Defining the complete set of Significant Weather codes

Significant weather (w'w') is reported within FM 15 METAR Aerodrome routine meteorological report, FM 16 SPECI Aerodrome special meteorological report and FM 51 TAF Aerodrome Forecast.

Please refer to technical regulation WMO No. 306 MANUAL ON CODES Volume I.1 Part A - Alphanumeric Codes for details.

Code-table 4678 is organised into 5-categories from which terms can be combined to form appropriate significant weather codes (e.g. +DZSNRA 'Heavy' 'Drizzle', 'Snow' and 'Rain').

The combinations of terms must be meteorological valid, and in some cases are further restricted through the business rules specified in the technical regulation (e.g. "forecast weather" is only permitted to report a subset of terms).

Publication of code-table 4678 as a web-accessible register

Assessment of significant weather codes to determine their validity is complex. As a result, the GML/XML implementation of METAR/SPECI and TAF will use a register of predefined codes for significant weather. Validation of significant weather codes can then be simply achieved by checking whether the term appears on within the register.

The register containing the complete set of valid terms derived from code-table 4678 shall be published by WMO as a web accessible resource. The URI of this register shall be:

• http://data.wmo.int/def/306/4678

Each of the valid significant weather codes will also be assigned an HTTP URI within the namespace of the register; e.g. (moderate) 'showers' 'in vicinity' shall be identified as:

• http://data.wmo.int/def/306/4678/VCSH

Both the register and the significant weather codes will resolve to provide useful information (e.g. human-readable web-pages or machine-readable content; the format provided will be determined using HTTP content negotiation). In the case of the register the set of member codes will be provided, whilst each significant weather code shall resolve to a definition of that weather phenomenon.

Note that the register and terms are not yet available to support 1.0RC1. These resources are anticipated to be available to support the 1.0RC2 release.

The identifiers for significant weather codes can be constructed by appending the term derived from code-table 4678 to the base URI <http://data.wmo.int/def/306/4678/>. The standard behaviour for intensity qualifiers is retained; e.g. for drizzle

• -DZ : drizzle (light) <http://data.wmo.int/def/306/4678/-DZ>
• DZ : drizzle (moderate) <http://data.wmo.int/def/306/4678/DZ>
• +DZ : drizzle (heavy) <http://data.wmo.int/def/306/4678/+DZ>

In addition to the intensity qualified terms above, it is anticipated that the register <http://data.wmo.int/def/306/4678> will also include a set of more generic terms for weather phenomena that are 'unqualified'. These terms may be used where the intensity qualification "light", "moderate" or "heavy" is not known or not possible to determine. These terms will be allocated human-readable identifiers; e.g.

• Drizzle : drizzle (without intensity qualification) <http://data.wmo.int/def/306/4678/Drizzle>

TODO: assess whether alternate WMO Code-tables should be used to provide authoritative references for weather phenomenon without intensity qualification.

Publication of restricted sets of significant weather codes

The technical regulation places a set of additional restrictions on the use of terms from code-table 4678 for recent and forecast weather: only a subset of the valid combinations may be used.

To cater for this restriction, three additional registers are defined whose members are the specific subset of terms permitted for use in METAR/SPECI and TAF for:

• recent weather - <http://data.wmo.int/def/49-2/AerodromeRecentWeather>
• present weather - <http://data.wmo.int/def/49-2/AerodromePresentWeather>
• forecast weather - <http://data.wmo.int/def/49-2/AerodromeForecastWeather>

Each of these registers is intended to support the business rules defined in the ICAO technical regulation for meteorology (ICAO Annex 3 / WMO No. 49 Volume 2) hence their URIs are allocated within the <http://data.wmo.int/def/49-2> namespace.

The members of these registers shall be the subset of applicable terms from the <http://data.wmo.int/def/306/4678> register.

For example, the term <http://data.wmo.int/def/306/4678/TS> ("Thunderstorm") is a member of the "Recent weather" register <http://data.wmo.int/def/49-2/AerodromeRecentWeather>.

FM 15 METAR / FM 16 SPECI

METAR and SPECI code-forms require that significant weather is encoded using the scheme defined in code-table 4678.

Significant weather is reported for present weather and recent weather within an aerodrome observation and for forecast weather within a trend forecast.

Please refer to the technical regulation for full details. Extracts are provided below:

15.8 Group w'w'

15.8.1

One or more groups w´w´, but not more than three, shall be used to report all present weather phenomena observed at or near the aerodrome and of significance to aeronautical operations in accordance with Code table 4678.

Appropriate intensity indicators and letter abbreviations (Code table 4678) shall be combined in groups of two to nine characters to indicate present weather phenomena.

15.13 Supplementary information - groups

15.13.2 Recent weather phenomena of operational significance REw'w'

Up to three groups of information on recent weather shall be given by the indicator letters RE followed, without a space, by the appropriate abbreviations, in accordance with Regulation 15.8 (but no intensity of the recent weather phenomena shall be indicated) if the following weather phenomena were observed during the period since the last routine report, or last hour, whichever is shorter, but not at the time of observation:

• Freezing precipitation;
• Moderate or heavy drizzle, rain or snow;
• Moderate or heavy: ice pellets, hail, small hail and/or snow pellets;
• Blowing snow;
• Sandstorm or duststorm;
• Thunderstorm;
• Funnel cloud(s) (tornado or waterspout);
• Volcanic ash.

15.14 Trend forecasts

15.14.12

Inclusion of significant forecast weather w’w’, using the appropriate abbreviations in accordance with Regulation 15.8, shall be restricted to indicate:

(1) The onset, cessation or change in intensity of the following weather phenomena:

• Freezing precipitation;
• Moderate or heavy precipitation (including showers);
• Duststorm;
• Sandstorm
• Thunderstorm (with precipitation);
• Other weather phenomena – given in Code table 4678 as agreed by the meteorological authority and air traffic services authority and operators concerned.

(2) The onset or cessation of the following weather phenomena:

• Freezing fog;
• Ice crystals;
• Low drifting dust, sand or snow;
• Blowing dust, sand or snow;
• Thunderstorm (without precipitation);
• Squall;
• Funnel cloud (tornado or waterspout).

FM 51 TAF

Significant weather is also reported in the TAF. Similar to the METAR Trend Forecast, the permitted codes from table 4678 are restricted.

Extracts from the technical regulation state:

51.5 Group w'w' or NSW

51.5.1

Inclusion of significant forecast weather w'w', using the appropriate abbreviations in accordance with Regulation 15.8 ...

Code-table 4678

Code-table 4678 specifies the set of significant weather phenomena that may be reported for aeronautical purposes.

The code-table is organised into 5-columns:

1. INTENSITY OR PROXIMITY
2. DESCRIPTOR
3. PRECIPITATION
4. OBSCURATION
5. OTHER

An extract of the code-table from WMO No. 306 Volume I.1 is provided below:


The significant weather codes shall be constructed by considering columns 1-5 in the code-table in sequence, that is intensity, followed by description, followed by weather phenomena. An example would be: +SHRA (heavy shower(s) of rain).

The technical regulation provides the following notes on usage of the code-table:

1. Entries in this code table are based on the descriptions of hydrometeors and lithometeors found in the International Cloud Atlas (WMO-No 407), Volume I (Manual on the observation of clouds and other meteors).
2. Regulation 15.8 shall apply.
3. More than one form of precipitation shall be combined, the dominant type of precipitation being reported first, for example +SNRA.
4. More than one phenomenon other than a precipitation combination noted shall be reported in separate w´w´ groups in the order of the columns, for example –DZ FG.
5. Intensity shall be indicated only with precipitation, precipitation associated with showers and/or thunderstorms, duststorm or sandstorm, and funnel clouds.
6. Not more than one descriptor shall be included in a w´w´ group, for example –FZDZ.
7. The descriptors MI, BC and PR shall be used only in combination with the letter abbreviation FG, for example MIFG.
8. The descriptor DR (low drifting) shall be used for dust, sand or snow raised by the wind to less than two metres above the ground. BL (blowing) shall be used to indicate dust, sand or snow raised by the wind to a height of two metres or more above the ground. The descriptors DR and BL shall be used only in combination with the letter abbreviations DU, SA and SN, for example BLSN.
9. When blowing snow is observed with snow falling from cloud, both phenomena are reported, e.g. SN BLSN. When due to blowing snow the observer cannot determine whether or not snow is also falling from cloud, then only BLSN shall be reported.
10. The descriptor SH shall be used only in combination with one or more of the letter abbreviations RA, SN, GS, GR and UP to indicate precipitation of the shower type at the time of observation, for example SHSN.
11. The descriptor TS, if not used on its own, shall be used only in combination with one or more of the letter abbreviations RA, SN, GS, GR and UP to indicate thunderstorm with precipitation at the aerodrome, for example TSSNGS.
12. The descriptor FZ shall be used only in combination with the letter abbreviations FG, DZ, RA and UP for example FZRA.
13. The proximity qualifier VC shall be used only in combination with the letter abbreviations TS, DS, SS, FG, FC, SH, PO, BLDU, BLSA, BLSN and VA.
14. UP is to be used only in reports from fully automated stations unable to distinguish precipitation type.

Proposed authoritative set of significant weather codes

A spreadsheet is available that lists four possible sources of the "authoritative list".

• Column 1 (Apply 4678 "rules") attempts to list all the possible combinations from table 4678. Forecasters have pointed out that the list contains entries that mix precipitation from layer cloud and from convective cloud that should be excluded on meteorological grounds. This list also only permits two types of precipitation to be reported in a group - forecasters have commented that reports sometimes contain three types of precipitation.
• Column 2 is a list of combinations that a group of aviation forecasters thought are used in practice.
• Column 3 is the list that appears in the "METAR/SPECI template" in Annex 3
• Column 4 is the list that appears in the "TREND template" in Annex 3.

Additional information

Comments on usage of code-table 4678:

• Use of GR or GS – we form from CBs so would expect these to always be prefixed by SH or TS.
• Use of PL with SH – WMO regard PL as forming from layer cloud, so should not be used with SH or TS (though the USA do use with SH/TS, but is a non conformance)
• FZ can only be used with RA, DZ and FG
• You cannot combine specifically dynamic and specifically convective precipitation in a single group (i.e. ICGR, ICGS, DZGR, DZGS, PLGR, PLGS are not meteorologically sound)
• You can only combine TS or SH with RA, SN, GS or GS – all other precipitation is solely from layer cloud
• Any precipitation in the vicinity should be reported as VCSH
• The following weather can be preceded by – or +: DZ, RA, SN, SG, PL, DS, SS, FZDZ, FZRA, FC, SHGR, SHGS, SHRA, SHSN, TSGR, TSGS, TSRA, TSSN (and combinations of ppn of course)
• The dominant precipitation shall be reported first.