SIGMET Guide: The Warnings That Apply to Every Aircraft

A SIGMET is the top tier of area weather warning. Where an AIRMET says some aircraft will find this hazardous, a SIGMET says all of them will.

That single distinction is what this SIGMET guide turns on. Size, equipment and rating do not exempt you from what a SIGMET describes.

What a SIGMET covers

The hazard list is short, and everything on it is dangerous to any aircraft.

Severe and extreme turbulence. Severe icing. Widespread dust storms or sandstorms reducing visibility below three miles. Volcanic ash.

Thunderstorms are covered too, but through a separate product with its own rules and its own two hour window.

Two of these have no AIRMET equivalent at all. There is no moderate volcanic ash, and no moderate sandstorm, because neither has a version worth flying through.

The valid periods

How long a SIGMET lasts tells you how fast the hazard moves.

Most run four hours. Volcanic ash and tropical cyclone warnings run six, because those hazards persist and drift rather than building and dying.

Convective SIGMETs run two hours and are reissued hourly. A storm that did not exist an hour ago can be at its peak now.

They are unscheduled, apart from the convective product. Nothing goes out on a clock. So the existence of a SIGMET is itself information: somebody decided the criteria were met.

Reading the area

The area is given in one of three forms, and which one is used tells you something about the hazard.

A polygon of points, joined into a boundary. That suits a patch of severe turbulence or an ash cloud.

A line with a width, such as within 60 nautical miles either side of a line between two points. That suits a front or a squall line.

Or the whole region, when the hazard is that widespread.

Whichever form is used, the area is broad. A SIGMET never describes a single airfield, and there will be usable air inside the boundary.

Reading the levels

The level line is the one that most often changes a plan, and it is easy to skim past.

FL180/300 puts the hazard between flight level 180 and flight level 300. SFC/FL100 runs from the surface to 10,000 feet.

Many turbulence and icing SIGMETs occupy a band with clear air above it. If your aircraft can get on top, the warning may not apply to your flight at all.

That is the first question to ask of any SIGMET on your route. Is there a level above or below the band, and can you reach it?

Movement and trend

Two more fields tell you what happens next.

Movement gives a direction and a speed. Read the wording rather than assuming: MOV E means heading east, while MOV FROM in some products means the direction it comes from. Our decoder tracks which form was written instead of guessing.

Trend is one of three words. Intensifying, weakening, or no change.

A weakening hazard that expires in an hour is a very different planning problem from an intensifying one heading toward your destination at 25 knots.

Using it with the other products

A SIGMET decides routing. Read it first, before anything else in the briefing.

Then read the AIRMETs, which tell you what the flight will be like rather than whether it can happen. Then read PIREPs, which are the only current information in the set.

Remember the order of confidence. A SIGMET is a forecast over a wide area for several hours. A PIREP is one aircraft at one place a few minutes ago. Where they conflict, the report is more current, though it covers far less sky.

US and international formats

The same product is written two ways, and the differences are worth knowing before you fly abroad.

US SIGMETs split into convective and non-convective, each with its own wording and valid period. They are numbered in sequence and cover regions of the country.

International SIGMETs follow ICAO Annex 3. They cover a Flight Information Region, use a fixed line order, and draw hazard phrases from a set list such as SEV TURB, EMBD TS and VA CLD.

The international form is stricter and easier to decode once you know the slots. The US form reads more like plain text.

One habit carries across both. Check whether an AIRMET exists alongside it, and remember that many countries issue none at all.

Verdict on this SIGMET guide

Treat a SIGMET as a routing input rather than a comfort warning. Every aircraft is included, so the response is to go around, over, under or later.

Read the level band first, because it often offers a way out. Then read the movement and the trend, which tell you whether the problem is coming to you and whether it is growing. And check the issue time, especially on a convective SIGMET, where two hours is the whole span.

Last updated August 29, 2026