In this guide I will provide and explain
the information you need to read a metar
in most cases and to fully comprehend it.
Metar reading may seem
complex and difficult
when you see a metar for
the first time,
but once you learn some
basic symbology you
will see that is not that
hard at all - actually
it is really easy.
Before proceeding it is
fundamental that
you first understand what
exactly a metar
is: a metar is a meteorological
observation
of the weather at an airport;
pay attention
to the fact that it is
not a forecast, it
is an istantaneous observation,
a snapshot
which shows how the weather
was when it had
been taken.
Because the metar is a
"weather snapshot"
and not a "weather
forecast", its
validity in time is zero
- it has no validity
in time. The aviation meteorological
message
which has validity in time
is the TAF, which
can be valid from nine
to twelve hours since
the moment in which is
released, but the
METAR, keep in mind that,
it has no validity
in time.
Metars are released every
60 minutes at minor
airports and every 30 minutes
at major airports.
The standard metar string
looks like this:
ICAO -> Date and zulu
time -> Wind
-> Prevailing horizontal
visibility ->
Vertical visibility ->
Temperature ->
Pressure -> Possible
remarks -> Possible
"trend" part
The above scheme is the
one that a metar
always respects; in case
one or more parts
are missing, it will just
pass to the next
one.
Now I am going to explain
each of these parts.
ICAO = is the four letters airport code.
Date and zulu time = day of the month and zulu time when the
metar has been released.
Format example: "071700Z"
- 07
is the day of the month;
1700 is the zulu
time (hour and minutes).
Wind = the main format in which wind is shown
is: example: "15012KT" - 150 is
the direction where it comes from; 12 is
the intensity; KT is the unit in which the
wind intensity is measured (it can be KT
for knots, MPS for meters per second, KMH
for kilometers per hour).
Of the wind, we can find
also other three
formats:
1. "15012KT 120V180"
- 120V180
indicates that the direction
where the wind
is coming from changes
between 120 and 180
degrees.
2. "VRB02KT"
- indicates that the
wind direction is variable
(usually you find
this with low intensity
values, below 3 kt).
3. "15012G25KT"
- G stands for
gusts, the number after
(in this example
25) stands for the maximum
intensity of the
gusts. Gusts are measured
for at least three
seconds, with an intensity
of at least 10
knots stronger than the
wind's.
Prevailing horizontal visibility = can be shown in meters, in SM (static
miles) or represented with one of the following
two symbols: "9999" when the visibility
is 10000 meters or more, or "CAVOK"
(which stands for "ceiling and visibility
okay") when the visibility is 10000
meters or more and there are no clouds below
5000 feet.
Format examples:
"7000" - horizontal
visibility
of 7000 meters;
"10SM" - horizontal
visibility
of 10 static miles;
"9999" - horizontal
visibility
of 10000 meters or more;
"CAVOK" - horizontal
visibility
of 10000 meters or more
and no clouds below
5000 feet. |
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Vertical visibility = is expressed with clouds' coverage and
their height from ground.
Clouds' coverage can be
of four types:
"FEW" - when
sky is covered between
1/8 and 2/8 of its area;
"SCT" - scattered
clouds, when
sky is covered between
3/8 and 4/8 of its
area;
"BKN" - broken
clouds, when sky
is covered between 5/8
and 7/8 of its area;
"OVC" - overcast,
when sky is completely
covered (8/8 of its area).
Clouds' height from ground
is written in
hundreds of feet.
Within the metar we can
find clouds written
even more than one time.
Format example: "SCT032"
- SCT
indicates there are scattered
clouds, 032
indicates that the clouds
are at 3200 feet.
When there are relevant
clouds like cumulus
nembus and towering cumulus
we find the symbols
"CB" for cumulus
nembus or "TCU"
for towering cumulus written
next to the
cloud's height.
Format example: "BKN027CB"
- BKN
indicates there are broken
clouds, 027 indicates
that the clouds are at
2700 feet, CB indicates
that the type of clouds
are cumulus nembus.
When there are no clouds
but still it is
not possible to use "CAVOK",
we
will find written "SKC"
which stands
for "sky clear".
Between the prevailing
horizontal visibility
and the vertical visibility
we can find one
or few symbols which will
indicate if there
are any particles in the
air (like water
or smoke for instance).
Here is a list of the symbols
you can find:
"+" heavy, "-"
light,
both used to indicate the
intensity of the
phenomenon.
"FZ" freezing,
"BL" blowing,
"DR" drifting,
"SH" shower,
all used to indicate the
way in which the
phenomenon happens.
"VC" in the vicinity
of the airport.
"BC" patches,
"DZ" drizzle,
"RA" rain, "SN"
snow,
"SG" snow grain,
"PE"
ice pellets, "IC"
diamond dust
that causes visibility
to be reduced to less
than 5000 meters, "GR"
hail with
diameter bigger than 5
milimeters, "GS"
small hail or snow pellets,
"PY"
sea spray, "FG"
fog, "BR"
mist, "SA" sand,
"DU"
dust, "HZ" haze,
"FU"
smoke, "VA" volcanic
ash, "PO"
dust devil, "SQ"
squall, "FC"
funnel cloud, "DS"
dust storm,
"SS" sand storm,
"TS"
thunder storm, are all
phenomenons.
"UP" unknown
precipitation.
Format examples of vertical
visibility strings:
"BR FEW015 BKN070"
- mist, few
clouds at 1500 feet, broken
clouds at 7000
feet.
"FEW008CB BKN175"
- few cumulus
nembus at 800 feet, broken
clouds at FL175.
"BR SKC" - mist,
sky clear.
"+BLSN SCT013 OVC017"
- heavy blowing
snow, scattered clouds
at 1300 feet, overcast
at 1700 feet.
"SHRA BKN007CB"
- showering rain,
broken cumulus nembus at
700 feet.
Temperature = it is written the temperature of the air
and the dew point, both in celsius degrees.
If the temperature is below zero then the
symbol "M" is placed in front of
the number.
Format example:
"15/09" - air
temperature plus
15 degrees, dew point plus
9 degrees.
"03/M01" - air
temperature plus
3 degrees, dew point minus
1 degree.
Closer to the air temperature
the dew point
is then more humidity is
it in the air.
Pressure = can be shown in two formats, in Hg inches
or in Hpa.
The reference point of
the atmospheric pressure
is: 29.92 inches Hg = 1013
Hpa.
If it is shown in Hg inches
the format will
look like "A2992",
while in Hpa
the format will look like
"Q1013".
Usually low pressure values
(below 29.92
= 1013) are sign of bad
weather while high
pressure values (above
29.92 = 1013) are
sign of good weather.
Possible remarks = this section it is optional and starts
with the symbol "RMK". Here are
indicated any additional possible information.
Possible "trend" part = in this optional section is indicated
the weather tendency. Its validity starts
from the time when the metar had been released
and lasts for two hours.
The "trend" section
can have any
of the following symbols:
"FM" from a specific
time, "TL"
until a specific time,
"AT" at
a specific time, "BECMG"
becoming,
"TEMPO" temporary
change which
lasts less than one hour,
"NO SIG"
no significant change of
the observed weather
in the following two hours.
By Claudio Adriano Dobre, © 2010 |
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