How to read a TAF

A TAF is a coded forecast of the weather expected at an aerodrome over a fixed period, and reading one well is what turns a pile of numbers into a plan for your flight.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

Forecast, not observation

A TAF (Terminal Aerodrome Forecast) is a prediction, where a METAR is a measurement. That single distinction shapes everything about how a TAF is written. Because the future is uncertain, the code includes ways to say "this will change", "this might change", and "expect this for short spells", which a METAR never needs.

Like the METAR, the TAF is an international code form defined in the WMO Manual on Codes (WMO No. 306) under the meteorological service framework of ICAO Annex 3. A TAF describes the weather expected close to the aerodrome, not across the whole region. The FAA AIM makes a US TAF a station forecast for about a 5 statute mile radius of the centre of the runway complex; the wider 5 to 10 statute mile band (roughly 8 km) is the AIM's "in the vicinity" (VC) range, used for phenomena reported near rather than at the aerodrome. Either way, the TAF is a vicinity forecast, so do not read it as an area or en route product.

A worked example

TAF EGLL 181100Z 1812/1918 24012KT 9999 BKN025 FM181500 27015G25KT SCT030 TEMPO 1818/1822 6000 -RA BKN012 PROB30 1900/1904 3000 BR

Reading it through:

• TAF is the report type, EGLL the aerodrome (Heathrow), and 181100Z the issue time: the 18th at 1100 UTC.
• 1812/1918 is the validity period: valid from the 18th at 1200 UTC until the 19th at 1800 UTC, a 30-hour TAF. Each part is a day-and-hour pair, so read both halves carefully across midnight and month ends.
• 24012KT 9999 BKN025 is the prevailing forecast at the start: wind from 240 degrees at 12 knots, visibility 10 km or more, broken cloud at 2500 ft.
• FM181500 27015G25KT SCT030 is a FM (from) group: from the 18th at 1500 UTC the wind veers to 270 degrees at 15 knots gusting 25, and cloud becomes scattered at 3000 ft.
• TEMPO 1818/1822 6000 -RA BKN012 says that, temporarily between the 18th at 1800 and 2200 UTC, expect visibility down to 6000 m in light rain with broken cloud at 1200 ft.
• PROB30 1900/1904 3000 BR gives a 30 per cent probability of visibility 3000 m in mist between the 19th at 0000 and 0400 UTC.

The units follow the same regional conventions as the METAR: metres and hectopascals in the ICAO form, statute miles and inches of mercury in the US form.

Try it yourself below. Paste a TAF into the decoder and it explains each group, including the validity period and the FM, BECMG, TEMPO and PROB change groups. It only explains the text you enter; it does not fetch live weather.

The change groups

The grammar of a TAF lives in its change groups, defined by the same WMO and ICAO code form:

• FM (from) marks a rapid and more or less complete change at a stated time, written as FMDDHHMM. Everything after it replaces the previous conditions until the next change group.
• BECMG (becoming) marks a gradual change expected to take place over the stated period, with the new conditions reached by the end of that window and then continuing.
• TEMPO (temporary) marks temporary fluctuations expected to come and go. Per the WMO and US NWS conventions, each instance lasts less than about an hour and the fluctuations in total cover less than half the stated period; in between, conditions return to the prevailing forecast.
• PROB30 and PROB40 give a 30 or 40 per cent probability of the conditions that follow. Only those two values are used: a higher likelihood is written as the main forecast, and a lower one is not considered worth a group.

How long a TAF is valid

Routine TAFs are commonly valid for 24 or 30 hours and are issued at fixed times, as the FAA AIM and Met Office both describe, though some aerodromes issue shorter ones. When a forecast is updated before its normal time, an amended TAF is issued and marked AMD; a corrected one is marked COR. Both supersede the version before them, so always brief from the latest issue.

Common pitfalls

• TEMPO and PROB are not invitations to ignore the bad weather. Plan for the worse case inside the window, especially around your planned arrival and any alternate.
• A TAF is vicinity only. Pair it with the latest METAR for current conditions and with area or significant-weather products for the route.
• Mind the validity boundaries. A flight that lands just after the TAF expires is not covered by it.

In Pilot EFB

Pilot EFB shows the decoded TAF alongside the raw text, with the raw forecast always kept so you can trace each change group back to the original code. A forecast you have already pulled stays readable offline, because your device holds what you have saved; fetching a fresh or amended TAF needs a connection. Pilot EFB is offline-first, not a certified Electronic Flight Bag, so cross-check the official forecast before you commit to a plan.