SIGMET Structure and Decoding for Aviation Weather
In the world of aviation, safety depends on the rapid and accurate communication of hazardous weather. SIGMETs (Significant Meteorological Information) are standardized alerts used internationally by the International Civil Aviation Organization (ICAO) and the World Meteorological Organization (WMO). These messages notify pilots and dispatchers of weather phenomena that could significantly affect the safety of aircraft and flight operations.
Because these alerts must be transmitted quickly across global networks, they rely on a highly structured format and a specific set of abbreviations to ensure clarity regardless of the language spoken by the recipient.
Key Facts
- Standardization: SIGMETs follow ICAO and WMO protocols for global consistency.
- Three-Part Layout: Every message is divided into a Header, a Summary, and a Main Body.
- Validity: Standard weather (WS) SIGMETs generally have a maximum validity of 4 hours.
- Scope: They cover specific Flight Information Regions (FIR) or Control Areas (CTA).
- Dynamic Updates: Messages are renewed with sequence numbers if the hazard persists or cancelled using a specific CNL code.
Standardized Abbreviations
To keep messages concise, SIGMETs use a set of universal abbreviations. Understanding these is essential for decoding the actual weather threat.
| Abbreviation | Meaning | Abbreviation | Meaning |
|---|---|---|---|
| ABV | Above | CNL | Cancel or cancelled |
| CTA | Control area | FCST | Forecast |
| FIR | Flight Information Region | FL | Flight level |
| FT | Feet | INTSF | Intensify or intensifying |
| KT | Knots | KMH | Kilometres per hour |
| MOV | Moving | NC | No Change (in intensity) |
| NM | Nautical Miles | OBS | Observed |
| SFC | Surface | STNR | Stationary |
| TOP | Top (of CB cloud) | WI | Within (area) |
| WKN | Weakening (intensity) | Z | Coordinated Universal Time |
The Anatomy of a SIGMET
A complete SIGMET follows a strict sequence: 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>=
1. The Header
The header provides the administrative data required to identify the alert. It includes the data type identifier (TT), where VA stands for volcanic ash, WV for volcanic ash, and TC for tropical cyclones; otherwise, WS is used. It also contains the country code (AA), bulletin number (ii), the 4-character ICAO location of the office issuing the alert (CCCC), and the date and time of issuance in UTC. If a correction is made, a correction indicator (CCx) is added.
2. The Summary
The summary line identifies the affected area and the validity period. A key element here is the sequence number ([n][n]n), which increments if the SIGMET remains active past 0001UTC or is renewed. This helps meteorologists and pilots track how long a hazard has been present. For convective SIGMETs in the United States, this section will specify the region (Eastern, Central, or Western) as SIG[E/C/W].
3. The Main Body
The body contains the actual weather data. It begins with the ICAO location and the affected Flight Information Region (FIR) or Control Area (CTA), followed by the specific meteorological phenomenon.
Phenomena Codes- Thunderstorms: OBSC TS (Obscured), EMBD TS (Embedded), FRQ TS (Frequent), or SQL TS (Squall line). These may also include GR (Hail).
- Severe Hazards: SEV TURB (Severe turbulence), SEV ICE (Severe icing), SEV MTW (Severe mountain wave), or SEV ICE (FZRA) for freezing rain.
- Atmospheric Conditions: HVY DS (Heavy duststorm), HVY SS (Heavy sandstorm), or RDOACT CLD (Radioactive cloud).
- Convective Specifics: AREA TS (Area-wide thunderstorms), LINE TS (Thunderstorm line), TDO (Tornado), FC (Funnel Cloud), WTSPT (Waterspout), and HVY GR (Heavy Hail).
The body specifies whether the event is observed (OBS) or forecasted (FCST) at a specific Zulu time. Location is typically given in latitude and longitude. Altitude (Level) is expressed in several ways:
- FLnnn / nnnnM / nnnnFT: A specific flight level, meter, or foot altitude.
- SFC/FLnnn: From the surface up to a specific altitude.
- FLnnn/nnn: Between two specific altitudes.
- TOP / ABV: Above a certain point or cloud cover.
Finally, the body may include the movement of the front (MOV) with a compass direction and speed (KT/KMH), the change in intensity (INTSF, WKN, or NC), and the expected position at the end of the validity period.
Renewal and Cancellation
SIGMETs are dynamic. If a weather phenomenon persists beyond the expiration time, the SIGMET is renewed with a new sequence number. In the United States, non-convective SIGMETs are reissued at least every 4 hours if the conditions continue.
If the hazard clears before the expiration time, a cancellation message is issued using the format: CNL SIGMET [n][n]n YYGGgg/YYGGgg. Any necessary amendments are issued as a new SIGMET with the next sequence number, while minor corrections are marked with COR.
Frequently Asked Questions
What is the maximum validity for a WS SIGMET?
A WS (standard weather) SIGMET cannot be active for more than 4 hours. If the weather phenomenon persists, it must be renewed or reissued.
How can I tell if a SIGMET has been corrected?
Corrections are identified in the header by a correction indicator (CCx), where x is a letter from A-Z, or by the use of the COR identifier.
What does the sequence number in a SIGMET indicate?
The sequence number increments each time a SIGMET is renewed or remains effective past 0001UTC, helping users determine how long the weather event has been active.
What is the difference between OBS and FCST in a SIGMET?
OBS indicates that the meteorological phenomenon has been observed, while FCST indicates that the phenomenon is forecasted to occur.
How is altitude represented in a SIGMET?
Altitude can be represented as a single flight level (FL), a range from the surface (SFC) to a level, a range between two levels, or as being above (ABV) or at the top (TOP) of a cloud cover.