SIGnificant METeorological Information
Weather advisory that contains meteorological information concerning the safety of all aircraft. There are two types of SIGMETs: convective and non-convective.
The criteria for a non-convective SIGMET to be issued are severe or greater turbulence over a 3,000-square-mile (7,800 km2) area, severe or greater icing over a 3,000-square-mile (7,800 km2) area or IMC over a 3,000-square-mile (7,800 km2) area due to dust, sand, or volcanic ash.
This information is usually broadcast on the ATIS at ATC facilities, as well as over VOLMET stations. They are assigned an alphabetic designator from N through Y (excluding S and T).[2] SIGMETs are issued as needed, and are valid up to four hours. SIGMETS for hurricanes and volcanic ash outside the CONUS are valid up to six hours.
Source: SIGMET – Wikipedia
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