How to Read a TAF

What Is a TAF?#

A TAF (Terminal Aerodrome Forecast) is a coded aviation weather forecast describing expected meteorological conditions within a 5‑statute‑mile radius of an airport, typically valid for 24 or 30 hours. This guide is part of Aviatopia's Aviation Weather Explained series.

Where a METAR reports observed weather, a TAF forecasts how wind, visibility, weather phenomena, and cloud cover are expected to evolve over time. It is issued in UTC (“Z” time) and used by pilots, dispatchers, and air traffic services for operational decision‑making.

If you are not already familiar with surface weather observations, review our guide on How to Read a METAR before decoding forecasts.

Why It Matters in Aviation#

A TAF directly affects:

• Alternate airport requirements under IFR
• Fuel planning and contingency fuel
• Approach selection and minima planning
• Crosswind limitations
• Diversion and delay risk assessment

Under instrument flight rules (IFR), forecast ceiling and visibility at the estimated time of arrival determine whether an alternate is legally required. Airlines incorporate TAFs into dispatch releases, and general aviation pilots rely on them for cross‑country risk assessment.

A forecast deterioration—even before it is observed—can materially change the safety and legality of a flight.

For broader context on weather systems and operational interpretation, see Aviation Weather Explained.

How It Works#

A TAF is structured in chronological forecast segments. Each change group modifies or replaces the previous forecast.

Basic Structure Example#

TAF KJFK 151130Z 1512/1618 18012KT P6SM SCT020 BKN050
     FM151800 20015G25KT P6SM BKN035
     TEMPO 1520/1524 3SM TSRA BKN020CB
     FM160200 24010KT 5SM -SHRA OVC015

1. Header Line#

TAF KJFK 151130Z 1512/1618

The validity period always uses UTC.

2. Prevailing Conditions Block#

18012KT P6SM SCT020 BKN050

Wind direction in TAFs is referenced to true north, not magnetic.

Cloud heights are always reported in feet above ground level (AGL).

A ceiling is defined as the lowest BKN (broken) or OVC (overcast) layer. In this example, the ceiling is 5,000 ft AGL.

3. Change Groups#

Forecast changes are expressed through structured modifiers.

FM (From)#

FM151800 20015G25KT P6SM BKN035

FM indicates a rapid and complete change beginning at the specified time. All previous conditions are replaced.

From 1800 UTC:

• Wind shifts to 200° at 15 knots, gusting 25
• Broken layer at 3,500 ft AGL

BECMG (Becoming)#

Indicates a gradual transition occurring over a specified time window. Conditions change progressively rather than abruptly.

TEMPO (Temporary Fluctuations)#

TEMPO 1520/1524 3SM TSRA BKN020CB

TEMPO indicates temporary conditions expected for less than half of the specified period.

From 2000–2400 UTC:

• Visibility may drop to 3 SM
• Thunderstorms with rain (TSRA)
• Broken cumulonimbus at 2,000 ft AGL

Outside that window, prevailing conditions apply.

PROB Groups#

Additional Common Codes#

Wind shear groups are operationally critical, particularly during approach and departure phases.

Operational Example#

A pilot plans arrival at 2130 UTC.

Forecast includes:

• Prevailing VFR conditions
• TEMPO 2000–2400: 3 SM visibility, thunderstorms, BKN020CB

Because the temporary thunderstorm window overlaps the estimated time of arrival, operational considerations include:

• Potential holding or deviations
• Alternate fuel planning
• Monitoring convective SIGMETs
• Preparing for an instrument approach

Even when prevailing conditions remain VFR, temporary forecast deterioration can influence legal and safety margins.

Under IFR planning in the United States, if forecast conditions at ETA are below 2,000 ft ceiling or 3 SM visibility (the “1‑2‑3 rule”), an alternate airport is generally required.

Step-by-Step Breakdown#

Common Misconceptions#

1. A TAF predicts exact conditions at a specific minute. It provides forecast windows, not minute‑by‑minute precision.

2. TEMPO conditions can be ignored if brief. Temporary does not mean operationally insignificant.

3. Scattered clouds define a ceiling. Only BKN or OVC layers constitute a ceiling.

4. Wind direction is magnetic. Forecast wind direction is referenced to true north.

5. Forecast visibility guarantees legality. Observed weather at arrival determines final regulatory compliance.

Frequently Asked Questions#

Key Takeaways#

• A TAF is a time‑sequenced forecast for conditions within 5 SM of an airport.
• Validity periods and change groups must be read chronologically.
• FM replaces conditions; BECMG transitions gradually; TEMPO introduces temporary fluctuations.
• Ceiling is defined only by BKN or OVC layers and reported in feet AGL.
• Wind direction is referenced to true north.
• Forecast conditions influence alternate requirements and fuel planning.
• Observed weather ultimately governs regulatory compliance.

Sources & References#

• FAA Advisory Circular AC 00-45 — Aviation Weather Services — Official guidance on decoding TAF format, change groups, and forecast conventions.
• ICAO Annex 3 — Meteorological Service for International Air Navigation — International standards governing TAF issuance and validity.
• FAA Aeronautical Information Manual (AIM), Chapter 7 — Weather services, reports, and forecasts used in flight operations.

Related Guides#

• How to Read a METAR
• Density Altitude Explained
• Flight Delays & Cancellations Explained

Browse Directories#

• METAR & TAF Codes — Full reference of TAF change indicators, sky condition codes, and weather-phenomena abbreviations.

More in Aviation Weather#

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