Significant Meteorological Information, or SIGMET, is a specialized, non-scheduled weather advisory. It is issued to alert air traffic control and pilots to hazardous atmospheric conditions that could seriously affect flight safety. SIGMETs communicate information about widespread, severe weather events within a designated Flight Information Region (FIR) to help aircraft operators plan safe routes and maneuvers.
Criteria for Issuance
A SIGMET is issued only when a meteorological event reaches an intensity threshold that poses a substantial risk to aircraft of all sizes. The criteria for these advisories focus on weather phenomena that are severe enough to necessitate immediate changes to a flight plan. The conditions that trigger a non-convective SIGMET include severe or extreme non-convective turbulence, which is not associated with thunderstorms, or severe airframe icing.
Severe turbulence is dangerous because it can occur in clear air, making it virtually impossible to see. This violent motion can cause structural damage or result in serious injury to passengers and crew. Severe icing occurs when supercooled water droplets freeze rapidly upon contact with the aircraft’s surfaces, altering the wing’s aerodynamics and adding significant weight.
Widespread dust storms or sandstorms also necessitate a SIGMET when they reduce surface or in-flight visibility to less than three statute miles. This low visibility creates hazards during takeoff, landing, and en-route flight by obscuring terrain and other air traffic. The presence of volcanic ash is another mandatory criterion for issuance, as the abrasive particles can quickly clog air filters and cause catastrophic failure of jet engines.
Volcanic ash is considered one of the most severe threats to aviation; it can melt in the hot section of a jet engine, re-solidify, and coat internal components, leading to a complete loss of thrust. Non-convective SIGMETs are generally issued for phenomena that are widespread, covering an area of at least 3,000 square miles. These advisories ensure pilots are aware of significant hazards that cannot be easily avoided by minor course adjustments.
Categorization of SIGMETs
The aviation community generally recognizes three main categories of SIGMETs, which classify the type of hazard being reported. The primary distinction is between weather caused by thunderstorms, known as convective activity, and all other severe weather phenomena. This classification allows for specific operational responses tailored to the nature of the threat.
Convective SIGMETs (WST in the United States) are issued for weather directly related to active thunderstorms.
Convective Hazards
These hazards include:
- Tornadoes
- Hail measuring three-quarters of an inch or larger in diameter
- Surface wind gusts of 50 knots or greater caused by thunderstorm activity
- Lines of thunderstorms that are at least 60 miles long, with storms affecting 40% of the length
The non-convective SIGMET, identified as WS, covers all severe weather events not associated with thunderstorms, such as severe clear air turbulence or severe icing. This category also includes widespread dust storms, sandstorms, and mountain waves, which are powerful vertical air currents caused by wind flowing over mountainous terrain. International regulations outside the contiguous United States also recognize a category for tropical cyclones.
The third distinct type is the Volcanic Ash SIGMET (WV), issued specifically for the presence of volcanic ash clouds. This separation reflects the unique danger ash poses to aircraft engines and systems. These advisories are important because ash clouds can travel vast distances and remain suspended in the atmosphere for extended periods.
Format, Validity, and Dissemination
SIGMET advisories are distributed in a standardized, coded format to ensure rapid and clear communication across international aviation networks. They are issued by Meteorological Watch Offices (MWOs), specialized weather centers responsible for forecasting within designated Flight Information Regions. The message structure includes a header, a summary, and a main body detailing the weather hazard.
The coded message contains specific identifiers, such as “WS” for non-convective or “WST” for convective, followed by a sequence number and the validity period. Geographic references, often using navigational coordinates, define the affected area. The message also specifies the altitude or flight level where the phenomenon is occurring, such as “FL260” for 26,000 feet.
The duration of a SIGMET depends on its type. Non-convective SIGMETs are typically valid for up to four hours, with a new advisory issued if conditions persist. Convective SIGMETs in the United States have a shorter validity of up to two hours and are updated every hour at 55 minutes past the hour.
SIGMETs for volcanic ash clouds and tropical cyclones are exceptions, often having a maximum validity period of six hours due to the slower-moving and more enduring nature of these phenomena. Once issued, these advisories are disseminated primarily to aircraft in flight through Air Traffic Control (ATC) voice broadcasts and automated systems like VOLMET (meteorological information for aircraft in flight). This ensures pilots receive the most current hazardous weather information as they navigate their routes.