A SIGMET is short for “significant metrological information”. It’s significant weather information, which applies to the safety of all aircraft. SIGMET’s consists of two different kinds; convective and non-convective.
Convective SIGMET’s are issued when there is convection over the continental United States. This SIGMET is issued anytime there is convective activity such as thunderstorms covering an area of more then 3000 square miles. Convective SIGMET’s are also issued when there is a line of thunderstorms at least 60 nautical miles long called a squall line, or when there are severe thunderstorms affecting a particular area longer then 30 minutes.
Non-Convective SIGMET’s are less serious but can be safety hazards for smaller aircrafts. The aircraft being affected will depend on the type of the non-convective SIGMET being issued. In order for a non-convective SIGMET to be issued there must be severe or greater icing, turbulence, or instrument conditions due to dust covering a 3000 square mile area provided its not caused by thunderstorms or the presence of volcanic ash. In Hawaii or Alaska SIGMET’s are also issued for Tornadoes, squall lines, embedded thunderstorms, or hail greater then ¾ inches.
SIGMET’s are identified by one letter ranging from “N” to “Y” excluding the letters “S” and “T”. The letters “S” and “T” are reserved for AIRMET’s. No same to weather phenomena across the country can have the same two letter identifiers at the same time so different letters are reserved for different phenomena. A SIGMET is unscheduled and can be updated or corrected as necessary. SIGMET’s are issued for a period of 3 hours but can be updated or corrected as deemed necessary.
Last but still serious is the International SIGMET. These differ in the amount of time they last fore. An International SIGMET for volcanic ash will last up to 12 hours, for hurricanes up to 6 hours, and for all other metrological events requiring a SIGMET up to 4 hours. These are designated with one letter from “A” to “M”. They are issued for the same criteria of a convective or non-convective SIGMET.