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The TAF tells you what an aerodrome is expected to do over the coming hours. Understanding its change groups is what separates reading a forecast from planning against one.
Monalisa DuttaIf the METAR describes the present, the Terminal Aerodrome Forecast describes the expected future. It is the document upon which fuel decisions, alternate selection and departure timing ultimately rest, and it rewards careful reading.
A TAF is a concise statement of the meteorological conditions expected at an aerodrome during a specified period. In India, TAFs are issued by the India Meteorological Department for designated aerodromes, generally with validity periods of nine, eighteen or twenty-four hours depending upon the station, and amended whenever conditions depart materially from those forecast.
Let us consider a representative example:
TAF VABB 191100Z 1912/2012 26012KT 6000 HZ SCT018
TEMPO 1914/1918 25018G30KT 2000 TSRA BKN012 FEW020CB
BECMG 2000/2003 27008KT 8000 NSWTAF VABB 191100Z 1912/2012 identifies the report type, the aerodrome — Chhatrapati Shivaji Maharaj International, Mumbai — and the time of issue, 1100 UTC on the 19th.
The final group is the validity period, and it is expressed differently from the issue time. 1912/2012 means the forecast is valid from 1200 UTC on the 19th until 1200 UTC on the 20th: a twenty-four hour period. Each half of the group gives a day and an hour.
26012KT 6000 HZ SCT018 describes the base conditions expected to prevail through the period unless a change group states otherwise: wind from 260 degrees at 12 knots, visibility 6,000 metres in haze, scattered cloud at 1,800 feet.
The encoding of wind, visibility, weather and cloud follows exactly the same conventions as the METAR, which is the principal reason I would encourage anyone new to these products to become thoroughly comfortable with the METAR first.
This is where a TAF becomes genuinely informative, and where misreading carries the greatest consequence.
TEMPO 1914/1918 25018G30KT 2000 TSRA BKN012 FEW020CB describes temporary fluctuations expected between 1400 and 1800 UTC on the 19th.
The formal definition matters. TEMPO indicates fluctuations which are each expected to last less than one hour, and which in aggregate are expected to cover less than half the stated period. It does not mean the condition will persist for those four hours; it means the condition may occur at any time within them, briefly, and repeatedly.
BECMG 2000/2003 27008KT 8000 NSW indicates a permanent change expected to occur gradually at some point between 0000 and 0300 UTC on the 20th, after which the new conditions prevail for the remainder of the forecast.
The distinction from TEMPO is fundamental. BECMG describes a transition to a new state; TEMPO describes fluctuation around an existing state. NSW denotes nil significant weather, indicating the thunderstorm activity is expected to have cleared.
A group beginning FM, such as FM201500, indicates a rapid and complete change occurring at the stated time, generally within less than one hour. Everything forecast before it is superseded entirely. Where BECMG describes a gradual transition, FM describes a distinct switch, most commonly associated with the passage of a front or a well-defined change in air mass.
These groups indicate a 30 or 40 per cent probability of the stated condition occurring. They are frequently used for thunderstorm activity in situations where development is plausible but by no means certain.
Probabilities above 50 per cent are not expressed this way; if a condition is more likely than not, it is stated directly in the prevailing conditions or in a BECMG group. A PROB30 TEMPO combination, which is occasionally seen, indicates a 30 per cent probability of temporary fluctuations of the stated kind.
Three limitations deserve to be understood clearly.
A TAF describes the aerodrome, not the region. The forecast applies to conditions within approximately eight kilometres of the aerodrome reference point. Weather ten kilometres away, on the approach path or along the departure track, is outside its scope. Route and area forecasts, significant weather charts and radar imagery serve that purpose.
A TAF is a forecast, and forecasts carry uncertainty. This is particularly true of convective weather during the Indian pre-monsoon and monsoon seasons, when thunderstorm development is often driven by local factors which no model resolves precisely. The forecaster is describing the most probable evolution, not a guaranteed one.
A TAF may be amended. An amended TAF, identified by TAF AMD, supersedes the previous issue entirely. I would recommend checking for amendments immediately before departure rather than relying upon a briefing taken some hours earlier.
Mumbai, valid 1200 UTC on the 19th to 1200 UTC on the 20th. Generally west-south-westerly at 12 knots with 6 kilometres visibility in haze and scattered cloud at 1,800 feet. Between 1400 and 1800 UTC, temporary thunderstorms with rain, wind increasing to 18 knots gusting 30, visibility falling to 2,000 metres, a broken ceiling at 1,200 feet and cumulonimbus present. Improving from around 0000 to 0300 UTC on the 20th to 8 kilometres visibility with no significant weather.
Read operationally, this is a forecast which identifies a clearly defined convective window during the afternoon and evening, with conditions improving overnight. An arrival planned into that window would warrant careful consideration of alternate fuel and holding capability. An arrival after midnight UTC would face a considerably more benign situation.
That is the value of the TAF. It does not merely describe weather; it identifies when the weather matters, which is precisely the information required for a sound plan.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →