How to Read a TAF: Change Groups, Windows and Probability
People who can read a METAR often stall on a TAF, and the reason is a category error: they read it as a sequence of future METARs. It is not. A TAF describes a baseline and then the ways that baseline is expected to change, and every group after the first is an amendment to what came before it.
Once you read the change groups as edits rather than as separate observations, the format becomes straightforward — and considerably more informative, because it tells you not just what is expected but how confident the forecaster is.
What a TAF is and is not#
A TAF is a forecast of conditions in the vicinity of a single aerodrome, generally within about five nautical miles, for a defined validity period — usually 24 or 30 hours, issued four times a day. It includes only weather expected to be operationally significant. That is why a TAF can look sparse next to a METAR: everything not mentioned is expected to stay as described in the prevailing conditions.
- Covers one aerodrome and its immediate vicinity, not a region.
- Has an explicit validity window; outside it the forecast says nothing at all.
- Mentions only significant weather — the absence of a group is information.
- Is amended whenever the forecast changes materially, and the amendment replaces the original entirely.
An amended TAF carries AMD. When one appears, discard your reading of the previous issue completely rather than merging them.
The header and the baseline#
The first line sets the scene: station, issue time, validity period, then the prevailing conditions at the start. EGLL 121103Z 1212/1318 24012KT 9999 SCT030 means Heathrow, issued at 11:03 on the 12th, valid from 12:00 on the 12th to 18:00 on the 13th, initially wind from 240 at 12 knots, visibility 10 km or more, scattered cloud at 3,000 feet.
| Element | Example | Reads as |
|---|---|---|
| Station and issue | EGLL 121103Z | Heathrow, issued 11:03 UTC on the 12th |
| Validity | 1212/1318 | From 12:00 on the 12th to 18:00 on the 13th |
| Prevailing wind | 24012KT | From 240° at 12 knots |
| Prevailing visibility | 9999 | 10 km or more |
| Prevailing cloud | SCT030 | Scattered at 3,000 ft |
The validity period is the group people skip and then misread the whole forecast. A TAF outside its window is history.
The change groups, and what each one commits to#
| Group | Meaning | How to plan against it |
|---|---|---|
| BECMG 1418/1420 | A permanent change occurring gradually within that window | Assume the new conditions from the end of the window onward |
| TEMPO 1500/1504 | Fluctuations lasting under an hour each, less than half the period | Plan for it happening; do not plan on it persisting |
| PROB30 1600/1620 | 30 per cent probability of the stated conditions | Significant enough to carry fuel for, not enough to rely on |
| PROB40 TEMPO | 40 per cent chance of temporary conditions | The most cautious group in common use |
| FM121500 | From that time, a rapid and complete change | Everything before it is superseded at that minute |
BECMG and FM are permanent; TEMPO and PROB are not. Mixing those two families up is the most common misreading of a TAF, and it is the one that leaves an aircraft short of fuel.
Probability, honestly read#
PROB30 and PROB40 exist because forecasters are being explicit about uncertainty, and no higher value is used — a 50 per cent or greater expectation is stated as BECMG or TEMPO instead. That makes PROB groups unusually informative: their presence means the forecaster considered the scenario likely enough to write down but was not willing to commit to it.
- PROB30 TSRA on an afternoon TAF is the standard summer convective hedge.
- PROB40 0400 FG on a clear autumn evening should be read as a serious fog warning.
- No PROB group does not mean no risk, only that nothing crossed the threshold for one.
- Two competing scenarios sometimes appear as a BECMG plus a PROB — read them as the forecaster showing their working.
Reading one for a real decision#
- Check the validity window covers your arrival time, plus the holding you would realistically do.
- Read the baseline. This is what you get if nothing else in the TAF happens.
- Apply the permanent groups in order — FM and BECMG — to build the expected conditions at your time.
- Layer the TEMPO and PROB groups on top as things that may happen at your time, and check whether the worst of them is still workable.
- Compare the result against the minimum for the approach you expect, and against your alternate’s own TAF.
- Re-check for an AMD before departure. Amendments cluster exactly when conditions are changing.
Step six is the one that catches people. A TAF read two hours ago may have been replaced twice since, and amendments are issued precisely when the situation is deteriorating.
Why TAFs are wrong, and in which direction#
A TAF is a forecast and inherits every limitation of one. Frontal timing is often out by a couple of hours even inside a day. Radiation fog is notoriously hard to time and easy to under-forecast in strength. Convection can be forecast as a probability for a region but not placed on a specific aerodrome a day ahead. Forecasters know all of this, which is why TAFs in uncertain situations widen rather than sharpen — a vague-looking TAF is usually an honest one.
Where a TAF is systematically conservative, that is deliberate. The cost of an under-forecast approach minimum is much higher than the cost of carrying extra fuel.
Frequently asked questions
What is the difference between a TAF and a METAR?
A METAR is an observation of what is happening at an aerodrome now; a TAF is a forecast for that aerodrome over a stated period, usually 24 or 30 hours. A METAR describes the whole picture each time it is issued. A TAF describes a baseline and then the changes expected to it, in change groups, and mentions only weather significant for operations. Reading a TAF as a list of future METARs is the most common mistake and leads directly to misjudging fuel.
What do BECMG, TEMPO and PROB30 mean in a TAF?
BECMG marks a permanent change happening gradually within the stated window — after it, assume the new conditions. TEMPO marks fluctuations that last under an hour at a time and cover less than half the period; plan for them occurring but not persisting. PROB30 and PROB40 give the probability of the stated conditions, and no higher figure is used, because anything more likely is written as BECMG or TEMPO. The permanent group family and the temporary one are planned against completely differently.
How far ahead is a TAF reliable?
It varies by phenomenon rather than by hours. Wind and general cloud trends hold up reasonably across the whole validity period. Frontal timing is often uncertain by several hours even within 24. Radiation fog is hard to time and easy to under-forecast. Individual thunderstorm placement is not knowable a day ahead at all, which is why convection appears as PROB groups and why radar in the last hours matters more than yesterday’s forecast. Always check for an amended TAF before you rely on one.
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