SIGMET vs AIRMET: Severity, Scope and Which One Stops You
SIGMET vs AIRMET is the most useful comparison in aviation weather, because the two products answer different questions about the same sky.
One asks whether the weather is bad enough to trouble some aircraft. The other asks whether it is bad enough to trouble all of them. Everything else follows from that line.
The severity line
An AIRMET warns of weather hazardous to some aircraft. Light aircraft, aircraft without ice protection, pilots without an instrument rating.
A SIGMET warns of weather hazardous to all aircraft. Size, equipment and rating do not exempt you.
That is why the same hazard appears in both products at different strengths. Moderate turbulence is an AIRMET. Severe turbulence is a SIGMET. Moderate icing is an AIRMET. Severe icing is a SIGMET.
The hazard did not change. The question changed from who might struggle to who is at risk.
What each one covers
AIRMETs come in three types, and each owns a set of hazards.
Sierra covers instrument conditions and mountain obscuration. Tango covers moderate turbulence, sustained surface winds of 30 knots or more, and non-convective low-level wind shear. Zulu covers moderate icing and publishes the freezing level.
SIGMETs cover the severe end and a few hazards that have no moderate version at all.
Severe and extreme turbulence. Severe icing. Widespread dust storms or sandstorms that cut visibility below three miles. Volcanic ash. And, through the convective product, thunderstorms.
Volcanic ash is the clearest case of a hazard with no AIRMET. There is no moderate amount of ash worth flying through, so there is nothing for a lower-tier product to say.
How long each lasts
The valid periods differ, and they tell you how fast each hazard moves.
An AIRMET runs six hours. It goes out on a schedule, four times a day, with amendments between when things change.
A SIGMET runs four hours. Volcanic ash and tropical cyclone SIGMETs run six. It is unscheduled, issued when the criteria are met.
A convective SIGMET runs two hours and is reissued hourly. Storms build and die too fast for anything longer to be honest.
So the shorter the period, the faster the weather. A two hour product is telling you the situation will look different by the time you land.
The area test cuts both ways
Both products need the conditions to cover a wide area, and this is the part most often misread.
An AIRMET needs conditions over at least 3000 square miles. A SIGMET needs a similarly broad footprint.
So neither product describes your airfield. Inside an AIRMET Sierra there will be fields in clear air. Inside a turbulence SIGMET there will be smooth patches.
And the reverse matters more. No AIRMET does not mean no low cloud. A single valley in fog will never trigger one, and it will still stop you landing there.
Use the advisories for the scale of the day and the METARs and TAFs for the field.
Using both together
The two products work best read in sequence rather than separately.
Start with the SIGMETs. If one covers your route, that is a routing decision before anything else, because no aircraft is exempt.
Then read the AIRMETs. These tell you what the flight will be like rather than whether it can happen. Sierra tells you whether visual flight is available. Tango tells you how rough. Zulu tells you where the ice is and where the freezing level sits.
Then go to PIREPs. Both products are area forecasts valid for hours. A pilot report is one aircraft, one place, one moment, and it is the only thing in the system that tells you what the air actually did.
Where the reports and the advisories disagree, the reports are more current.
Outside the United States
The comparison works differently abroad, and it catches people out.
Many countries issue SIGMETs and no AIRMETs at all. There is no Sierra, Tango or Zulu, and no moderate-level advisory of any kind.
That means a quiet advisory feed abroad carries far less information than the same quiet feed at home. Conditions that would produce three AIRMETs in the US may produce nothing.
Where AIRMETs are missing, the significant weather charts and the aerodrome forecasts have to do the work instead.
Verdict on SIGMET vs AIRMET
Sort them by who is at risk. An AIRMET says some aircraft will find this hazardous. A SIGMET says all of them will.
Read SIGMETs first, because they decide routing. Read AIRMETs next, because they decide how the flight will feel and whether visual flight is on. Then read PIREPs, because they are the only current thing in the set. And when you fly outside the US, remember that a missing AIRMET may simply mean nobody issues them there.
Frequently asked questions
What is the main difference between a SIGMET and an AIRMET?
Severity. An AIRMET warns of conditions hazardous to some aircraft, mainly lighter ones and those without ice protection. A SIGMET warns of conditions hazardous to all aircraft, whatever they are flying.
Which hazards are AIRMET only?
Moderate turbulence, moderate icing, sustained surface winds of 30 knots or more, widespread instrument conditions, and mountain obscuration. Each of these belongs to one of the three AIRMET types.
Which hazards are SIGMET only?
Severe and extreme turbulence, severe icing, widespread dust or sandstorms, volcanic ash, and thunderstorms through the convective product. These threaten every category of aircraft.
Are AIRMETs issued outside the United States?
Often not. Many countries issue SIGMETs only, so there is no moderate-level advisory at all. Outside the US, the absence of a warning says much less about the day than it would at home.
How long is each one valid?
An AIRMET runs six hours and is issued on a schedule four times a day. A SIGMET runs four hours, or six for volcanic ash and tropical cyclones, and is issued as needed. A convective SIGMET runs two hours.