A Significant Meteorological Information (SIGMET) is a weather advisory issued to alert pilots to the presence or expectation of hazardous weather conditions that could affect aircraft safety. These advisories focus on non-routine, severe weather phenomena occurring over a wide area and not covered by other forecasts. The primary goal of a SIGMET is to provide timely, concise information to flight crews and air traffic control so they can plan routes to safely avoid affected regions. Because the weather conditions described are severe and rapidly changing, the duration for which a SIGMET is valid is intentionally limited to ensure the advisory remains current and accurate.
Standard Validity Periods
The time limit for a SIGMET’s validity is directly tied to the type of hazard it describes and the typical persistence of the weather event. For standard non-convective hazards, such as severe icing or severe turbulence, the advisory is valid for a maximum of four hours. This four-hour duration is the rule for most en-route weather phenomena under international (ICAO) standards, ensuring the meteorological watch office updates or reissues the information promptly.
A longer validity period of up to six hours is applied to SIGMETs issued for tropical cyclones and volcanic ash clouds. These phenomena, such as hurricanes or extensive ash plumes, are more spatially widespread and evolve slower than isolated turbulence or icing events. This extended six-hour period is used primarily for warnings in oceanic regions or for volcanic activity, reflecting the persistent nature of these hazards.
The United States employs a more restrictive time limit for Convective SIGMETs, which address severe thunderstorm activity. These advisories are issued on a scheduled basis and are valid for a maximum of two hours. This shorter time frame acknowledges the dynamic and localized nature of severe thunderstorms, where conditions can change drastically. Convective SIGMETs are reissued every hour if the severe weather persists, making them one of the most frequently updated aviation weather products.
Criteria for Issuance
SIGMETs are issued only when specific, high-intensity weather criteria are met, categorized as either non-convective or convective hazards. Non-convective SIGMETs warn of conditions that pose a severe threat to flight safety but are not related to thunderstorms. The criteria include severe or extreme turbulence not associated with convection, a condition often encountered in clear-air turbulence at high altitudes.
Another non-convective trigger is severe icing, which can rapidly accumulate on an aircraft’s wings and control surfaces, degrading performance. Widespread dust storms or sandstorms also necessitate a SIGMET if they reduce visibility to less than three statute miles over a significant area. Volcanic ash is a serious criterion, as its abrasive nature can cause catastrophic engine failure and impair flight controls, triggering a SIGMET regardless of visibility.
Convective SIGMETs address severe thunderstorm activity within the contiguous United States. They are issued for tornadoes, hail three-quarters of an inch or larger in diameter, or surface wind gusts of 50 knots or greater. A Convective SIGMET is also warranted for lines of thunderstorms at least 60 miles long, with storms affecting 40% of the length, or for an area of active thunderstorms covering at least 3,000 square miles. Embedded or severe thunderstorms must generally persist for more than 30 minutes before a routine convective advisory is issued.
Amendment and Cancellation Procedures
Although a SIGMET is issued with a fixed maximum validity time, the actual duration of the hazard is managed through dynamic procedures. If the meteorological phenomenon changes intensity or geographical coverage before expiration, the issuing authority can release an amendment. An amendment updates the original advisory to reflect current, evolving conditions, which is important for rapidly moving systems.
When the hazard is predicted to continue past the initial four- or six-hour maximum validity period, the advisory is managed through a renewal process. A renewal involves issuing a completely new SIGMET with an updated sequence number and a new validity period. This procedure ensures continuous coverage while forcing the forecaster to reassess the hazard every four or six hours.
Conversely, if the hazardous weather dissipates or falls below the criteria that necessitated the advisory before expiration, the SIGMET is cancelled. Forecasters at the meteorological watch office constantly monitor conditions, and promptly canceling an advisory that is no longer valid removes unnecessary restrictions on air traffic. The true duration of a SIGMET is not the fixed maximum time limit but the persistence of the severe weather itself.