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minimum value, e.g.
R24/M0050.
Cloud
Up to 4 cloud groups may be
included, in ascending order of
bases. A group has 3 letters for
the amount (FEW = 1 to 2
oktas, SCT, or scattered = 3 to 4
oktas; BKN, or broken, = 5 to 7
oktas, and OVC, or overcast = 8
oktas) and 3 for the height of
the cloud base in hundreds of
feet above aerodrome level.
Apart from significant
convective clouds (CB) cloud
types are ignored. Cloud layers
or masses are reported so the
first group represents the lowest
individual layer; the second is
the next individual layer of more
than 2 oktas; the third is the
next higher layer of more than 4
oktas, and the additional group,
if any, represents significant
convective cloud, if not already
reported, e.g. ‘SCT010 SCT015
SCT018CB BKN025’.
CAVOK and SKC
CAVOK will replace visibility,
RVR, weather and cloud groups
when visibility is 10 km or more,
there is no cloud below 5000
feet or below the highest MSA,
whichever is greater, and no cunims;
with no precipitation,
thunderstorm, shallow fog or
low, drifting snow. Otherwise,
the cloud group is replaced by
‘SKC’ (sky clear) if there is no
cloud to report.
Air Temperature and Dewpoint
Shown in °C, separated by an
oblique stroke. A negative value
is indicated by an ‘M’ in front of
the appropriate digits, e.g. 10/03
or ‘01/MOI’
Pressure Setting
QNH is rounded down to the
next whole millibar and reported
as a 4-figure group preceded by
‘Q’. If less than 1000 Mb, the
first digit will be ‘0’.
Recent Weather
Significant weather since the
previous observation, but not
current, will be reported with
the standard present weather
code preceded by the indicator
‘RE’, e.g. ‘RETS’.
72 Operational Flying
Windshear
Included if windshear is
reported in the lowest 1600 feet,
beginning with ‘WS’: ‘WS
TKOF RWY20’, ‘WS LDG
RWY20’.
Runway State
For snow or other runway
contamination, an 8-figure
group may be added at the end
of the METAR.
Trend
For when significant changes are
forecast during the next 2 hours.
The codes ‘BECMG’
(becoming) or ‘TEMPO’
(temporarily) may be followed
by a time group (in hours and
minutes UTC) preceded by one
of ‘FM’ (from), ‘TL’ (until) or
‘AT’ (at). These are followed by
the expected change using
standard codes, e.g. ‘BECMG
FM 1100 250/35G50KT’ or
‘TEMPO FM 0630 TL0830
3000 SHRA’. Where no
significant changes are expected,
the trend group will be replaced
by the word ‘NOSIG’.
DENEB
Fog dispersal is in progress.
Aerodrome Weather Forecasts (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
as 2 figures. Many groups in
METARs are also found in TAFs,
but differences are noted below:
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).
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,
the abbreviation ‘NSW’ (no
significant weather) is inserted.
Cloud
For clear sky, the cloud group
will be replaced by ‘SKC’ (sky
clear). When no CB, or cloud
below 5000' or the highest MSA,
whichever is greater, are
forecast, but ‘CAVOK’ or ‘SKC’
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