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 convert|3000|sqmi|km2|sing=on area, or severe or greater icing over a convert|3000|sqmi|km2|sing=on area or IMC conditions over a convert|3000|sqmi|km2|sing=on area due to dust, sand, or volcanic ash [http://www.aviationweather.gov/exp/product_overlay/help/p-sigmets.html] .

This information is usually broadcast on the ATIS at ATC facilities. 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. [http://www.faa.gov/ATpubs/AIM/Chap7/aim0701.html#7-1-6]

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 convert|3000|sqmi|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. [http://aviationweather.gov/exp/product_overlay/help/p-csigmets.html]

ee also

*AIRMET
*PIREP