A TAF is the forecast of an aerodrome. It is written with the groups of a METAR — wind, visibility, weather, clouds — but cut up in time: base conditions, then periods where they change. Reading it is knowing what applies at the hour of your flight.
The header and the validity
TAF EIDW 101100Z 1012/1112: the type, the aerodrome's indicator, the time of issue — the 10th at 11:00 UTC — then the validity period, from the 10th at 12:00 to the 11th at 12:00. Twenty-four hours here; thirty at major aerodromes, nine or twelve at some small ones.
TAF AMD is an amended forecast, which replaces the previous one; TAF COR a correction; CNL after the validity cancels the forecast.
TAF EIDW 101100Z 1012/1112 17008KT 9999 SCT030 TEMPO 1012/1018 4000 -SHRA BKN014 BECMG 1018/1021 32010KT PROB30 TEMPO 1100/1106 3000 BR
The base conditions
Right after the validity come the conditions forecast at the start of the period: 17008KT 9999 SCT030 — wind from 170 at eight knots, visibility ten kilometres or more, three to four oktas at three thousand feet. No temperature, no QNH: a TAF forecasts neither.
These groups read exactly like those of a METAR. What is specific to the TAF is what follows: the changes.
The changes: BECMG, TEMPO, FM, PROB
BECMG 1018/1021 32010KT: a gradual and lasting change, settling in between 18:00 and 21:00 — at 21:00 the wind is from 320 at ten knots, and stays so until the end of the validity or the next change.
TEMPO 1012/1018 4000 -SHRA BKN014: temporary fluctuations between 12:00 and 18:00 — each lasting less than an hour, and in total less than half of the period. In between, the base conditions return.
FM101800 32010KT 9999 SCT030: a sharp and complete change from the 10th at 18:00 — everything that follows replaces everything before. PROB30 or PROB40 before a period gives its probability, thirty or forty per cent, alone or with TEMPO: PROB30 TEMPO 1100/1106 3000 BR.
The timeline
On an aerodrome's page and in the decoder, the site draws these periods on a timeline: one bar per change group, laid over the hours it covers, and a bar you touch points to its sentence in the list. You see at a glance what overlaps — a TEMPO above a BECMG — and what applies at a given hour.
At the hour of the flight, read the base conditions or the last FM first, then what a BECMG changed in them, then what a TEMPO or a PROB may add at times.
What a TAF does not say
No QNH, no temperature, no exact hour for a thunderstorm: a TEMPO says it may come within the period, not when. It is a forecast: the METAR next to it says what is observed — and when the two diverge, the observation is right.
An issued TAF only holds until the next one or its amendment: an aerodrome's page shows the age of its own beside its title.