How to use it
- Paste a TAF into Terminal forecast (TAF), or keep the sample that is already there.
- Read Validity: the period the forecast covers, in UTC.
- Go through the Timeline of the periods: one row for each period, with the wind, visibility, weather and cloud in force then.
- Where a value says carried over, the period does not state it and it comes from the period before. Open Decoded group by group to see each group as it was sent.
How a TAF is built
A TAF gives the expected weather at an airport for 24 or 30 hours, from the issue time and the validity period (AIM 7-1-28). The first group after the validity gives the wind, the visibility and the sky. Weather is included only when it is significant to aviation. Each change of conditions is a new period, starting with one of the change words.
FM gives a new set of conditions from a day, hour and minute on. BECMG gives a gradual change within a period, TEMPO a change that lasts less than an hour and less than half of the period, and PROB30 or PROB40 a probability of the conditions.
- Visibility is in metres (four digits; 9999 is 10 km or more) or in statute miles, where P6SM is more than 6 statute miles and M1/4SM is less than a quarter of one.
- The sky is an amount, a height in hundreds of feet and an optional type: BKN015 is broken cloud at 1,500 ft. The wind is a direction and a speed; 00000KT is calm, and gusts are shown after G.
How a period carries over the one before
AIM 7-1-28 says: with the exception of an FM group, the new time period includes only those elements which are expected to change; the forecast wind remains the same as in the previous period. The decoder follows that. A BASE or FM group gives the whole forecast. A BECMG group changes what it states and the rest carries on. A TEMPO or PROB group changes its own period only, and the period after it is what it was before.
- Elements that carry over are marked in the timeline, so you can see which values the period states and which it takes from before.
- A CAVOK in a group gives 10 km or more visibility, no cloud below 5,000 ft and no significant weather.
What this decoder does not cover
- Forecast times and temperature groups (TX and TN), which are shown as not decoded.
- The flight category. A TAF period is not decided here; use the minima of your operation and the official briefing.
- Amended and corrected forecasts beyond showing the AMD or COR flag, and the codes of other countries that differ in small details.
- This decoder is for training and planning practice, not for real flight. Use the official weather briefing for a real flight.
Worked example: the KORD forecast of the AIM, three of its periods
The base forecast is valid from day 5, 12:00 UTC to day 6, 18:00 UTC: 5 SM with mist and a broken cloud base at 3,000 ft. The TEMPO from 13 to 16 UTC is 1 1/2 SM, and it takes the cloud from the base, which the table marks as carried over.
The PROB40 from 00 to 06 UTC on day 6 is a 40 percent chance of thunderstorms with moderate rain, with cumulonimbus at 800 ft and the wind of the 23 UTC group carried over.
| Period | Visibility | Weather | Cloud |
|---|---|---|---|
| Base forecast | 5 SM (8,047 m) | mist | broken (5-7/8) at 3,000 ft |
| Temporary (TEMPO) | 1 1/2 SM (2,414 m) | mist | broken (5-7/8) at 3,000 ft (carried over) |
| Probability 40% (PROB40) | 2 SM (3,219 m) | thunderstorm with moderate rain | overcast (8/8) at 800 ft, cumulonimbus |
Frequently asked questions
What is a TAF?
A TAF is a terminal aerodrome forecast: a concise statement of the expected weather at an airport over a period of usually 24 hours, with changes shown as periods in the same code as the METAR reports.
What is the difference between BECMG, TEMPO and FM?
FM starts a new set of conditions at a fixed time and replaces everything. BECMG is a gradual change that stays. TEMPO is a temporary change of less than an hour and less than half of the period, and the conditions return after it.
What does PROB40 mean?
PROB40 is the probability, 40 percent, that the conditions in its group occur during its period. It does not change the conditions after it, so the period after it is what it was before.
Why does a period say carried over?
The period does not state that element, so it takes the value from the period before, as AIM 7-1-28 describes. The decoder marks it so that you can see what the period states itself.
Can I use the decoder to plan a real flight?
No. It is for training and planning practice, not for real flight. Use the official weather briefing of your authority and the aircraft's manual.
Sources
Figures and text last checked on Tuesday, November 10, 2026.