TAF decoder, free, in plain English

report type

TAF

An aerodrome forecast.

A TAF forecasts the weather at one aerodrome for a stated span, usually 24 or 30 hours. It is a forecast, not an observation: the METAR tells you what the weather is, the TAF tells you what it is expected to be.

Exam trap. A TAF and a METAR use the same group format, so a paper can hand you either one and ask you to read it. Check the first word and look for a validity span before you answer.

station

VIDP

Delhi Indira Gandhi.

VIDP is the four letter ICAO location indicator of the aerodrome the forecast covers, Delhi Indira Gandhi. A TAF covers that aerodrome and roughly eight kilometres around it, and nothing beyond that.

Exam trap. A TAF is for one aerodrome. It says nothing about the weather en route, and a question that hands you a TAF and asks about the cruise is asking you to say so.

issue time

080500Z

Issued on the 8th at 05:00 UTC.

This is when the forecaster wrote the forecast, not when it starts. The forecast usually starts a little after it was issued, which is what the next group tells you.

Exam trap. The issue time is not the start of the validity. Reading the first time group as the start of the forecast puts every later timing answer out by an hour or more.

validity

0806/0912

Valid from the 8th at 0600 UTC to the 9th at 1200 UTC.

The forecast covers 30 hours: it begins on the 8th at 0600 UTC and ends on the 9th at 1200 UTC. Every change group later in the forecast sits inside this span. The pair before the slash is day then hour, and so is the pair after it.

Exam trap. Both halves are day and hour, never hour and minute. 0806/0912 is the 8th at 0600 to the 9th at 1200, not six minutes past eight.

BECMG 0810/0812

Becoming, between the 8th at 1000 UTC to the 8th at 1200 UTC

The weather changes gradually to this, at some point inside the span.

BECMG means the change happens progressively during the span and then holds. After the span has passed, the new conditions apply for the rest of the forecast or until the next change group. Only the elements written after BECMG change, and everything else carries on from before.

Wind from 120 at 8 knots. Visibility 5 km.

Exam trap. BECMG is a permanent change and TEMPO is not. Swapping the two forecasts a condition ending when it was going to persist. The span is also when the change happens, not when the new weather starts.

TEMPO 0818/0821

Temporarily, between the 8th at 1800 UTC to the 8th at 2100 UTC

The weather dips to this from time to time inside the span, then goes back.

TEMPO means temporary fluctuations, each lasting under an hour, and in total under half the span. Between the fluctuations the previous conditions apply. It is a swing, not a new state.

Visibility 2 km. Mist. Broken at 1,500 ft.

Exam trap. TEMPO fluctuations each last under one hour and together cover less than half the span. Whether an aerodrome is usable often turns on that, because a TEMPO below minima does not close it the way a BECMG below minima does.

PROB30 0821/0903

30% chance, between the 8th at 2100 UTC to the 9th at 0300 UTC

There is a 30% chance of these conditions inside the span.

PROB30 is the forecaster's stated confidence: a 30 in 100 chance of these conditions during the span. It is used for conditions the forecaster cannot commit to, most often thunderstorms and fog.

Visibility 500 m. Fog. Sky obscured, vertical visibility 200 ft.

Exam trap. Only PROB30 and PROB40 exist. PROB is also never written in front of BECMG, only on its own or in front of TEMPO.

FM090400

From the 9th at 04:00 UTC

Everything changes at 04:00 UTC on the 9th.

FM is a clean break. At that time the whole forecast is replaced by what follows, and every element of the previous period stops applying, including the ones this period does not mention. FM carries minutes as well as hours, which no other change group does.

Wind from 270 at 4 knots. Visibility 3 km. Mist. Scattered at 2,000 ft.

Exam trap. FM replaces the entire previous forecast, not only the groups it repeats. If the earlier period had a cloud layer and this one does not mention cloud, the cloud is gone, not carried over.