SIGMET, or Significant Meteorological Information, is a 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). 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.
A Convective SIGMET is issued for convection over the Continental U.S. Convective SIGMETs are issued for an area of embedded thunderstorms, a line of thunderstorms, thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3000 square miles, and severe surface weather including surface winds greater than or equal to 50 knots, hail at the surface greater than or equal to 3/4 inches in diameter, and tornadoes. Severe thunderstorms are characterized by tornado(s), hail 3/4 inches or greater, or wind gusts 50 knots or greater. A Convective SIGMET is valid for 2 hours and they are issued every hour + 55 min.