Codes

Codes, coding, plotting etc.

 

Information on codes and coding

Try Dave Wheeler's site - a wealth of information on the SYNOP code - at: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm
The site also hosts a display of the 'ww' symbols for present weather. Dave was awarded the MBE in the Queen's Birthday Honours in 2002 for services to meteorology on Fair Isle.

For the 'new' automatic weather types output from the latest generation of AWS (they use a different present weather code), & a plotting matrix of said code figures see here.

Information to decode a METAR

There is a FAQ entry devoted to decoding METAR reports here.

Amongst other useful information, and links to other sites, data sources etc., this site has a decode page relating to the METAR and TAF code: http://www.ukweather.freeserve.co.uk ... then follow the route from the 'Aviation' button to 'Weather Codes Explained'.
 

Snow groups on SYNOP & METAR reports

SYNOP reports (i.e. 5-figure numeric codes exchanged internationally).
Snow depths (& state of ground when snow covered) are reported in the group 4E'sss in the sub-section with a start indicator '333'. The frequency of report varies from country to country - in the UK, reports are hourly if conditions are relevant.

E' records the state of snow cover in the following code:
0: Ground predominantly covered by ice.
1: Compact or wet snow (with or without ice) covering less than one-half of the ground.
2: As for 1 (above), covering at least one-half of ground, but not completely.
3: Even layer of compact or wet snow covering the ground completely.
4: Uneven layer of compact or wet snow covering ground completely.
5: Loose dry snow covering less than one-half of the ground.
6: As for 5 (above), covering at least one-half of ground, but not completely.
7: Even layer of loose dry snow covering ground completely.
8: Uneven layer of loose dry snow covering ground completely.
9: Snow covering ground completely; deep drifts.

sss records the depth of snow:
001 1cm
002 2cm
003 3cm (etc.) ...

996 996cm
997 Less than 0.5cm
998 Snow cover not continuous
999 Measurement impossible or inaccurate.


METAR reports carry information relating to the state of ice/snow ON THE RUNWAY only, in a group properly called a 'runway state' report, but often referred to as a 'SNOWTAM'. The group should follow the TREND group. These groups have little relevance to general snow conditions in the vicinity of the airfield & care must be exercised when looking at these.
The SNOWTAM (from the NOTAM - 'Notice to airmen' root) group takes the following format:

nnCLRDxx

nn: runway designator (50 added to indicate 'right' runways; 88=all runways; 99=repeat of previous report)
C: Type of deposit (0=clear/dry; 1=damp; 2=wet/puddles; 3=rime or frost covered; 4=dry snow; 5=wet snow; 6=slush; 7=ice; 8=compacted or rolled snow; 9=frozen ruts or ridges; /=type of deposit not reported)
L: Extent of runway contamination (1=10% or less; 2=11 to 25%; 5=26-50%; 9=51-100%; /=extent not reported)
RD: Depth of deposit (note: millimetres NOT cm) 00=less than 1mm; 01=1mm etc. through to 90=90mm; 91=not used; 92=10 cm; 93=15 cm .. then 5 cm steps to 97=35 cm; 98=40 cm or more; 99=runway(s) not operational due to snow or runway clearance. //=not measurable or not significant.

When the runway (nn=specific runway or 88=all runways) is/are declared operational, the group has the four letters "CLRD" within it, with the braking action xx encoded.
xx: Friction coefficient/braking action.

Rainfall groups in SYNOP reports

Rainfall (or melted snowfall) is reported in SYNOP reports in a group: 6RRRt r, which should come immediately after the 5 (pressure tendency) group in the main section of the SYNOP, and before the 7 (weather group) if reported. (However, a few countries, mainly in South America, place this group in the 333 section .. this can be confusing, see below.)

RRR=rainfall total in mm, 990=trace, 991 to 995=0.1 to 0.5mm, /// not recorded for any reason.
tr=period over which rainfall recorded in the following code: (ending at the time of the report)
1=6 hours; 2=12 hours; 3=18 hours; 4=24 hours; 5=1 hour; 6=2 hours; 7=3 hours; 8=9 hours; 9=15 hours; /=accumulated rainfall in a period notified nationally. For example, Blocks 41 & 42 (India & nearby island groups), report accumulated rainfall since 0300 UTC.
Note that this group may also be used in the '333' (supplementary) section to indicate rainfall for other periods: for example, in the main SYNOP, a station might have a group 60422, indicating 42mm has fallen in the past 12hr, but in the '333' section, the group 60067 would indicate that 6mm of that total has fallen in the past 3hr.

In addition, many countries report 24hr total precipitation in the group 7RRRR, in the 'supplementary' (or '333') section of the SYNOP: this is agreed regionally - European countries reporting (mainly) at 06Z, but elsewhere at other 'main' SYNOP hours, sometimes every 6 hours. The rainfall/melted snowfall is in mm & tenths, so 70467 in the '333' section would indicate 46.7mm has been recorded in the past 24hr.

Wind reports carried in SYNOP reports

The mean wind direction and speed is carried in the SYNOP report in the ' Nddff ' group (for the structure of the SYNOP, consult Dave Wheeler's excellent guide: http://www.zetnet.co.uk/sigs/weather/Met_Codes/codes.htm )

The wind direction is that FROM which the wind is blowing (e.g. if dd=27, then the wind is blowing from the west; if dd=36, the wind is coming from the north etc.)
The wind speed is averaged over 10 minutes (but see below), and is given in units of either knots (kn) or metres per second (m/s).
You need to inspect the group that comes at the head of a collective of observations in the form YYGGiw (where YYGG = month-date & hour-time);
if iw=1, then the wind speeds are given in m/s (double to convert roughly to knots), and if iw=4, then the values are in knots.

Re: FM 12 (SYNOP), FM 13 (SHIP), FM 14 (SYNOP MOBIL)
Regulation: 12.2.2.3.1 " The mean direction and speed of the wind over the 10-minute period immediately preceding the observation shall be reported for ddff. However, when the 10-minute period includes a discontinuity in the wind characteristics, only data obtained after the discontinuity shall be used for reporting the mean values, and hence the period in these circumstances shall be correspondingly reduced. "

If recording/logging equipment is not available, then it's much looser, but the same principal (should) apply ... you take an estimate at the beginning of the 10 mins, one at the end and use a 'mean', or simply make a Beaufort estimate at ob-time and convert to appropriate value.

Thus, for practical purposes, it should be assumed that 10 minutes prior to observation time is the 'standard' for reports in the main section of a SYNOP report (i.e. in the 'Nddff' section).
However .... (life isn't that simple) .... in the United States, and presumably territories where they have an influence in the provision of equipment, setting up services etc., the period is 2 minutes. I'm informed that for the US (Block 72) specifically, the mean wind direction & speed (ddff) is derived from the METAR report, which is known to be a 2 minute mean.

... when we move away from the main section of the SYNOP, there is some variability allowed for in the 'additional' data section as under:

Code 3778 ('9-group' sections)
910ff Highest gust during the 10-minute period immediately preceding the observation.
911ff Highest gust .... (see note below*)
912ff Highest mean wind speed .... (see note below*)
913ff Mean wind speed .... (see note below*)
914ff Lowest mean wind speed .... (see note below*)

*Note: " ... during the period covered by W1W2 (i.e. past weather) in group 7wwW1W2, unless a different period of reference is indicated by group 907tt; or during the 10-minute period immediately preceding the time of observation indicated by group 904tt "
where tt =
00 at observation time
01 .. 09 units of 6 minutes before observation (i.e. 02=12 minutes before etc.)
10 1 hr before observation (the most often used in this context)
11 .. 60 continuing the scheme of units of 6 minutes before observation. (i.e. 39=3hr 54mins)
61 6 to 7 hrs
62 7 to 8 hrs etc. ... up to
66 11 to 12 hrs
67 12 to 18hr
68 more than 18hr
69 time unknown
70 began during observation
71 ended during observation

Sunshine groups carried in SYNOP reports from automatic stations

Where Weather Stations are suitably equipped, and are reporting in the SYNOP code for inclusion in international bulletins, then "sunshine" duration (mainly using radiation sensors) is carried in the following groups:
At 0600Z ONLY .... In Section 333: the group 55SSS shows the duration of sunshine, in tenths of an hour, for the 24 hours of the previous day.
At ALL hours .... In Section 333, the group 553SS is reported, showing the sunshine total for that hour in tenths of an hour.

The local software integrates direct solar radiation received by the AWS sensor in order to estimate the sunshine duration. For day-to-day work, the data are accurate and timely, and will in time form the standard database for bright sunshine records. The sensors respond quickly and accurately to sunshine, and eliminate the problems experienced with the Campbell-Stokes recorder (CSR). This latter uses a glass sphere to focus the rays of sunlight on a card, which is charred upon strong heating - much as you would focus sunlight by a magnifying glass to try and light a fire. There are well-known problems with the CSR instrument, in particular with intermittent, short-duration sunshine, and the subjectivity used in estimating the burn-lengths. The figures obtained from the AWS and CSR units are obtained & processed in totally different ways and care must be applied when comparing the two sets of records. (Also see 'Sunshine recording' in the Glossary)

 

Code 4680

FM 12-IX Ext. SYNOP code form

In reports from fully automatic stations (THAT ARE EQUIPPED WITH THE APPROPRIATE SENSORS), a separate 'present weather' code table is used (wawa): After the station number group (IIiii), comes a group which contains information on whether or not 'weather' groups are included, and which code table is used. The important figure is the second figure in the group [ iRiXhVV ] as under:

 iX  Type of station  7 ('weather')- group
 1  Manned  Included
 2  Manned  Not-included (no sig.Wx to report)
 3  Manned  Not-included (data not available)
 4  Automatic  Included using 'old' wwWW present weather coding
 5  Automatic  Not-included (no sig. Wx to report)
 6  Automatic  Not-included (data not available)
 7  Automatic  Included using 'new' wawa Code table (as below)

 

A scanned display of the the plotting convention for this code type will be found here.


 01-09  Description
 01  Clouds generally dissolving or becoming less developed (during past hour)
 02  State of sky generally unchanged
 03  Clouds forming or developing (during past hour)
 04  Haze, smoke or suspended dust (vis >=1.0km)
 05  [as 04 but vis < 1.0km ]
 06  -
 07  -
 08  -
 09  -

 10-19  Description
 10  Mist
 11  Diamond dust (not yet used)
 12  Distant lightning (not yet used)
 13  -
 14  -
 15  -
 16  -
 17  -
 18  Squalls
 19  -

Code 20-26: used to report precipitation, fog (or ice fog) or thunderstorm at the station in the past hour, but not at time of observation. 27-29 appear to be current weather!

 20-29  Description
 20  Fog
 21  Precipitation
 22  Drizzle (non-freezing) or snow grains
 23  Rain (non-freezing)
 24  Snow
 25  Freezing drizzle or rain
 26  Thunderstorm (with/without ppn) (not yet used)
 27  Blowing/Drifting snow/sand (not yet used)
 28  Blowing/Drifting snow/sand: vis >=1.0km (not yet used)
 29  Blowing/Drifting snow/sand: vis < 1.0km (not yet used)

 30-39  Description
 30  Fog
 31  Fog/Ice Fog in patches
 32  Fog/Ice Fog - thinning in past hour
 33  Fog/Ice Fog - no change
 34  Fog/Ice Fog - set in, or thickened in past hour
 35  Fog - depositing rime
 36  -
 37  -
 38  -
 39  -

 40-49  Description
 40  Precipitation (PPN)
 41  PPN - slight/moderate
 42  PPN - heavy
 43  Liquid PPN - slight/moderate
 44  Liquid PPN - heavy
 45  Solid PPN - slight/moderate
 46  Solid PPN - heavy
 47  Freezing PPN - slight/moderate
 48  Freezing PPN - heavy
 49  -

 50-59  Description
 50  Drizzle
 51  Drizzle, not freezing - slight
 52  Drizzle, not freezing - moderate
 53  Drizzle, not freezing - heavy
 54  Drizzle, freezing - slight
 55  Drizzle, freezing - moderate
 56  Drizzle, freezing - heavy
 57  Drizzle & rain mixed - slight
 58  Drizzle & rain mixed - moderate/heavy
 59  -

 60-69  Description
 60  Rain
 61  Rain, not freezing - slight
 62  Rain, not freezing - moderate
 63  Rain, not freezing - heavy
 64  Freezing rain - slight
 65  Freezing rain - moderate
 66  Freezing rain - heavy
 67  Rain/drizzle & snow mixed - slight
 68  Rain/drizzle & snow mixed - moderate/heavy
 69  -

 70-79  Description
 70  Snow
 71  Snow - slight
 72  Snow - moderate
 73  Snow - heavy
 74  Ice Pellets - slight
 75  Ice Pellets - moderate
 76  Ice Pellets - heavy
 77  Snow grains
 78  Ice crystals
 79  -

 80-89  Description
 80  Intermittent PPN
 81  Intermittent rain - slight
 82  Intermittent rain - moderate
 83  Intermittent rain - heavy
 84  Intermittent rain - violent
 85  Intermittent snow - slight
 86  Intermittent snow - moderate
 87  Intermittent snow - heavy
 88  -
 89  Hail (not yet used)

 90-99  Description
 90  Thunderstorm (TS)(not yet used)
 91  TS, slight/moderate - no PPN (not yet used)
 92  TS, slight/moderate - rain/snow showers (not yet used)
 93  TS, slight/moderate - hail (not yet used)
 94  TS, heavy - no PPN (not yet used)
 95  TS, heavy - rain/snow showers (not yet used)
 96  TS, heavy - hail (not yet used)
 97  -
 98  -
 99  Tornado !!! (not yet used)

 

NB: This is not an official publicising of the WMO code form. It is purely intended for interested amateurs who want to be aware of the relevant code forms. If you have a professional requirement to use these codes, you should refer to the appropriate WMO publications for the official listing, or contact your national meteorological service for more details. I will not be responsible for any omissions, errors etc.

Code 4680 Plotting Matrix

(For the CURRENT/'old-style' symbols, go to Dave Wheeler's site)

Plotting matrix of Code 4680

METAR Decode

This page is designed to help Internet users decode a METAR (Meteorological Aviation Report). It will not necessarily cover the finer points of coding (though I have tried to cover many such), and therefore should not be quoted as the 'final authority': users wishing to know the correct procedures for coding, usage etc., should consult their national 'meteorological authority' for guidance, or the appropriate ICAO or WMO regulations.


The code-form follows the pattern:
LOCATOR - DATE/TIME - WIND - VISIBILITY - CLOUD - TEMPERATURE - PRESSURE - ADDITIONAL INFORMATION - TREND FORECAST


In the list below, I have tried to build up from the 'basic' version (via CAVOK) to the more complex issues. I have colour-coded as follows:

   always included in a report (provided station so equipped).
   always included, unless for a CAVOK report.
   added/inserted when necessary, as required by coding convention.
   optional: added as available - often to national or regional (ICAO) rules.

Although the basic code-form should always be recognisable, many nations have adapted the METAR (or merged / integrated with their own national form of same) to produce hybrids. Where possible, I have tried to indicate these variants. Military users in particular will have extra information - e.g. Colour States. Also note that there is an increasing use of automatic weather stations to provided some or all of the data when airfields are closed, or an observer is not available for some reason. In these cases the word AUTO is inserted after the date/time group - other variations follow and are listed in the appropriate sections below. However, in general, if there is a failure of all/some of the systems with these units, then the appropriate element will be replaced by solidi in varying numbers.


 METAR:-  "BASIC/MINIMUM"

EGKK 021950Z 24015KT 8000 FEW008 SCT012 BKN020 18/15 Q1007
EGKK 021350Z 17009KT 8000 NSC 23/12 Q1018

EGKK .. airfield identifier according to the ICAO listing. This one is Gatwick airport.

021950Z .. date/time of report, in this case 2nd day of month, at 1950 GMT (or UTC).
[in the second example, 1350 GMT]

24015KT .. 'surface' wind direction (usually at 10 metres, but some take as low as 2 metres) and mean speed: wind blowing from 240deg True, with an averaged speed (over 10 minutes usually, but some use 1 or 2 minute sampling periods) of 15 knots (KT); some countries use metres per second (MPS) or kilometres per hour (KPH). If the wind direction is 'variable', then the direction is replaced by VRB; dead-calm would be 00000KT.
[in the second example wind direction from 170deg, speed 9 kt]

8000 .. horizontal prevailing visibility** representative of the airfield in metres up to 9 km (9000): if the prevailing visibility (as defined below) is 10 km or more, then this group is given as 9999 & if < 50 metres, it is coded as 0000.
[ ** the 'prevailing visibility' is defined as the value that is reached or exceeded over at least 50% of the horizon (contiguous or in fragments), or within at least half of the airfield / airport surface. See examples below for the reporting of significant variations from this value.
** prior to November 2004, the convention long adopted (except in the US & Canada), was to report the lowest visibility as the primary value, with 'better' values appended to defined rules. Note, however, that this use of 'prevailing visibility' is only applicable to the METAR code; SYNOP visibility reports continue to show the lowest visibility figure.

FEW008 SCT012 BKN020 .. amounts and height-of-base of clouds over/in near vicinity of airfield. SKC (some use CLR) =  no cloud; FEW = 1 or 2 eighths cover; SCT = 3 or 4 eighths cover; BKN = 5, 6 or 7 eighths cover & OVC = 8/8 cover. Heights are given in 100's of feet above airfield level, thus 008 = 800ft, 012 = 1200ft, 020 = 2000ft etc.
[ When there is more than one layer of cloud, the convention for inclusion of cloud groups is ....
(a): the lowest layer
(b): the next highest layer, covering 3 oktas or more of the sky (SCT or more)
(c): the next highest layer, covering 5 oktas or more of the sky (BKN or more)
(d): any CB not already included by these rules - the group being inserted in 'natural' height order.]

When fog or heavy snow is occurring, and it is not possible to determine cloud structure, then these groups are replaced by VVhhh or VV///, where either the vertical visibility can be determined (hhh) in hundreds of feet, or impossible to determine (///)

NSC: increasingly (outside North America), cloud information above 5000 ft / 1500m (or the highest minimum sector altitude, whichever is greater) is being omitted from METAR reports, unless it is 'significant' e.g. when CB or TCU are observed (i.e. the bases of these latter may be above 5000 ft / 1500 m). You will therefore see such replacing the cloud groups: note carefully, that 'NSC' (no significant cloud), means just that .... it does NOT mean no cloud at all!
NCD: in reports from automatic stations (e.g. 'AUTO' obs.), then you may see this abbreviation used to mean .. "no cloud detected". It is important to understand that such means exactly that - there is no cloud below 5000 ft / 1500 m overhead the cloud sensor: there may indeed be cloud floating around well below this level - e.g. 2 oktas of stratus over the airfield approach, or 1 okta cumulus associated with a nearby shower. Also, because of the coding convention now used, there may be cloud higher up, but it will not now be reported under these rules.
[ The formerly-used abbreviation 'SKC', meaning sky clear of cloud, should no longer be used under any circumstances. ]

18/15 .. air and dew-point temperatures (screened), both generally around 1.25m above station level, in degC. Negative values preceded by 'M', thus M02/M05 would indicate air temperature minus 2degC, and dew-point minus 5degC.
[in the second example, air temperature 23 and dew point 12 degC]

Q1007 .. atmospheric (i.e. mean sea level) pressure/QNH, in whole mbar (or hPa). In North America in particular (and associated reporting stations), then this is reported in inches of mercury multiplied by 100, thus A2997 = 29.97 inches. The value in mbar may then be appended to the end of the report as SLPppp, where ppp = QNH in whole mbar, with the 'thousand & hundred' figures missed off where necessary: thus SLP987 would be 998.7 mbar, SLP030 would be 1003.0 mbar.
[in the second example, QNH=1018]


 METAR:-  "CAVOK version"

EGKK 132020Z 22013KT CAVOK 18/15 Q1016

Provided the visibility is >= 10 km, AND the height of the lowest cloud (any amount) is >=5000 ft (or highest minimum sector altitude) AND there are no cumulonimbus clouds (CB, any height) within sight AND there is no significant weather (see list below), then the visibility and cloud part of the standard METAR is replaced by CAVOK (say "cav-oh-kay": 'Ceiling And Visibility OK'). (not used by certain countries, e.g. the United States)


 METAR:-  with additional WIND information

EGKK 312355Z 24028G42KT 210V280 9999 SCT018 12/06 Q0984

24028G42KT.. if over the period that the mean wind is assessed (1, 2 or 10 minutes), the 'peak' gust is greater than the mean by 10KT or more (or equivalent in MPS or KPH), then the gust is appended as 'Gff' .. in above, gust is 42 knots. Note carefully when comparing with SYNOP data, the period of the gust is only for the past 10 minutes at most, NOT the past hour, 3-hours, 6-hours etc., as in SYNOP. For this reason, METAR gusts do not give the true picture of peak winds for any one synoptic situation.

210V280 .. if over the period of observation the wind direction is varying between defined limits, and is in excess of a pre-set level (generally 3 knots or more - national variations), then the 'outer limits' of variation are given, in a clockwise direction: .. in this example between 210degT and 280degT.

 METAR:-  Example of WIND SHEAR report

EDDS 120820Z 24028G45KT 210V280 9999 SCT018 12/06 Q0984 WS RWY28

Some (not all) airfields add wind shear information to the end of the METAR when above a certain threshold: this example would be interpreted as " critical wind shear has been exceeded on approach to runway 28 ".


 METAR:-  with additional VISIBILITY information

EGZZ 231020Z 02006KT 4000 0900NE R27/0600U R32/0150D PRFG OVC007 12/11 Q1028

4000 .. 0900NE .. The first figure given is the 'prevailing visibility', which can be regarded as the 'best' figure that can be applied to at least 50% of the horizon (contiguously or otherwise). So, for example, if the visibility varies from 8km down to 4000m for at least half of the visible horizon, the prevailing visibility is 4000m. It is important to note that the visibility may be lower than this figure elsewhere, but for deviations to be reported, they must obey certain rules: if the visibility in one particular direction is less than 1500m or is less than half of the prevailing figure, then the lowest visibility observed (900m in above) is reported, with the direction shown (NE). If the lowest value applies in several directions, then the 'most operationally significant' direction is given. If the visibility is fluctuating wildly (e.g. rapid shower transistion), then only the lowest visibility is reported. Where the observation is a fully-automated one (e.g. an 'AUTO' ob.), then no variation with direction of visibility is usually possible, and the letter-group NDV is appended to the visibility value.

R27/0600U .. R32/0150D.. The visibility given above is not necessarily the most useful indicator of what a pilot would actually 'see' along a particular runway. To try and overcome this limitation, for airfields so equipped, the 'Runway Visual Range' (RVR) is given when general visibility is poor. In this example, the RVR along runway 27 is 600metres, and along runway 32 it is 150metres. In addition, the letters U, D & N are sometimes used to denote respectively increasing (Up), decreasing (Down) and unchanged (Nochange) RVR values since last report. If the RVR is less than 50 metres, then the group is coded as M0050. If the RVR is more than 2000 m (but general visibility is poor), then the group is coded as P2000: however, many systems only have a maximum operating limit of 1500 m, so you will then seen P1500 in these cases. There are other variants - see current regulations for exact style, meaning etc.


 METAR:-  with additional WEATHER information

EGKK 111150Z 24018KT 8000 -RA SCT012 OVC015 12/10 Q0984 RETS

Weather information follows the format:
Intensity .. Description .. Precipitation .. Obscuration .. Other
This table sets out the full list. (See note 4 below).

INTENSITY DESCRIPTION PRECIPITATION OBSCURATION OTHER
- Light MI shallow DZ drizzle BR mist (see note 5) PO well developed dust / sand whirls
Moderate (no symbol) PR partial (e.g. fog bank - see note 1 below) RA rain FG fog SQ squalls
+ Heavy BC patches (see note 1 below) SN snow FU smoke (see note 5) FC funnel clouds, inc tornadoes / waterspouts
VC Vicinity (see note 3 below) DR low drifting (less than 2 m height) SG snow grains VA volcanic ash SS sandstorm
  BL blowing (2 m or more agl; i.e. affecting visibility significantly) IC ice crystals DU widespread dust haze (see note 5) DS duststorm
  SH showers PL ice pellets SA sand  
  TS thunderstorm GR hail HZ haze (see note 5)  
  FZ freezing (see note 2 below) GS small hail PY spray  
    UP unknown
(automatics)
   

.. up to three weather groups can be included.
.. the individual categories are used from left-to-right in the table above, when more than one applies: thus ... -SHRA light shower of rain; +TSRA heavy thunderstorm with rain; DZ BR moderate drizzle and mist; SN moderate snow; VCPO dust/sand whirls in vicinity (but not over airfield); -SHRAPL FC light shower of rain, with ice pellets and funnel cloud observed.

Note 1: When used with FG, the qualifier 'PR' is used for fog banks, i.e. an extensive area of fog impinges upon an airfield, reducing visibility over part of same to less than 1 km; 'BC' [ patches ] would be used when a discrete, small-scale area of fog drifts/forms over the airfield, again reducing visibility below 1 km but not in all directions. In practice, it is not easy to tell the two apart!
Note 2: When used with FG, DZ, RA etc., the qualifier 'FZ' is now used to mean BOTH fog (drizzle, rain) depositing rime AND fog (drizzle, rain) occurring with an air-temperature below zero deg.C; this latter may or may not be depositing rime-ice. This definition is therefore not the same as that for the SYNOP code (but I wonder how carefully the distinction is going to be maintained!)
Note 3: all phenomena are considered to be 'at the station' unless prefixed by VC = vicinity; thus VCFG is fog in the area, but not affecting the airfield. Vicinity is defined as within 8 km of the airfield. )
Note 4: in observations from fully-automated units (e.g. 'AUTO' obs.), then if present weather cannot be assessed due to failure (or otherwise) of the system, then the present weather will be given at //. If the system is functioning correctly but there is no 'significant' weather, then as for manual observations, the 'weather' group is blank.
Note 5: the present weather mist (BR), dust (DU), smoke (FU) and haze (HZ) should only be encoded when the prevailing visibility is 5000 metres or less (though in practice this rule is not often followed).

.. RExx.. significant 'recent' weather may be added after the pressure group, using the list below.

 Thunderstorm  RETS  Freezing Rain  REFZRA
 Freezing Drizzle  REFZDZ  Moderate/heavy rain  RERA
 Moderate/heavy snow  RESN  Moderate/heavy small hail  REGS
 Moderate/heavy snow pellets  REGS  Moderate/heavy ice pellets  REPL
 Moderate/heavy hail  REGR  Moderate/heavy snow grains  RESG
 Moderate/heavy rain showers  RESHRA  Moderate/heavy snow showers  RESHSN
 Moderate/heavy small hail shower  RESHGS  Moderate/heavy snow pellet shower  RESHGS
 Moderate/heavy ice pellet shower  RESHPL  Moderate/heavy hail shower  RESHGR
 Moderate/heavy ice crystals  REIC  Moderate/heavy blowing snow (visibility significantly reduced)  REBLSN
 Sandstorm  RESS  Dust Storm  REDS
 Funnel Cloud  REFC  Volcanic Ash  REVA
 Unidentified precipitation (AUTO obs. only)  REUP    

 METAR:-  with additional CLOUD information

ENBR 232350Z 32017G36KT 270V010 9999 FEW009 SCT018TCU 02/M03 Q0992 WS RWY28

SCT018TCU .. this group indicates 3 or 4 eighths of cloud with base 1800ft, cloud type 'towering Cumulus' (TCU); the only other cloud type allowed (in the official METAR code) is Cumulonimbus (CB). However, some national services use additional types: CBMAM .. Cumulonimbus mammatus (implying turbulent air in the vicinity); ACC .. Altocumulus castellatus (medium level vigorous instability); CLD .. standing lenticular or rotor clouds.
Where the observation is an 'AUTO' ob, and as such cannot determine such variations, then the group /// is added after each cloud group.


 METAR:-  with TREND appended

EGKK 151550Z 24018KT 8000 -RA SCT012 0VC015 12/11 Q0984 RERA TEMPO 3000 RA BKN008

For some major airports (and many military airfields with forecasters attached), a TREND forecast is added to airfield weather reports, usually covering the following 2 hours. If there is no significant change expected (as defined in both international and national regulations), then "NOSIG" is added. Otherwise, the TREND will indicate the expected change in wind direction/speed, visibility, significant weather and cloud base using conventions applied to the TAF code. Some abbreviations you might see: BECMG = becoming (with time groups); TEMPO = temporarily; NSW = no significant weather; AT = at (time); TL = until (time); NSC = no significant cloud.


 METAR:-  additional (non-standard) variants

Some nations (e.g. US, Australia), add more information to the end of the METAR. This list below is not exhaustive but covers some of the more usual inclusions:
RMK .. additional remarks are added after this group.
Volcanic eruptions .. plain language, to include name of volcano, lat/long or direction/distance, date/time of eruption, size/description of ash cloud etc.
Funnel cloud .. format: Type B(hh)mm LOC: type can be one of TORNADO, FUNNEL CLOUD, WATERSPOUT; B(hh)mm: beginning time (can also be used to show end time/E(hh)mm). hh is the hour of the sighting, which can be left out if redundant. mm is the minute of occurrence; LOC: location or direction of movement.
Type of automatic station .. A01 - without precipitation sensor; A02 - with precipitation sensor.
Peak wind .. format: PK WND dddff(f)/hh)mm: ddd wind direction; ff(f) wind speed in knots; (hh)mm time at which wind speed occurred, with hh being left out if redundant.
Wind shift .. format: WSHIFT (hh)mm [FROPA]: (hh)mm time wind shift occurred; FROPA added if windshift is at frontal passage; again, hh is left off if redundant.
Visibility .. format: TWR VIS vvvvv - shows ATC tower visibility; SFC VIS vvvvv shows surface visibility; VIS lllllVuuuuu - shows variable visibility, lower to upper; VIS [DIR] - sector visibility with additional direction (e.g. VIS S 1 1/2 .. visibility to south 1 and a half miles).
Lightning .. format: Frequency LTG(type) [LOC]: frequency can be one of OCNL - less than 1 flash per minute, FRQ - 1 to 6 flashes per minute & CONS - more than 6 flashes per minute; CG: cloud-to-ground, IC: in-cloud, CC: cloud-to-cloud, CA: cloud-to-air.
Tstttsddd .. the actual temperature and dew-point temperature in degrees and tenths (C), with "s" indicating the 'sign' of temperature: 0 for positive, and 1 for negative: thus T00081016 would be interpreted as air temperature +00.8degC and dew-point temperature -01.6degC.
1sTxTxTx 2sTxTxTx .. as for the SYNOP code, where TxTxTx is the maximum temperature and TnTnTn is the minimum temperature - but the period is the last 6 hours. Code 's' indicates the sign of temperature, 0 for at or above zero degC, 1 for below.
4sTxTxTxsTnTnTn .. 24hr maximum and minimum temperature, with s indicating sign as before.
Prrrr.. hourly precipitation amount.
6rrrr.. 3 and 6 hour precipitation amounts (3hr at intermediate hours, 6hr at 'main' hours).
7rrrr.. 24hr precipitation amount.
4/sss.. snow depth on ground (not runway).
CIGhhh.. ceiling in hundreds of feet (hhh), generally the lowest layer with 5 oktas or more, but USAF bases have variations on this.


Colour states: METAR reports from military airfields operated by the RAF, some USAF and others may have a 'colour-code' appended (usually only when ATC is open), which describes the airfield 'fitness': these run from BLU best, through WHT GRN YLO (1 and 2), AMB and RED. The colour is based on the lowest cloud base (usually 3 oktas or more cover, but some use 5 oktas) and the horizontal 'MET' visibility. BLACK is also used, for airfield closed for non-weather reasons.