The reporting is based on the following World Meteorological Organization (WMO) and National Weather Service (NWS) classification:
- The SYNOP code is based the WMO code from table 4680 (WaWa, Present Weather reported from an automatic weather station).
- The METAR code is according to the WMO code table 4678 (METAR) format.
| Precipitation / obscuration type | SYNOP 1 | METAR | NWS |
|---|---|---|---|
| Clear | 00 | − | C |
| Haze or smoke, or dust in suspension in the air, visibility ≥ 1 km (3280 ft) | 04 | HZ | − |
| Haze or smoke, or dust in suspension in the air, visibility < 1 km (3280 ft) | 05 | DU | − |
| Mist | 10 | BR | − |
| Code figures 20 to 25 are used, if precipitation or fog was observed during the preceding hour but not at the time of observation 2 | |||
| Fog | 20 | − | − |
| PRECIPITATION | 21 | REUP | − |
| Drizzle (not freezing) or snow grains | 22 | REDZ | − |
| Rain (not freezing) | 23 | RERA | − |
| Snow | 24 | RESN | − |
| Freezing rain or freezing drizzle | 25 | REFZRA | − |
| REFZDZ | − | ||
| The following code figures are used if precipitation or fog is observed at the time of observation 3 | |||
| FOG | 30 | FG | − |
| Fog or ice fog, in patches | 31 | BCFG | − |
| Fog or ice fog, has become thinner during the past hour | 32 | FG | − |
| Fog or ice fog, no appreciable change during the past hour | 33 | FG | − |
| Fog or ice fog, has begun or become thicker during the past hour | 34 | FG | − |
| Fog, depositing rime (freezing fog) | 35 | FZFG | − |
| PRECIPITATION | 40 | UP | P |
| Precipitation, slight or moderate | 41 | −UP | P− |
| Precipitation, heavy | 42 | +UP | P+ |
| DRIZZLE | 50 | DZ | L |
| Drizzle, not freezing, slight | 51 | −DZ | L− |
| Drizzle, not freezing, moderate | 52 | DZ | L |
| Drizzle, not freezing, heavy | 53 | +DZ | L+ |
| Drizzle, freezing, slight | 54 | −FZDZ | ZL− |
| Drizzle, freezing, moderate | 55 | FZDZ | ZL |
| Drizzle, freezing, heavy | 56 | +FZDZ | ZL+ |
| Drizzle and rain, slight | 57 | −DZRA | RL− |
| Drizzle and rain, moderate or heavy | 58 | DZRA | RL |
| +DZRA | RL+ | ||
| RAIN | 60 | RA | R |
| Rain, slight | 61 | −RA | R− |
| Rain, moderate | 62 | RA | R |
| Rain, heavy | 63 | +RA | R+ |
| Rain, freezing, slight | 64 | −FZRA | ZR− |
| Rain, freezing, moderate | 65 | FZRA | ZR |
| Rain, freezing, heavy | 66 | +FZRA | ZR+ |
| Rain (or drizzle) and snow, slight 4 | 67 |
−RASN −FZRASN |
RS− |
| Rain (or drizzle) and snow, moderate or heavy 4 | 68 |
RASN FZRASN |
RS |
|
+RASN +FZRASN |
RS+ | ||
| SNOW | 70 | SN | S |
| Snow, slight | 71 | −SN | S− |
| Snow, moderate | 72 | SN | S |
| Snow, heavy | 73 | +SN | S+ |
| Ice pellets, slight | 74 | −PL | IP− |
| Ice pellets, moderate | 75 | PL | IP |
| Ice pellets, heavy | 76 | +PL | IP+ |
| Snow grains | 77 | −SG | SG− |
| SG | SG | ||
| +SG | SG+ | ||
| Ice crystals | 78 | − | IC− |
| IC | |||
| IC+ | |||
| SHOWERS OR INTERMITTENT PRECIPITATION 5 | 80 | UP | − |
| Intermittent rain, slight | 81 | −RA | − |
| Intermittent rain, moderate | 82 | RA | − |
| Intermittent rain, heavy | 83 | +RA | − |
| Intermittent rain, violent (> 32 mm/h) (> 1.26 in/h) | 84 | − | |
| Intermittent snow, slight | 85 | −SN | − |
| Intermittent snow, moderate | 86 | SN | − |
| Intermittent snow, heavy | 87 | +SN | − |
| Snow pellets, according to WMO 4677 | 88 |
−SHGS SHGS +SHGS |
−SP SP +SP |
| Hail | 89 | SHGR | A |
When using humidity information, the visibility and humidity affect the METAR code responses as follows:
SYNOP codes 80–87 indicate intermittent precipitation but not showers. They are reported as 15 min and 60 min codes, not as instant code.
SYNOP codes 88–89 indicate showers. They are also reported as instant code.
RA and SN is never reported as shower in METAR code. GS and GR is always reported as shower in METAR code.
The method used to determine
whether the visibility reduction is caused by moisture or dry particles depends on the
sensor configuration:
When using humidity, the visibility and humidity affect the METAR code responses as follows:
When using forward scatter measurements, it is used to determine obscuration type instead of humidity. The visibility limits are the same as shown in the table above. |
The observation history data for reporting SYNOP, METAR or NWS codes differ from each other. This may appear as a difference in reporting onset and cessation of a precipitation, especially in case of intermittent or showery events.
The following table shows the observation history used for reporting the weather codes. For example: the instant SYNOP code for RAIN is reported based on 1 minute of data, but the instant SYNOP code for FOG is based on 10 minutes of data. For METAR reporting the periods are 15 minutes and 10 minutes respectively.
| Weather reporting | Precipitation observation data history | Visibility observation data history |
|---|---|---|
| SYNOP code (Instant) | 1 min | 10 min |
| SYNOP code (15 min) | 15 min | 10 min 6 / 60 min 7 |
| SYNOP code (60 min) | 60 min | 10 min 6 / 60 min 7 |
| METAR code (Instant) | 15 min | 10 min |
| METAR code (Recent) | 60 min | - |
| NWS code | 1 min | - |