Airman’s Meteorological Advisory (AIRMET)

Definition

An AIRMET advises pilots of the occurrence of weather phenomena, which may affect the safety of aircraft operations and which were not already included in the graphic area forecast (GFA). It describes potentially hazardous weather conditions up to and including 24,000 ft (FL 240).

Issuance Criteria

AIRMETs are issued when the following criteria occur or are expected to occur and were not forecast in the graphic area forecast (GFA) and a significant meteorological information (SIGMET) message is not warranted:

Use of Letters

All eight flight information regions (FIRs) share 25 letters of the alphabet (T is used only for tests). The letter used cannot currently be in service in any other FIR. The letter Z will wrap back to A if necessary.

Use of Numbers

The following describes the rules when numbering an AIRMET:

• Numbering of an event begins at 1 (e.g. A1)
• The number is incremented by 1 when updating a message, including cancellation (e.g. A2)
• The numbering is reset at 0000Z

Validity

The period of validity of an AIRMET is 4 hours and it may be issued up to 4 hours prior to the start of the validity period (i.e. expected time of occurrence of the phenomenon).

Description of Location

The location of the phenomenon is depicted as an area using coordinate points. The description always begins with the abbreviation WTN (within) and the area can be described as a circle, a line or a polygon.

Movement

Direction of movement is given with reference to one of the 16 points of compass (radials) and speed is given in knots. The abbreviation QS (quasi stationary) is used if no significant movement is expected.

Intensity

The expected evolution of a phenomenon’s intensity is indicated by one of the following abbreviations:

• INTSFYG Intensifying
• WKNG Weakening
• NC No change

Updates

An updated AIRMET, when issued, automatically replaces the previous AIRMET in the same series (i.e. the previous AIRMET with the same letter). An AIRMET must be updated every 4 hours

Cancellation

An AIRMET must be cancelled when, during its validity period:

• The phenomenon for which the AIRMET had been issued is no longer occurring or no longer expected to occur
• The phenomenon for which the AIRMET had been issued strengthens such that significant meteorological information (SIGMET) is now required
• The new issue of the graphic area forecast (GFA) has been transmitted and now includes the phenomenon.

An AIRMET does not cancel itself automatically at the end of its validity period. A cancellation AIRMET with the abbreviation CNCL must be issued.