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, or severe or greater icing over a 3,000-square-mile (7,800 km2) area or IMC conditions 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. A SIGMET is a forecast valid for up to four hours. They are assigned an alphabetic designator from N (November) through Y (Yankee), excluding S and T. 
A Convective SIGMET is issued for convection over the Continental U.S. Convective SIGMETs are issued for an area of thunderstorms affecting an area of 3,000 square miles (7,800 km2) or greater, a line of thunderstorms at least 60 nm long, and/or severe or embedded thunderstorms affecting any area that are expected to last 30 minutes or longer. A Convective SIGMET is valid for 2 hours and they are issued every hour + 55 min. 
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