SIGMET

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 km) area, severe or greater icing over a 3,000-square-mile (7,800 km) area or IMC over a 3,000-square-mile (7,800 km) area due to dust, sand, or volcanic ash.