rising. Once
you suspect whiteout, immediately
climb or level off towards an area
where you can see things properly.
Three common causes are:
· Water Fog. Thin clouds of
supercooled water droplets
contacting a cold snow surface.
· Blowing Snow. Winds over 20 kts
picking up fine snow from the
surface, diffusing sunlight.
· Precipitation. Small wind-driven
snow crystals coming from low
clouds with the Sun above
them. Light is refracted and
objects obscured.
Sandstorms
To be avoided. On the ground,
aircraft should be under cover, or at
least have engine blanks and cockpit
covers fitted, as well as those for
instrument intakes and probes.
These should be carefully removed
before flight so accumulations of
dust are not deposited in the places
the covers are designed to protect.
The stuff gets everywhere!
Volcanic Ash
Flight through this can cause
abrasion to all forward facing parts
of aircraft, enough to impair
visibility through the windshields
and severely damage aerofoil and
control surface leading edges.
Airspeed indications may also be
completely unreliable through
blocking of pitot heads, and engines
may become choked enough to shut
down. Known areas of ashproducing
volcanic activity are found
in NOTAMs, as deduced by a Cray
computer. Flight into them should
be avoided, particularly at night or in
IMC when ash clouds won't be
seen—don't expect weather radar to
help. If you end up in one, the
immediate action is to keep all or
some of the engines running and
find the shortest route out, which
may be downwards.
Met Services
There are all sorts of facilities
available over the Internet, by
telephone, fax or radio (our site at
www.electrocution.com/aviation.
htm has useful links). Mostly,
though, you will be dealing with your
local Flight Service Station (FSS), which
will provide the Aviation Weather
Information Service, or AWIS (W2 in
the CFS). The Aviation Weather
Briefing Service (AWBS), however, is
more advanced because it is used for
long range flights as well. It is not
always found at an FSS – you mostly
see it at AES offices (W1 in CFS).
The PATWAS is the Pilot's Automatic
Telephone Weather Answering Service,
which pretty much speaks for itself –
you use a touch tone telephone to
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