A SIGMET is short for "significant metrological information." It’s a significant weather information, which applies to the safety of all aircraft. SIGMETs are classified into two types: convective and non-convective.
Convective SIGMETs are issued when there is convection over the continental United States. It is issued anytime there is convective activity such as thunderstorms, covering an area of more than 3000 square miles. Convective SIGMETs 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 for more than 30 minutes.
Non-convective SIGMETs are less serious, but they can be dangerous for smaller aircraft. The aircraft being impacted will depend on the type of 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 it's not caused by thunderstorms or the presence of volcanic ash. In Hawaii or Alaska, SIGMETs are also issued for tornadoes, squall lines, embedded thunderstorms, or hail greater than ¾ inches.
SIGMETs 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. 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. They 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. An international SIGMET for volcanic ash will last up to 12 hours, for hurricanes up to 6 hours, and for all other meteorological events requiring a SIGMET for up to 4 hours. These are designated with one letter from "A" to "M". They are issued for the same conditions as convective or non-convective SIGMET.