SIGMET & AIRMET
A SIGMET and an AIRMET are in-flight hazard warnings issued by a meteorological watch office when the weather turns worse than the GFA showed. The split is severity.
- SIGMET carries the severe hazards: frequent or squall-line thunderstorms, severe turbulence or icing, severe mountain wave, low-level wind shear, volcanic ash, tropical cyclone, heavy dust or sandstorm, radioactive cloud.
- AIRMET carries less-severe weather not already shown in the GFA: surface wind over 30 kt, surface visibility under 3 SM, broken or overcast cloud based under 1,000 ft AGL, isolated or occasional thunderstorms or towering cumulus, and moderate turbulence, icing, or mountain wave.
- Both are valid 4 hours (volcanic ash and tropical cyclone SIGMETs run 6 hours), and both amend the GFA.
Trap: severe goes in a SIGMET, moderate in an AIRMET. A thunderstorm SIGMET already implies its icing and turbulence, so no separate SIGMET is issued for those.
Source note: the TC AIM defines the split by severity (and by AIRMET covering weather not already in the GFA). The idea that a SIGMET warns all aircraft while an AIRMET is aimed at lighter aircraft is ICAO convention, not stated in the TC AIM.
References
- TC AIM MET 5.0 (AIRMET)
- TC AIM MET 6.0 (SIGMET)