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Codes, coding, plotting etc.


Information on codes and coding

Try Dave Wheeler's site - a wealth of information on the SYNOP code - at:
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: ... 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:


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: )

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: " 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)