Sigmet

A SIGMET (Significant Meteorological Information) is a weather advisory issued by a Meteorological Watch Office to warn pilots and operators of significant weather phenomena that may affect the safety of all aircraft operations. SIGMETs are more severe than AIRMETs and cover hazards that threaten both VFR and IFR flights.

SIGMETs are issued for:

• Severe turbulence (not associated with convection)
• Severe icing (not associated with convection)
• Volcanic ash clouds — one of the most critical SIGMET types, as volcanic ash can destroy jet engines
• Tropical cyclones
• Widespread dust storms or sandstorms reducing visibility below 5 km
• Radioactive cloud (following nuclear incidents)

SIGMETs are valid for up to 4 hours (6 hours for volcanic ash and tropical cyclones). They include the location, extent, altitude range, movement, and expected intensity change of the hazard.