SIGMET & AIRMET heading & identifier line

WMO heading opens a SIGMET or AIRMET message, then the identifier line, then the phenomenon line. Heading order: two-letter prefix + number, originating office, date/time group.

Prefix = message type (MET 5.15, 6.15):

Number = format. Each FIR has a fixed number pair (MET 5.15, 6.15):

Identifier line (MET 5.16, 6.16): <FIR> <AIRMET/SIGMET> <letter><number> VALID <start>/<end> <office>

• e.g. CZEG AIRMET H1 VALID 251315/251715 CWEG

VALID <start>/<end>:

• Two DDHHMM groups (day, hour, minute UTC): start, then end.
• 251315 = the 25th at 1315Z.
• Z = UTC, never local time.

Letter:

• Identifies the event.
• Never changes through updates or cancellation.
• Not in service in another FIR until retired 24 h.
• Not reused for a widely separated recurrence of same phenomenon in one FIR.
• AIRMET and SIGMET alphabets separate: same letter can be active in both at once, never in two SIGMETs.

Number:

• Starts at 1 for a new event (e.g. B1).
• Rises by 1 on every update, including the cancellation message.
• Resets to 1 at 0000Z each day (count of messages for that event since 0000Z).
• 0000Z alone triggers no update.
• e.g. SIGMET A4 updated after next 0000Z becomes A1: same event, same letter.

Update: replaces previous message with same letter (MET 5.12, 6.12).

Cancellation and test messages:

• CNL and CNCL not interchangeable.

Trap: letter never changes on update, only number does; ICAO number (01 to 07) on a national-format message, which uses 21 to 27.

Practice