Significant Meteorological Information

DefinitionNaming CriteriaValidityDescription of LocationDescription of Movement and IntensityUpdates and CancellationsSample Report

Definition

Message disseminated to advise pilots of the occurrence or expected occurrence of specified weather phenomena, which may affect the safety of aircraft operations.

Issuance Criteria

A SIGMET is issued in response to the following criteria:

Frequent thunderstorms
Squall Lines
Severe turbulence
Severe icing
Severe mountain wave

Low level wind shear
Heavy sand/dust storm
Radioactive cloud
Volcanic Ash
Tropical Cyclone

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 a SIGMET:

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 a WS SIGMET is 4 hours and it may be issued up to 4 hours prior to the commencement of the phenomenon in the corresponding flight information region (FIR). There is an exception for volcanic ash and tropical storm SIGMETs which are valid for 6 hours and may be issued up to 12 hours before they enter the corresponding FIR.

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 significant meteorological information (SIGMET) message, when issued, automatically replaces the previous SIGMET in the same series (i.e. the previous SIGMET with the same letter). A typical SIGMET must be updated every 4 hours, whereas a SIGMET relating to volcanic activity or tropical storms must be updated every 6 hours

Cancellation

If, during the validity period of a SIGMET message, the phenomenon for which the SIGMET had been issued is no longer occurring or no longer expected to occur, this SIGMET should be cancelled. A cancellation SIGMET will be issued and will include the abbreviation CNCL.