How to read a TAF
A TAF - terminal aerodrome forecast - is a forecast for one aerodrome over the next day or so, not the weather now. Paste one below and it is decoded period by period; the guide under it explains each group in the words of the code's own rulebooks.
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Decode a TAF
Decoded in your browser - the forecast you paste is never sent anywhere, and it works with no connection. For an airport's current forecast, look it up on the METAR and TAF page.
Forecast - TAF EGLL
SampleThis is a forecast for future periods, not the weather now. Each period below carries its own forecast category, scoped to the times shown with it.
Valid from day 12 18:00Z to day 13 24:00Z.Issued day 12 at 16:58 UTC.
- BASEFrom the start of the forecast until day 13 02:00ZForecast: MVFR
The conditions expected from the start of the validity period until the first FM group changes them.
- Wind from 210 degrees at 14 kt.
- Visibility 6000 metres.
- Weather: light rain.
- Broken clouds at 1200 ft AGL.
- Ceiling 1200 ft AGL. Forecast flight category for this period: MVFR.
- BECMGday 12 20:00Z to day 12 22:00ZForecast: MVFR
A GRADUAL change: the stated values are reached at some point inside this window and then persist. Elements not restated carry on unchanged.
- Wind from 240 degrees at 10 kt.
- Visibility 10 km or more.
- NSW: significant weather is forecast to end.
- Broken clouds at 2500 ft AGL.
- Ceiling 2500 ft AGL. Forecast flight category for this period: MVFR.
- FMFrom day 13 02:00Z until day 13 24:00ZForecast: VFR
SUPERSEDES everything before it. From this time the whole forecast is the line below - wind, visibility, weather and cloud are all restated.
- Wind from 270 degrees at 8 kt.
- Visibility 10 km or more.
- Scattered clouds at 3500 ft AGL.
- No ceiling forecast. Forecast flight category for this period: VFR.
- TEMPOday 13 02:00Z to day 13 08:00ZForecast: IFR
INTERMITTENT fluctuations, each lasting under an hour and together less than half this window. The prevailing forecast still applies the rest of the time.
- Wind from 270 degrees at 8 kt (unchanged from earlier in this forecast).
- Visibility 3000 metres.
- Weather: mist.
- Broken clouds at 600 ft AGL.
- Ceiling 600 ft AGL. Forecast flight category for this period: IFR.
- PROB30 TEMPOday 13 06:00Z to day 13 10:00ZForecast: LIFR
30 percent probability. A PROBABILITY that TEMPORARY fluctuations to these conditions occur in this window. Two hedges stacked: it may not happen, and if it does it is intermittent.
- Wind from 270 degrees at 8 kt (unchanged from earlier in this forecast).
- Visibility 800 metres.
- Weather: fog.
- Vertical visibility 200 ft (sky obscured).
- Ceiling 200 ft AGL. Forecast flight category for this period: LIFR.
- BECMGday 13 12:00Z to day 13 14:00ZForecast: VFR
A GRADUAL change: the stated values are reached at some point inside this window and then persist. Elements not restated carry on unchanged.
- Wind from 270 degrees at 8 kt (unchanged from earlier in this forecast).
- CAVOK: visibility 10 km or more, no significant cloud or weather.
- No ceiling forecast. Forecast flight category for this period: VFR.
Forecast temperatures: max 16 degC on day 13 at 14:00Z, min 9 degC on day 13 at 06:00Z.
The sample, as coded
TAF EGLL 121658Z 1218/1324 21014KT 6000 -RA BKN012 TX16/1314Z TN09/1306Z BECMG 1220/1222 24010KT 9999 NSW BKN025 FM130200 27008KT 9999 SCT035 TEMPO 1302/1308 3000 BR BKN006 PROB30 TEMPO 1306/1310 0800 FG VV002 BECMG 1312/1314 CAVOK
The header and the validity period
TAF EGLL 121658Z 1218/1324 reads: a TAF for London Heathrow (an ICAO location indicator), issued on the 12th of the month at 16:58 UTC, valid from 18:00 UTC on the 12th to midnight at the end of the 13th. A TAF codes the day of the month, never the month itself. TAF AMD is an amended forecast, TAF COR a corrected one; NIL means no forecast was issued and CNL that it was cancelled.
If the valid period's end time is midnight UTC, it is annotated as 24. For example, a 00Z TAF issued on the 9th of the month would have a valid period of 0900/0924.
The DDHH/DDHH validity has been the international form since 5 November 2008 (ICAO Annex 3 Amendment 74). A scheduled US TAF covers 24 or 30 hours (NWSI 10-813, Appendix B2.9). After the validity comes the forecast for its start, which must carry at least the elements the code requires:
The description of forecast conditions shall contain at least information about wind, visibility, weather and cloud or vertical visibility.
FM - from: a new forecast from this time
FM130200 means from the 13th at 02:00 UTC. It starts a new, self-contained forecast: wind, visibility and cloud are given again (weather too, when any is expected), and nothing coded before it applies any more.
When the group FMYYGGgg is used, all forecast conditions given before the group FMYYGGgg are superseded by the conditions indicated after the group.
The FM change indicator group (FMYYGGgg) is used to indicate when prevailing conditions are expected to change significantly over a period of less than one hour.
BECMG - becoming: a change at some time in the window
BECMG 1220/1222: at some time between 20:00 and 22:00 UTC on the 12th the conditions change to those given, and then persist. Anything the group does not restate carries on unchanged.
The change groups TTTTT YYGG/YeYeGeGe in the form of BECMG YYGG/YeYeGeGe shall indicate a change in forecast meteorological conditions expected to occur at either a regular or irregular rate at an unspecified time within the period YYGG to YeYeGeGe. The duration of the period YYGG to YeYeGeGe shall normally not exceed two hours and in any case shall not exceed four hours.
Because the change can come at any moment in the window, the decoder's timeline shows the worse of the old and the new conditions until the window ends. US TAFs do not use BECMG (NWSI 10-813, Appendix E2).
TEMPO - temporarily: short spells, then back
TEMPO 1302/1308 3000 BR BKN006: between 02:00 and 08:00 UTC on the 13th, spells of 3,000 m visibility in mist under a broken layer at 600 ft - each under an hour, and together less than half of those six hours. The rest of the time, the forecast before it applies.
The change groups TTTTT YYGG/YeYeGeGe in the form of TEMPO YYGG/YeYeGeGe shall indicate frequent or infrequent temporary fluctuations in forecast meteorological conditions which are expected to last less than one hour in each instance and, in the aggregate cover, less than half of the period indicated by YYGG/YeYeGeGe.
US TAFs add that a TEMPO condition has a “high percentage (greater than 50%) probability of occurrence” and that “TEMPO groups do not exceed four hours” (NWSI 10-813, Appendix B2.9.3). In Australia, “TEMPO is used for periods of 30 minutes or more but less than 60 minutes” (Bureau of Meteorology) - shorter spells are INTER.
PROB30 and PROB40 - a probability, not a forecast
PROB30 TEMPO 1306/1310 0800 FG VV002: a 30 percent probability of temporary spells of fog, 800 m, sky obscured with vertical visibility 200 ft, between 06:00 and 10:00 UTC on the 13th. The probability is stated as coded: ICAO gives it “in tens of per cent” (Annex 3, Appendix 5, 1.4), so PROB30 is 30 percent - not a 30 to 39 percent band, which no ICAO text gives.
For C2C2, only the values 30 and 40 shall be used to indicate the probabilities 30 and 40%, respectively. [...] A probability of less than 30% of actual values deviating from those forecast is not considered to justify the use of the group PROB. When the possibility of an alternative value is 50% or more, this should be indicated by the use of BECMG, TEMPO or FM as appropriate.
A probability can qualify a TEMPO (PROB30 TEMPO, regulation 51.9.2) but never a BECMG or an FM (51.9.3). In the US, “PROB30 is the only PROB group used in NWS TAFs”, and it is not combined with TEMPO (NWSI 10-813, Appendix B2.9.4).
INTER - intermittent (Australia)
INTER 1220/1302 4000 SHRA BKN012: between 20:00 UTC on the 12th and 02:00 UTC on the 13th, spells of 4,000 m in showers of rain under a broken layer at 1,200 ft, each lasting less than 30 minutes - shorter than an Australian TEMPO.
The terms TEMPO and INTER are used to indicate significant temporary or intermittent variations from the prevailing conditions previously given in the TAF. [...] INTER is used for periods less than 30 minutes.
INTER is not one of WMO's FM 51 change groups, so expect it in Australian TAFs, not elsewhere.
WS - low-level wind shear (US)
WS015/23045KT: non-convective wind shear from the surface up to 1,500 ft above the ground, at the top of which the wind is from 230 degrees true at 45 kt.
Forecasts of Non-Convective LLWS in the TAF refer only to non-convective LLWS from the surface up to 2,000 ft AGL. LLWS is always assumed to be present in convective activity. [...] After the solidus (/), the five-digit wind group is the wind direction and speed at the top of the wind shear layer. It is not a value for the amount of shear.
It is an NWS group, absent from WMO's FM 51 code form, and once coded it stays in force until the next FM group or the end of the TAF (Appendix B2.8).
TX and TN - the forecast maximum and minimum temperature
TX16/1314Z TN09/1306Z: a maximum of 16 °C on the 13th at 14:00 UTC and a minimum of 9 °C on the 13th at 06:00 UTC. An M in front of the figure means minus.
To indicate forecast maximum and minimum temperatures expected to occur at the time indicated by YFYFGFGFZ, the letter indicator TX for the maximum forecast temperature and TN for the minimum forecast temperature shall precede TFTF without a space.
They are used by regional air navigation agreement (WMO FM 51, note 3), and NWS offices are not required to include them (NWSI 10-813, Appendix E2).
NSW - the weather is forecast to end
BECMG 1220/1222 24010KT 9999 NSW BKN025: the rain coded earlier stops some time in that window. Without NSW, weather coded before a change group carries on.
To indicate the end of significant weather phenomena w'w', the abbreviation NSW (Nil Significant Weather) shall replace the group w'w'.
NWSI 10-813 spells it “No Significant Weather”; the meaning is the same.
CAVOK - ceiling and visibility OK
One word in place of the visibility, weather and cloud groups. It does not mean cloudless, and it does not cover wind.
When it is expected that the following conditions will apply simultaneously, the code word CAVOK shall be included in place of the groups VVVV, w'w' and NsNsNshshshs or VVhshshs: (a) Visibility: 10 km or more; (b) No cloud below 1 500 metres (5 000 ft) or below the highest minimum sector altitude, whichever is greater, and no cumulonimbus and no towering cumulus; (c) No significant weather phenomena (see Code table 4678).
US TAFs do not use CAVOK (NWSI 10-813, Appendix E2). In the decoder, a CAVOK period is forecast VFR: visibility of 10 km or more is above 5 statute miles, and no cloud below 5,000 ft leaves no ceiling at or below 3,000 ft.
Planning with a TAF
- Every value is the most probable one, not a promise.
the specific value of any of the elements given in a forecast shall be understood by the recipient to be the most probable value which the element is likely to assume during the period of the forecast.
WMO-No. 306, FM 51-XV TAF, note 2 - Plan on the worst line, including the TEMPO and PROB groups.
Forecasters should remember the lowest meteorological condition contained in a TAF, regardless of any conditional language, including those forecasted in the PROB or TEMPO groups drive user operational decisions.
NWS Instruction 10-813, Appendix B2.9 - There is no single category for a whole TAF. The decoder gives each period its own forecast VFR, MVFR, IFR or LIFR, from the same FAA AIM 7-1-7 thresholds as the METAR badge, labelled as a forecast and tied to that period's times. A period it cannot classify is shown as unknown, never as fine.
- A decode is not a briefing. Get the current TAF and an official briefing before you fly; a decoded forecast here is a reading aid.
What the checkride and the theory exam ask
Reading a TAF is examined on both sides of the Atlantic. Practising it here is all KeepBlueUp claims - nothing on this site is approved by the FAA, EASA or any other authority.
- FAA-S-ACS-6C, PA.I.C.K2c - Acceptable weather products and resources required for preflight planning, current and forecast weather for departure, en route, and arrival phases of flight such as: “Terminal Aerodrome Forecasts (TAF)”
- EASA PPL theory syllabus (AMC1 FCL.210; FCL.215): 3. Meteorology - Aviation weather messages
Sources
- WMO-No. 306, Manual on Codes, Volume I.1, FM 51-XV TAF (2011 edition, updated 2013, in the copy hosted by the University of British Columbia), regulations 51.1 to 51.10 - read 12 October 2026. The current 2019 edition was not read.
- NWS Instruction 10-813, Terminal Aerodrome Forecasts (30 October 2024), Appendices A, B2.3, B2.8, B2.9 and E2 - read 12 October 2026.
- Bureau of Meteorology, Aerodrome forecast (TAF and TAF3) (updated 24 January 2024) - read 12 October 2026.
- ICAO Annex 3, Meteorological Service for International Air Navigation, 20th edition (July 2018), Appendix 5, in an unofficial copy, and the Hong Kong Observatory's note on Amendment 74 (the 5 November 2008 TAF format change) - read 7 October 2026.
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