How to read a TAF and decide whether you'll fly

It's Friday evening, you've got the aircraft booked for tomorrow morning, and you'd like to know whether it's worth setting the alarm. The METAR can't tell you. It only reports what the weather was doing a few minutes ago. For tomorrow you need the TAF (terminal aerodrome forecast), the forecaster's view of the weather at one aerodrome for a set period of up to 30 hours.

A TAF looks like a longer, scarier METAR, and mostly it's the same codes. What's new is four change groups (FM, BECMG, TEMPO and PROB). Once you've got those, the rest reads like the METARs you already know.

What a TAF covers#

A TAF is a coded forecast of wind, visibility, weather and cloud at one aerodrome for a set period. The Met Office describes its TAFs as 24 to 30 hour forecasts issued four times a day, and it also publishes 9-hour TAFs for some aerodromes.

Elsewhere the rules are much the same. In Australia, the Bureau of Meteorology's TAF covers the aerodrome and the area within 8 kilometres of the aerodrome reference point, and New Zealand's CAA gives the same 8 km radius. In Ireland, Met Éireann issues the TAFs for the main airports, and the Irish AIP applies ICAO Annex 3 (the international standard behind the code) with no listed differences for TAFs.

Before you start, fix three things in your head. They catch people out more than any code does.

• Every time is UTC. Convert to local time before you compare it with your booking.
• Cloud heights are above the aerodrome, not above sea level, in UK, Irish, Australian and New Zealand TAFs.
• Visibility is in metres. 9999 means 10 kilometres or more.

The wind, visibility, weather and cloud groups use the same codes as a METAR, and our guide on how to read a METAR covers them. So the only genuinely new parts are the header and the change groups.

A worked example from Prestwick#

The Met Office prints this example TAF for Glasgow Prestwick in its GetMet booklet. We'll use it all the way through:

EGPK 110500Z 1106/1212 13010KT 9000 BKN010
BECMG 1106/1108 BKN018
PROB30 TEMPO 1108/1116 17025G40KT 4000 TSRA BKN012CB
BECMG 1118/1121 3000 BR NSC=

In plain words, that's a grey start with the cloud lifting a bit by breakfast, a chance of thunderstorms through the middle of the day, and mist settling in during the evening.

The first line after the validity period is the starting forecast. Every line after that is a change group, and each kind means something different.

The four change groups#

FM: from a set time#

FM followed by a day, hour and minute (FM030800 is 0800 UTC on the 3rd) is a clean break. The Bureau of Meteorology calls it the start of a self-contained forecast. Whatever follows FM replaces everything before it and holds until the end of the TAF or the next FM or BECMG. So don't read it as a tweak to the old forecast. Read it as a fresh one.

BECMG: becoming, some time in the window#

BECMG means conditions move to a new, lasting state at some point inside the window. New Zealand's CAA describes it as a change "at a regular, or irregular, rate, and at an unspecified time within a period". That really does mean any time. In the Prestwick example, BKN018 could turn up at 0601 or at 0759. Once the window closes, the new conditions are the forecast.

In the UK example, BECMG lists only what changes. The wind and the 9 km visibility from the first line carry on as they were, and the Met Office decode describes the group as a change in "some or all of the elements". Australia does it differently. There the Bureau gives wind, visibility, weather and cloud after every FM or BECMG, unless the group is CAVOK or forecasts fog.

You'll also meet NSW (no significant weather) in a BECMG or TEMPO group. It means the weather forecast before it, such as rain, mist or thunderstorms, is expected to stop. The Met Office's own decode example has BR (mist) on the first line and BECMG 0412/0415 9999 NSW after it, so the mist clears. The flip side matters too. In a UK TAF, where BECMG and TEMPO list only what changes, a weather code with no NSW after it carries on.

TEMPO: temporary spells within the window#

TEMPO marks spells that come and go inside the window while the main forecast still stands. New Zealand's CAA defines them as fluctuations lasting "less than one hour in each instance", infrequent enough that the prevailing conditions remain those originally forecast. The WMO's international TAF code (FM 51) adds that the spells together cover less than half the period.

This is the bit people most often read too kindly. A TEMPO is a forecast that the spells will happen. The forecaster just can't say exactly when.

Australia splits this into two codes. The Bureau of Meteorology uses TEMPO for spells of 30 minutes or more but less than 60 minutes, and INTER for spells of less than 30 minutes. An Australian TAF can carry both, as in this Bureau example for Albury (remarks, temperature and QNH lines left out):

TAF YMAY 022230Z 0300/0312 35010KT CAVOK
FM030800 31018KT 9999 SHRA BKN025 OVC100
INTER 0308/0312 31020G40KT 3000 +TSRA BKN010 SCT040CB

Albury starts the day looking lovely. CAVOK on the first line means visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no cumulonimbus or towering cumulus and no weather significant to aviation. Then from 0800 UTC the forecast changes to showers under broken cloud at 2,500 ft. And until 1200 UTC there are intermittent thunderstorms of less than 30 minutes at a time, bringing gusts to 40 knots and 3,000 m in heavy rain. We wouldn't plan a cross-country into that.

PROB30 and PROB40: a stated chance#

PROB gives the probability that the conditions after it happen in the window. The Met Office uses only 30% and 40%. The Bureau of Meteorology doesn't give probabilities under 30% and drops the PROB once a probability reaches 50%. The Bureau also makes a point we'd underline. PROB is the chance of the weather occurring, not a measure of how confident the forecaster feels.

PROB can stand alone or sit in front of TEMPO (or INTER in Australia). PROB30 TEMPO, as at Prestwick, means a 30% chance of temporary spells. A PROB on its own, as in this New Zealand CAA example, is a chance that the conditions set in:

PROB30 1117/1121 0500 FG

That's a 30% chance of 500 m visibility in fog between 1700 and 2100 UTC on the 11th. In Australia, the Bureau uses PROB only for thunderstorms or for poor visibility from fog, mist, dust, smoke or sand, and a PROB with no INTER or TEMPO means the conditions are expected to last an hour or more.

Using the TAF to decide whether to fly#

Back to Prestwick. Say you're booked on a local flight on the 11th, 1000 to 1200 UTC, and you're reading the TAF over a cup of tea the night before. Take it one group at a time against your booking.

The base and the first BECMG. By 0800 at the latest, cloud is forecast at broken 1,800 ft with 9 km visibility and a 10-knot wind. Whether 1,800 ft is enough depends on the flight you planned and your own minima. On its own, though, it's a workable forecast for the circuit or the local area. So far, so good.

The PROB30 TEMPO. Here's the problem. It runs from 0800 to 1600, so it covers the whole booking. In those spells you'd get thunderstorms, cumulonimbus at 1,200 ft, 4,000 m and gusts to 40 knots from 170 degrees. The chance is only 30%. But if it comes off, that's a light single in the circuit under a cumulonimbus with 40-knot gusts. For most private pilots, and almost every training flight, that's a no-go. At best, plan to check the latest TAF, METAR and radar at 0930, and honestly, expect to cancel.

The second BECMG. It doesn't touch your booking, but it matters if you'd planned a late finish. From some time between 1800 and 2100 the forecast is 3,000 m in mist. If you were due back at 1830, plan on the mist being there.

The same approach works on any TAF:

Read the TAF for the whole time you might be in the air, from departure to well after your planned arrival. Flights run late (the refuel always takes longer than you think). For a trip away, the flight planner puts the departure and arrival TAFs side by side, so you can read both for the same hours.

And check the issue time before you rely on any of it. TAF AMD means the forecast has been amended since it was first issued, so always brief from the latest one.

When your airfield has no TAF#

Most small aerodromes and farm strips don't issue one, which is a shame, because that's where a lot of us fly. Read the TAFs for the nearest aerodromes that do, ideally on both sides of you and upwind, along with the area forecast your met service publishes for the route. Bear in mind a TAF only covers the aerodrome it's written for. The weather between two good TAFs can still close a route.

See the forecast on a timeline#

Type an ICAO code into the METAR and TAF decoder and it shows the raw TAF, each period in plain English, and a timeline coloured by the US flight categories (VFR, MVFR, IFR and LIFR). The prevailing conditions sit above and the TEMPO and PROB periods below, so you can see at a glance which hours drop to IFR. An Australian INTER shows on the timeline as TEMPO.

For a trip away, print the flight planner as your briefing. It puts the latest METAR and TAF for your departure and arrival and the wind on every runway on one page.

Holding aircraft for weather in Volanti#

A METAR tells a club about bad weather once it's here. A TAF gives you warning, and that's worth using. In Volanti, an administrator can hold an aircraft for a set period with weather recorded as the reason, so nobody books into the thunderstorm risk. When a booking is cancelled for weather, the aircraft or instructor can be held for everyone rather than only for the member who cancelled. And when the hold is released, the waiting list runs again on its own. See how in booking and scheduling.