Definition
The specific time window during which a Terminal Aerodrome Forecast (TAF) applies. It is shown on the TAF as a six-digit group giving the start day and hour, then the end day and hour, in UTC (Zulu) time. Routine TAFs typically have a valid period of 24 or 30 hours, beginning one hour after the forecast is issued.
Plain English
The block of time the forecast is good for. The TAF tells you what weather is expected at the airport between a stated start time and end time — outside that window, the forecast does not apply.
Context Anchor
Seen in TAF weather forecasts when checking whether the forecast covers your planned departure, arrival, or alternate airport time.
Derivation
From Latin validus, meaning 'strong' or 'effective.' A 'valid period' is the stretch of time during which the forecast is in force and can be relied upon for planning.
Why Pilots Care
Pilots must confirm their arrival time falls inside the valid period; an expired forecast can produce unsafe assumptions about weather.
Intuition Check
Do not read valid as guaranteed correct. Here, valid means applicable during the stated time window; the forecast can still be amended or replaced if conditions change.
Example Sentence 1
Before departure, she checked that her arrival time at the destination fell within the TAF's valid period.
Example Sentence 2
The valid period for this TAF runs from 0600Z today until 0600Z tomorrow.