SIGMETs are inflight advisories concerning nonconvective weather that is potentially hazardous to all aircraft.
SIGMETs report weather forecasts that include severe icing not associated with thunderstorms, severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms, dust storms or sandstorms that lower surface or inflight visibilities to below three miles, and volcanic ash. SIGMETs are unscheduled forecasts that are valid for 4 hours unless the SIGMET relates to a hurricane, in which case it is valid for 6 hours.
FAA AC 00-45H Aviation Weather Services pg. 5-16
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 13-12