METAR and TAF Decoder: Live Reports in Plain English

Type an airport's ICAO code to get its latest METAR and TAF, or paste any report, and every group is explained in plain English: wind, visibility, RVR, weather, cloud, temperature, pressure, trend and the FM, BECMG, TEMPO and PROB change groups of a TAF. ICAO and US formats, US remarks included, with the flight category of each period. For a whole flight, the Flight Briefing writes up the weather, runways, SIGMETs and terrain.

The decoder

Live data: the Aviation Weather Center of the US National Weather Service, through v1prep's server, at most 5 minutes old. Code: ICAO Annex 3 and WMO-No. 306; FAA AIM 7-1-28 and 7-1-29 for the US key and differences.

How to read a METAR

A METAR is the routine observation for an aerodrome, issued every hour or every half hour; a SPECI is the same report issued in between when conditions change significantly. The groups always come in the same order, and a group that does not apply is simply left out.

The wind is the mean of the last 10 minutes. A gust is reported when the maximum exceeds the mean by 10 kt or more, and two extreme directions appear when the direction varies by 60° or more. The older runway state group (for example R24/451293) was dropped from the ICAO METAR with the Global Reporting Format for runway surface conditions in 2021; the decoder still reads it in older reports. For the full picture of the report, see the Library articles on the METAR and SPECI and on visibility and RVR.

How to read a TAF

A TAF gives the forecast for the aerodrome over its validity period, which is up to 30 hours (in the US, 24 or 30 hours). The header gives the station, the issue time and the period: 2918/3024 is from the 29th at 18:00 to the 30th at 24:00 UTC. The base forecast follows in the same groups as a METAR, then the change groups.

In a METAR, a trend uses the same words with times of day only: BECMG FM0700 TL0800 5000 BR means becoming, from 07:00 until 08:00 UTC, 5 km in mist. The Library article on TAFs and trend forecasts covers how to use them in planning.

Weather codes

Fog (FG) is reported when the visibility is below 1,000 m, mist (BR) from 1,000 to 5,000 m; in the US the fog limit is 5/8 statute mile. The Library explains how fog forms.

Cloud, ceiling and flight categories

Cloud amount is given in eighths of the sky, oktas: FEW 1 to 2, SCT 3 to 4, BKN 5 to 7 and OVC 8. Heights are above the aerodrome in hundreds of feet. METARs name only two cloud types, CB and TCU; TAFs only CB. The ceiling is the lowest BKN or OVC layer, or the vertical visibility in an obscured sky.

• CAVOK: visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no CB or TCU, and no significant weather.
• NSC: nil significant cloud, none below 5,000 ft or the highest minimum sector altitude and no CB or TCU, when CAVOK does not apply.
• NCD: no cloud detected, from an automatic station.
• SKC and CLR (US): sky clear, reported by an observer; CLR from an automatic station, meaning no cloud below 12,000 ft.

These are the US categories of the FAA and the National Weather Service; the decoder applies them to ICAO reports too, converting metres to statute miles, as a quick guide. They are not the legal VMC minima: those depend on the airspace and are covered in VFR, IFR and meteorological conditions and cloud cover and ceiling.

US and ICAO differences

• Units: the US reports visibility in statute miles, RVR in feet and the altimeter setting in inches of mercury (A2992); ICAO reports use metres and hectopascals (Q1013). Temperatures are in degrees Celsius everywhere.
• RVR: reported in the US when the prevailing visibility is less than one mile or the RVR is 6,000 ft or less; in ICAO reports when the visibility or the RVR is below 1,500 m.
• No cloud: the US uses SKC and CLR; ICAO reports use CAVOK, NSC and NCD.
• Trends and TAFs: US METARs carry no trend forecast. National Weather Service TAFs use no BECMG groups and no temperature forecasts, and no PROB in the first 9 hours; they use FM groups instead, and add non-convective low-level wind shear (WS).
• Remarks: US METARs end with RMK and coded remarks: AO1 or AO2 (automatic station without or with a precipitation discriminator), SLP (sea-level pressure in hPa and tenths), T (temperature and dew point to the tenth), PK WND (peak wind), and $ (station needs maintenance).

Frequently asked questions

What is the difference between a METAR and a TAF? A METAR is an observation: the weather at the aerodrome at the time in the report. A TAF is a forecast for the aerodrome, usually valid for 9 to 30 hours, with change groups (FM, BECMG, TEMPO, PROB) for what is expected to change and when. A SPECI is a METAR issued between the routine reports because conditions changed significantly.

How often are METARs issued? ICAO requires a routine METAR every hour, or every half hour where regional agreement says so, as at many European aerodromes. United States stations report hourly. A SPECI is issued in between when conditions cross set thresholds.

Are METAR and TAF winds true or magnetic? True. Wind direction in a METAR, SPECI or TAF is in degrees true. The wind given by ATIS or by the tower is magnetic, because runways are numbered by magnetic direction.

What does CAVOK mean? Ceiling and visibility OK: visibility 10 km or more, no cloud below 5,000 ft or below the highest minimum sector altitude, whichever is higher, no cumulonimbus or towering cumulus, and no weather of significance to aviation. It replaces the visibility, weather and cloud groups.

What is a ceiling? The height of the lowest broken (BKN) or overcast (OVC) layer, or the vertical visibility (VV) when the sky is obscured. FEW and SCT layers do not make a ceiling. Cloud heights in METARs and TAFs are above the aerodrome, in hundreds of feet.

What do FM, BECMG, TEMPO and PROB mean in a TAF? FM (from) starts a new set of conditions at the time given, replacing everything before it. BECMG (becoming) means a change during the period given, after which the new conditions hold. TEMPO means temporary fluctuations, each lasting less than 1 hour and together less than half the period. PROB30 or PROB40 gives a 30% or 40% probability, alone or with TEMPO.

What are VFR, MVFR, IFR and LIFR? US flight categories used by the FAA and the National Weather Service. VFR: ceiling above 3,000 ft and visibility above 5 statute miles. MVFR: ceiling 1,000 to 3,000 ft and/or visibility 3 to 5 miles. IFR: ceiling 500 to below 1,000 ft and/or visibility 1 to below 3 miles. LIFR: ceiling below 500 ft and/or visibility below 1 mile. The decoder applies them to ICAO reports too, converting metres to statute miles, as a quick guide.

Is there a briefing for a whole flight? Yes: the v1prep Flight Briefing writes one for a departure time and two airports, with the METAR and TAF at each end for the times you are there, the runway most likely in use, SIGMETs along the route, elevations and terrain. It is €4.99 a month, or included with v1prep PRO, Lifetime and the Interview Pass.

Do I need an account? Not at first: the decoder can be used 3 times without an account. After that it asks for a v1prep account, which is free and takes a minute to create.

Can I use the decoder in my own app or website? Yes. The decoder is open source under the MIT licence: github.com/lebbars/metar-taf-decoder. It is one JavaScript file with no dependencies and no network access, for the browser and for Node, with its tests.

Where does the live data come from, and can I use it for flight planning? The live METAR and TAF come from the Aviation Weather Center of the US National Weather Service (aviationweather.gov), which publishes reports from aerodromes worldwide, and are at most 5 minutes old on v1prep. Use them to learn and for a quick look, not for flight planning: get your weather from an official briefing source, your operator or the aeronautical information service.