What a SIGMET is
A SIGMET is issued by a meteorological watch office to give timely information on en-route weather phenomena that affect the safety of flight operations within its area of responsibility, giving the location, extent, intensity and expected evolution of the phenomenon. They are issued per flight information region, and the rules come from ICAO Annex 3.
Two things follow from that definition and they shape how the layer should be read.
First, a SIGMET is primarily an in-flight service. The design intent is timely transmission to aircraft already airborne, with controllers passing relevant SIGMETs covering the route up to about two hours flying time ahead. Global dissemination through regional data banks is what makes it usable for planning as well.
Second, a SIGMET is a forecaster's warning polygon, not an observation. It encodes an office's judgment, informed by radar, satellite, model guidance, volcanic ash and tropical cyclone advisories and pilot reports. Treating the polygon as a measurement is the most common misreading. The polygon is also deliberately generous, because it has to cover the forecast evolution across its whole validity period.
The hazard categories
ICAO permits SIGMETs only for a closed list of phenomena, with fixed abbreviations and, in several cases, quantitative thresholds.
| Code | Meaning and threshold |
|---|---|
| OBSC TS | Thunderstorms obscured by haze or smoke, or not readily seen in darkness |
| EMBD TS | Thunderstorms embedded in cloud layers, not readily recognizable by the pilot |
| FRQ TS | Frequent thunderstorms, little or no separation, coverage above 75 percent |
| SQL TS | Squall line: thunderstorms with little or no space between cells |
| SEV TURB | Severe turbulence, not in connection with convective cloud |
| SEV ICE | Severe icing, not associated with convective cloud; FZRA variant for freezing rain |
| SEV MTW | Severe mountain wave, downdraft 3 m/s (600 ft/min) or more |
| HVY DS / SS | Heavy dust or sandstorm, visibility below 200 m with sky obscured |
| VA | Volcanic ash |
| TC | Tropical cyclone, 10 minute mean surface wind 17 m/s (34 kt) or more |
| RDOACT CLD | Radioactive cloud |
Note that the thunderstorm criteria are about whether a pilot can see and avoid the cell, which is why "obscured" and "embedded" are separate categories rather than intensity grades. Note also that the live feed does not contain only this exact list, because states implement different editions of the rules and the United States issues its international SIGMETs under its own national instruction with a slightly different vocabulary. The nine categories this layer colors are a normalized superset.
How long a polygon is valid
Validity is short, and this is the detail most often ignored by consumers of the data.
- A general SIGMET should be valid for no more than 4 hours, and is updated at least every 4 hours while the phenomenon continues.
- A volcanic ash or tropical cyclone SIGMET should be valid for no more than 6 hours, updated at least every 6 hours.
- A SIGMET can be cancelled during its validity, and can be amended.
A polygon past its validity time is not a weaker warning, it is not a warning at all, and leaving one on a map is actively misleading. Each feature here carries its own validity window, and the popup shows the raw SIGMET text alongside the decoded fields. The raw bulletin is the authoritative artifact; the polygon is a rendering of it.
SIGMET, AIRMET, and the gap over the United States
A SIGMET warns of phenomena hazardous to all aircraft and means severe or greater. An AIRMET covers phenomena at lower intensity that matter to low-level flight and to aircraft without de-icing or performance margin, and is issued on a schedule rather than on event. In the US these carry names: SIERRA for IFR and mountain obscuration, TANGO for turbulence and strong surface winds, ZULU for icing and freezing levels. CONUS text AIRMETs were discontinued in January 2025 in favor of graphical G-AIRMETs.
The structural point for this layer: it draws international SIGMETs only. The United States splits its domestic product, and handles thunderstorms through a separate convective SIGMET issued hourly for the eastern, central and western CONUS, each valid two hours or until superseded. Those are a different product and are not in the feed this layer reads.
The practical consequence is worth stating bluntly: during a US summer convective outbreak this layer can look empty over the CONUS while three convective SIGMET bulletins are active. Do not read a blank United States as quiet air.
What this layer does not tell you
Every data source has an edge. These are the ones that matter for reading this layer correctly.
- These are forecast warnings written by people, not measurements. The polygon is intentionally conservative in both horizontal extent and vertical band.
- This layer carries international SIGMETs only. US domestic convective SIGMETs and CONUS G-AIRMETs are not drawn, so the CONUS can appear empty during active convection.
- Issuance practice, timeliness and format compliance vary considerably between states. A quiet area may mean no hazard, no issuance, or a bulletin that failed to propagate.
- Validity is 4 hours for most phenomena and 6 for volcanic ash and tropical cyclone. An expired polygon is not a weak warning, it is not a warning.
- Mapping varied national codes onto a handful of colored categories loses nuance, including the obscured versus embedded distinction and the quantitative turbulence criterion.
- This is a secondhand rendering with added polling latency. The authoritative product is the raw bulletin from the issuing office.
AeroScope is an situational awareness and research tool. Nothing here is approved for air traffic control, navigation, flight planning or collision avoidance. For operational decisions, use the authoritative source named above.