AIRMET vs SIGMET: What Each Advisory Means

Here’s the difference in one sentence: an AIRMET warns of moderate weather that matters most to light aircraft and less-experienced pilots, while a SIGMET warns of severe weather hazardous to every aircraft, from a 152 to a 777.

Both are inflight weather advisories issued by the Aviation Weather Center, and both describe an area, not a point. That’s the mental shift from METARs and TAFs: those tell you what one airport is reporting or expecting, while an advisory draws a polygon on the map and says “somewhere in here, this hazard exists or is coming.” The AIM covers all of them in paragraph 7-1-6, and the details below come from there and from aviationweather.gov.

What an AIRMET Covers: Sierra, Tango, Zulu

AIRMETs describe weather of less severity than SIGMET criteria, but still operationally significant, and potentially hazardous to aircraft with limited capability: light singles, pilots without an instrument rating, anyone without deice equipment. They come in three flavors, each with a phonetic name that tells you the hazard before you read a word of the body:

• Sierra: IFR conditions (widespread ceilings below 1,000 ft and/or visibility below 3 SM) and extensive mountain obscuration.
• Tango: moderate turbulence, sustained surface winds greater than 30 knots, and non-convective low-level wind shear.
• Zulu: moderate icing, plus freezing level heights.

That last detail in Zulu is easy to overlook and worth the read even on a warm day: the freezing level tells you whether an escape to warmer air below exists at all.

Textual AIRMETs Are Gone Over the CONUS

If you learned to decode the old text AIRMET bulletin, that skill is now historical. The National Weather Service retired the textual AIRMET for the contiguous United States on January 27, 2025 (Service Change Notice 24-92). The G-AIRMET, the graphical version that had been running alongside it since 2010, is now the advisory of record for the lower 48. Alaska and Hawaii still get textual AIRMETs.

The change was more than a format swap. The old text product smeared a hazard across its whole six-hour valid period: if icing was forecast over Ohio at the start of the window and over Pennsylvania at the end, the text bulletin covered both for all six hours, and the area looked worse than any single moment actually was. A G-AIRMET instead gives you snapshots at three-hour intervals, out to 12 hours. Step through the frames on aviationweather.gov and you watch the polygon move. Suddenly “will that icing area still be over the ridge when I cross it at 1800Z?” has an answer instead of a shrug.

G-AIRMETs are issued four times daily, with updates whenever conditions demand one.

What Triggers a SIGMET

A SIGMET (Significant Meteorological Information) covers non-convective weather hazardous to all aircraft. Per AIM 7-1-6, that means any of:

• Severe icing not associated with thunderstorms
• Severe or extreme turbulence, or clear air turbulence, not associated with thunderstorms
• Widespread dust storms or sandstorms lowering surface visibility below 3 miles
• Volcanic ash

Note the word severe doing the work. Moderate icing gets a G-AIRMET Zulu; severe icing gets a SIGMET. Severe icing means ice accumulating faster than deice equipment can shed it, so a SIGMET for icing is a hazard even to a known-ice twin, not just to our no-boots singles. What that ice actually does to a wing, and how to get out of it, is covered in airframe icing explained.

A SIGMET is unscheduled, issued whenever the weather earns it, and valid for 4 hours, except SIGMETs for tropical cyclones and volcanic ash clouds, which run 6 hours. Each one gets a phonetic name: in the CONUS, November through Yankee, skipping Sierra and Tango because those belong to AIRMETs. The same weather system keeps its name through successive issuances, so “SIGMET Papa 3” is the third update on the phenomenon named Papa. If you picked up Papa 1 in your morning brief, Papa 3 before departure is the same beast, updated.

Convective SIGMETs: The Thunderstorm Product

Thunderstorms get their own advisory with its own rules. A Convective SIGMET is issued for:

• A severe thunderstorm: surface winds of 50 knots or greater, hail 3/4 inch or larger, or tornadoes
• Embedded thunderstorms
• A line of thunderstorms
• Thunderstorms producing heavy or greater precipitation affecting 40 percent or more of an area of at least 3,000 square miles

Any one of those conditions is a ride we want no part of; what a cell actually does to a light airplane, and how far to give one, is covered in thunderstorms and small aircraft. Three bulletins go out hourly at 55 minutes past the hour, one each for the Eastern, Central, and Western United States, and each is valid for up to 2 hours. Specials come out any time the weather outpaces the schedule. Numbering restarts daily and carries the region letter, so “Convective SIGMET 22C” is the 22nd issued for the Central region that day.

In plain English, a typical one reads like this: Convective SIGMET 22C, issued at 1455Z and valid until 1655Z, covers a line of thunderstorms in western Kansas, running from northeast of the Goodland VOR to southwest of Garden City, 20 NM wide, moving northeast at 20 knots, with tops to FL450. The raw bulletin (ours is invented; the format is real) says the same thing in fewer characters:

CONVECTIVE SIGMET 22C
VALID UNTIL 1655Z
KS
FROM 30NE GLD-40SSW GCK
LINE TS 20 NM WIDE MOV FROM 24020KT. TOPS TO FL450.

Two things every pilot should internalize about this product. First, per the AIM, any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear — the bulletin doesn’t need to spell those out, because a thunderstorm that qualifies contains all three. Second, the absence of a Convective SIGMET does not mean the absence of thunderstorms. The criteria are steep. An isolated air-mass cell that isn’t severe, embedded, or part of a line qualifies for nothing, and it can still tear a wing off a light aircraft. “No convective SIGMETs on the route” is a statement about organized convection, not a clear-sky certificate. Check the radar and the TAFs anyway.

Using Advisories in an Actual Brief

An advisory only means something when you intersect it with your specific flight, and that intersection has three parts: the polygon (checked against the clock), the altitude, and the severity.

The polygon. Does the advisory area touch your route, or sit 150 NM west of it? With G-AIRMET snapshots, check the frame closest to when you’ll actually be there, not just the current one. A Tango area drifting east at 30 knots may arrive over your destination right on schedule with you.

The altitude. Advisories carry vertical limits, and this is where many self-briefs go soft. A G-AIRMET Zulu from 10,000 ft to FL180 is a non-event for a coastal hop at 4,500 ft; the same advisory is a hard stop for a direct crossing of terrain that forces you to 12,000. Read the floor and the ceiling of the hazard against the floor and ceiling of your plan.

The severity, honestly assessed. “Moderate icing” sounds survivable on paper. The AIM’s definition (7-1-19) quietly assumes ice protection: an accretion rate of 1 to 3 inches per hour on the outer wing, demanding frequent cycling of the deice systems and an exit as soon as possible. In a piston single with no boots to cycle, the only item on that list available to us is the exit. For most of us flying normal category singles, a Zulu advisory across the route at our only usable altitudes is a no-go, however mild the word “moderate” sounds. The honest question isn’t “will I definitely pick up ice?” but “if I do, what’s my out?” No out, no launch.

Then close the loop with observations. Advisories are forecasts; PIREPs are pilots telling you what the air is actually doing, and a fresh negative-ice PIREP at your altitude inside a Zulu area is valuable information (as is the opposite). Pair the area picture with the point picture from METARs and TAFs, and slot the whole thing into the sequence laid out in our preflight weather briefing guide. Advisories are one layer of a brief, not the brief.

Quick Comparison

	G-AIRMET	SIGMET	Convective SIGMET
Severity	Moderate	Severe / extreme	Organized or severe thunderstorms
Hazards	IFR, mountain obscuration, turbulence, surface winds, LLWS, icing	Severe icing, severe/extreme turbulence, dust/sandstorms, volcanic ash	Severe TS, embedded TS, lines, wide-area heavy precipitation
Matters most to	Light aircraft, limited equipment or experience	All aircraft	All aircraft
Issuance	Four times daily, snapshots every 3 hr out to 12 hr	Unscheduled, when criteria are met	Hourly at H+55, plus specials
Valid for	Snapshots at discrete times	4 hr (6 hr tropical cyclone / volcanic ash)	Up to 2 hr
Implied extras	—	—	Severe turbulence, severe icing, LLWS

The habit worth building: read every advisory as a question about your flight. Where is it, when will it be there, what altitudes does it own, and what’s the out if the forecast verifies? Answer those four and the alphabet soup turns back into weather.

Common questions

Are textual AIRMETs still issued?

Not over the contiguous United States. The National Weather Service retired the textual AIRMET for the lower 48 on 27 January 2025 under Service Change Notice 24-92, and the graphical G-AIRMET became the advisory of record. Alaska and Hawaii still receive textual AIRMETs. G-AIRMETs give snapshots every three hours out to twelve, so you can watch the hazard area move.

How long is a SIGMET valid?

Four hours, except SIGMETs for tropical cyclones and volcanic ash clouds, which run six. Convective SIGMETs work differently: three bulletins go out hourly at 55 minutes past the hour, one each for the Eastern, Central and Western United States, valid up to two hours, with specials any time the weather outpaces the schedule.

Does no Convective SIGMET mean no thunderstorms?

No. The criteria are steep: a severe cell, embedded storms, a line of storms, or heavy precipitation affecting 40 percent of an area of at least 3,000 square miles. An isolated air-mass cell that meets none of those qualifies for nothing, and it can still tear a wing off a light aircraft. Check the radar and the TAFs regardless.