Significant Meteorological Message

warning of severe weather conditions (active thunderstorms, hail, severe turbulence, icing etc.) issued my Met offices.

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 3000 square mile area, or severe or greater icing over a 3000 sq mile area or IFR conditions over a 3000 sq mile area due to dust, sand, or volcanic ash http://www.aviationweather.gov/exp/product_overlay/help/p-sigmets.html.