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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
104 JAR Private Pilot Studies
impression of pitching nose up,
making you more likely to hit the
ground after corrective movements.
Be prepared for a missed approach
whenever you have the slightest
doubt about forward visibility. The
minimum RVR for landing 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
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 having the sun shining
above them. Light is refracted
and objects obscured.
Met Services & Information
ATIS (Automatic Terminal Information
Service) is typically broadcast on a
VOR frequency at major
aerodromes (you can use it as an ID
on instrument rides), although it may
have its own channel. You should
listen to it and take down the details
before you contact ATC.
VOLMET is usually transmitted
over HF for long-distance flights
(North Atlantic and Arctic for
Canada), but can be found
elsewhere. It consists of long
readouts of TAFs and METARs in a
sequence, so if you miss the
aerodrome you want, just wait for it
to come round again. Many airfields
have it available over the telephone.
Some codes (e.g. for wind velocity)
use the same figures as the values
being reported, so a wind from 280°
at 15 knots is 28015KT. Otherwise,
lettered abbreviations are used, as
described below.
TAFs
These describe forecast conditions at
an aerodrome for between 9 and 24
hours. The validity periods of many
longer forecasts may not start for up
to 8 hours after the time of origin,
and the details only cover the last 18
hours. 9-hour TAFs are updated and
re-issued every 3 hours, and 12- and
24-hour TAFs, every 6 hours, with
amendments issued as and when
necessary. They are not available for
offshore operations. A TAF may be
sub-divided into 2 or more selfcontained
parts by the abbreviation
‘FM’ (from) followed by the time
UTC to the nearest hour, expressed
Weather 105
as 2 figures. Many groups in
METARs are also found in TAFs,
but differences are noted below:
Message Type
TAF or TAF AMD, for amended.
The amended forecast will have
AMD inserted between TAF and the
aerodrome identifier, and will cover
the remainder of the validity period
of the original forecast.
Station Identifier
4-letter ICAO sign for aerodrome.
Date and Time of Issue
A 6-digit code, with the date as the
first two, then the time in UTC.
Validity Period
A METAR reports conditions at a
specific time, but the TAF contains
the date and time of origin, followed
by the start and finish times of its
validity period in whole hours UTC,
e.g. TAF EGLL 130600Z (date and
time of issue) 0716 (period of
validity 0700 to 1600 hours UTC).
Winds
To the nearest 10°, in knots.
000000KT is calm, VRB means
variable, less than 3 kts. Gusts are
given in 2 digits. WS means
windshear, when significant, with
speed and direction at a height.
Horizontal Visibility
Only minimum visibility is forecast;
RVR is not included.
Weather
If no significant weather is expected,
this is omitted. After a change group,
however, if the weather ceases to be
significant, 'NSW' (no significant
　
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