International SIGMET: The ICAO Format Decoded
An international SIGMET follows a stricter layout than the US products, and that strictness makes it easier to read once you know the order of the lines.
Every one is built the same way. Header, hazard, area, level, movement, intensity. Learn those six slots and the format stops being a wall of capitals.
The header line
The first line names who issued it and what it covers.
A header might read EGTT SIGMET 03 VALID 291200/291600 EGRR-. That is the
London FIR, SIGMET number three of the day, valid from 1200 to 1600 on the 29th,
issued by the Met Office at Exeter.
The four-letter codes are ICAO location identifiers. The first is the region the warning applies to. The one after the dash is the office that wrote it.
The valid period uses six digits each side: day, hour, minute. Most SIGMETs run four hours. Volcanic ash and tropical cyclone warnings can run six.
The hazard phrase
The next part names what the warning is about, using a fixed vocabulary.
The phrases you will meet most are SEV TURB for severe turbulence, SEV ICE
for severe icing, EMBD TS for embedded thunderstorms, SEV MTW for severe
mountain wave, VA CLD for a volcanic ash cloud, and TC for a tropical
cyclone.
Some carry a qualifier. SEV ICE (FZRA) marks severe icing caused by freezing
rain, which is worse than the same words without it.
Because the list is fixed, the hazard is unambiguous. There is no reading between the lines the way there is with a convective SIGMET, where the storm implies hazards nobody wrote down.
The area and level lines
Next comes where, and it is given in one of three ways.
A boundary of latitude and longitude points, joined into a polygon. A line with a width, such as within 60 nautical miles of a line between two points. Or the whole FIR, when the hazard is that widespread.
Then the level. FL180/300 means the hazard sits between flight level 180 and
flight level 300. SFC/FL100 runs from the surface to 10,000 feet.
The level line is the one to read first if you are deciding whether to route around or fly over. Many turbulence and icing SIGMETs occupy a band you can climb above.
Movement and intensity
The last two slots tell you what happens next.
Movement is written as a direction and a speed, such as MOV E 20KT. Read the
wording carefully. MOV E means moving toward the east. Some products use
MOV FROM, which reverses the meaning entirely, and a Center Weather Advisory
in the US uses the toward form as well.
Getting that backwards puts you on the wrong side of the hazard, so our decoder tracks which form was used rather than assuming.
Intensity is one of three words. INTSF means intensifying. WKN means
weakening. NC means no change expected.
A weakening hazard that expires in an hour is a different planning problem from an intensifying one moving toward your destination at 25 knots.
Where AIRMETs fit, or do not
This is the part that surprises pilots flying outside the United States for the first time.
Many countries do not issue AIRMETs at all. There is no moderate turbulence advisory, no moderate icing advisory, and no equivalent of Sierra, Tango or Zulu.
So the SIGMET is often the only area hazard product available, and it only fires at the severe end. Moderate conditions that would generate three AIRMETs at home may generate nothing at all.
That changes how you plan. Where AIRMETs are absent, the aerodrome forecasts and the significant weather charts carry more of the load, and the absence of a SIGMET says much less about the day than the absence of an AIRMET would.
Cancelling and updating
A SIGMET does not always run to its end time, and the wording for stopping one early is worth knowing.
A cancellation carries CNL and points at the number it is cancelling. Read as
the hazard has gone before the valid period ran out.
An update keeps the same number and revises the content. So a rising sequence number is a new hazard, while a repeated one is the same hazard restated.
Because the numbers run through the day per FIR, a high number early tells you something on its own. It means the office has already issued a lot of warnings, which is a fair sign of how the day is going.
Verdict on international SIGMET
Read it in slot order and it decodes quickly. Who and when, what hazard, where, what levels, moving which way, getting worse or better.
The two lines that change decisions most are the level band and the movement. The band tells you whether over the top is available. The movement tells you whether the problem is coming to you. And remember that outside the US, no SIGMET does not mean no hazard, because there may be no AIRMET behind it.
Frequently asked questions
How is an international SIGMET different from a US one?
The international format follows ICAO Annex 3 with a fixed line order and standard hazard phrases. US SIGMETs use their own convective and non-convective products with different wording and different valid periods.
What area does an international SIGMET cover?
A Flight Information Region, or FIR. The header names the FIR and the issuing office, and the area line inside describes the affected part of it using latitude and longitude or navigation points.
How long is an international SIGMET valid?
Usually four hours. Volcanic ash and tropical cyclone SIGMETs can run to six. The valid period appears in the header as a start and end time in day, hour and minute.
What does MOV E 20KT mean?
The hazard area is moving east at 20 knots. The direction is the way it is heading. Some products instead write MOV FROM, which gives the direction it is coming from, so read the wording rather than assuming.
What is the difference between SIGMET and AIRMET internationally?
SIGMETs cover hazards dangerous to all aircraft. Many countries do not issue AIRMETs at all, so outside the United States the SIGMET is often the only area hazard warning you will get.