This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
SIGMET
Significant Meteorological Information
A SIGMET is an authoritative weather advisory issued by a Meteorological Watch Office (MWO) to warn flight crews and dispatchers of significant meteorological conditions that may affect the safety of aircraft operations. SIGMETs cover phenomena including severe or extreme turbulence, severe icing not associated with thunderstorms, widespread dust or sand storms lowering visibility below three miles, and volcanic ash or radioactive material from a nuclear event. A specific subcategory - the Convective SIGMET, issued in the United States - covers thunderstorm activity, hail, and tornadoes.
SIGMETs are valid for up to four hours for most hazards, though volcanic ash and tropical cyclone SIGMETs can remain valid for up to six hours. They define the affected airspace by latitude and longitude or by reference to known fixes, specify the altitude band involved, and indicate the movement and expected change of the hazard. Flight dispatchers and crew use SIGMETs during pre-flight planning and in-flight decision-making to plan avoidance routes or request re-routes from ATC. SIGMETs are distinct from AIRMETs, which cover less severe conditions typically affecting lighter aircraft.