SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather. Types There are three main types of internationally recognized SIGMETs per ICAO: Volcanic ash (VA or WV SIGMET) Tropical Cyclone (TC SIGMET) Other En-route weather phenomenon (WS SIGMET), which may consist of Thunderstorm types Turbulences types Mountain waves Icing/Sleet/Hail Dust or sand storms Radioactive Cloud 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. Convective SIGMETs For airmen in the U.S., there is an additional category of SIGMET known as a convective SIGMET. These are issued for convection over the coterminous U.S.. There are three types of convective SIGMETs: There are also a few special issuance convective SIGMETs to cover extreme weather more common in the U.S., including Tornados Hail greater than or equal to 3/4 inches in diameter Wind gusts greater than or equal to 50 knots Structure SIGMETs are internationally used by ICAO and WMOs using standardized abbreviations and are split into three lines: Header Summary Main Body In sum, a standardized SIGMET will have the following structure: TTAAii CCCC YYGGgg [BBB] CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC- CCCC <FIR/CTA Name> FIR <Phenomenon> OBS/FCST [AT GGggZ] <Location> <Level> [MOV XXX xx KT/KMH] [INTSF/WKN/NC] <Forecast time and forecast position>= Header The header line consists of the following characters TTAAii CCCC YYGGgg [CCx] Summary The first line of the broadcast is a summary line consisting of the following characters CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC- {| class="wikitable" |- ! Component !! Description |- | CCCC || The 4-character ICAO location of the affected area. |- | SIGMET || Indicates that this is a SIGMET broadcast. [a] |- | [n][n]n || A sequence number of the form 1, ''01, A01, etc. which is incremented for each time the SIGMET remains effective past 0001UTC or upon renewals. Helps indicate how long the SIGMET has been active. |- | VALID YYGGgg/YYGGgg || Indicates the period the SIGMET is active (WS SIGMETs can not be active for more than 4 hours), where YY is the