What is the difference between BECMG and TEMPO?

BECMG marks a gradual, permanent change - by the end of the change window the new conditions are expected to persist. TEMPO marks a temporary fluctuation that lasts less than 60 minutes and occurs for less than half the forecast period.

Why does the TAF validity period use day numbers instead of dates?

TAFs are issued relative to the current UTC month. The day number refers to the day of the current UTC month, so 1612/1712 means from day 16 at 1200 UTC to day 17 at 1200 UTC.

What does 9999 mean for visibility?

9999 is the ICAO code for 10 km or more visibility - effectively unlimited for practical purposes. It does not mean exactly 9999 metres.

Weather Decoder

TAF Decoder

Paste any raw TAF to decode forecast periods, wind, visibility, clouds, and change groups into plain English.

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TAF = Terminal Aerodrome Forecast. Valid up to 30 hours. BECMG = permanent change, TEMPO = temporary fluctuation, PROB = probability percentage.

How to Use & Reference

About

Parses raw TAF (Terminal Aerodrome Forecast) strings into readable forecast periods. A TAF covers up to 30 hours at a single airport and is the standard pre-flight weather planning document for IFR operations worldwide.

How to Use

1Paste or type a raw TAF string into the text area - it must start with the word TAF.
2Use the Load Example button to see a sample EGLL TAF with multiple change groups.
3Read the base forecast card first, then step through each change group in chronological order.
4Check BECMG groups for permanent condition changes and TEMPO groups for temporary fluctuations.

Tips

BECMG indicates a gradual permanent change over the stated period - conditions will be as shown by the end of that window.
TEMPO means conditions may temporarily fluctuate for less than half the forecast period and less than 60 minutes at a time.
PROB30 or PROB40 indicates a 30% or 40% probability of the stated conditions occurring.
FM (from) groups replace all previous conditions from the stated time - treat them as a new base forecast.
CAVOK means ceiling and visibility OK: visibility 10 km or more, no clouds below 5000 ft, and no significant weather.

FAQ

What is the difference between BECMG and TEMPO?: BECMG marks a gradual, permanent change - by the end of the change window the new conditions are expected to persist. TEMPO marks a temporary fluctuation that lasts less than 60 minutes and occurs for less than half the forecast period.
Why does the TAF validity period use day numbers instead of dates?: TAFs are issued relative to the current UTC month. The day number refers to the day of the current UTC month, so 1612/1712 means from day 16 at 1200 UTC to day 17 at 1200 UTC.
What does 9999 mean for visibility?: 9999 is the ICAO code for 10 km or more visibility - effectively unlimited for practical purposes. It does not mean exactly 9999 metres.