AIRMET vs SIGMET vs Convective SIGMET, Explained

An AIRMET, a SIGMET and a Convective SIGMET are the same kind of product at three levels of trouble. All three are in-flight aviation weather advisories, forecasts issued to warn aircraft already en route that hazardous weather is developing (AIM 7-1-6, AIM Basic with Change 3, effective July 9, 2026). The SIGMET is reserved for weather expected to affect the safety of aircraft operations; the AIRMET covers the same hazards at intensities below SIGMET level. The Convective SIGMET is the thunderstorm product for the lower 48, where ordinary SIGMETs are not issued for convection at all.

The confusion usually starts with the names. Sierra, Tango and Zulu are AIRMET categories, not SIGMET series, and the product most pilots call a G-AIRMET is what the AIM now calls an AIRMET over the contiguous United States.

Three advisories, one table

Hazards, issuance and valid times are from AIM 7-1-6 and chapter 26 of the Aviation Weather Handbook (FAA-H-8083-28B). The Convective SIGMET valid time follows the handbook (26.2.4.2.3) and the AWC product reference; the AIM text still says the forecast portion is valid for up to 1 hour.

All heights in these products are MSL, except ceilings, which are AGL (AIM 7-1-6).

What a SIGMET warns about

A SIGMET over the lower 48 is issued for 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, and volcanic ash (AIM 7-1-6). It is valid for up to 4 hours and is reissued at least every 4 hours while conditions continue (FAA-H-8083-28B, 26.2.4.1.2). Amendments are not issued; a change gets the next number in the series (26.2.4.1.5).

Series names run November through Yankee, skipping Sierra and Tango, the AIRMET letters (AIM 7-1-6). Every continuing SIGMET resets to number 1 at 0000 UTC (FAA-H-8083-28B, 26.2.2). Here is the handbook's own example, reproduced from Figure 26-4. It is illustrative, not a live advisory.

SFOR UWS 100130
SIGMET ROMEO 1 VALID UNTIL 100530
OR WA
FROM SEA TO PDT TO EUG TO SEA
SEV TURB BTN FL280 AND FL350. CONDS CONTG BYD 0530Z.

Line by line: the San Francisco area, Romeo series, issued on the 10th at 0130 UTC; first issuance, valid until 0530 UTC; Oregon and Washington; the polygon Seattle to Pendleton to Eugene and back; severe turbulence between FL280 and FL350, expected to continue past the valid time (Table 26-1). A piston pilot at 7,500 feet is not in that layer. Read the vertical extent first with any SIGMET. The polygon on a map does not show it.

Outside the lower 48, SIGMETs follow the international format and do cover thunderstorms: obscured, embedded, widespread, squall line or isolated severe, along with tropical cyclones (26.2.5.1). A Hawaii or Alaska SIGMET for thunderstorms is normal; a lower-48 one does not exist, because that job belongs to the next product.

Convective SIGMETs are the thunderstorm product

A Convective SIGMET is issued in the lower 48 for any of three situations (FAA-H-8083-28B, 26.2.4.2.1). The first is a line of thunderstorms at least 60 miles long with storms affecting at least 40 percent of its length. The second is an area of active thunderstorms covering at least 40 percent of the area concerned, with very strong radar reflectivity or a significant satellite or lightning signature. The third is embedded or severe thunderstorms expected for more than 30 minutes, whatever the size of the area. A special issuance can follow a tornado, hail of 3/4 inch or more at the surface, or surface gusts of 50 knots or more (26.2.4.2.2; AIM 7-1-6).

Any Convective SIGMET implies severe or greater turbulence, severe icing and low-level wind shear (AIM 7-1-6). The forecaster does not have to spell those out, and a cell that looks small on a datalink picture still carries the implication. Small does not mean mild.

Bulletins go out at 55 minutes past every hour for the eastern, central and western regions, split at 87 and 107 degrees west longitude, and if nothing qualifies the bulletin says CONVECTIVE SIGMET... NONE (AIM 7-1-6). Each advisory is numbered from 1 at 0000 UTC with a region letter, so CONVECTIVE SIGMET 20C is the twentieth of the day for the central region (FAA-H-8083-28B, Table 26-2). The polygon is a snapshot at issuance and the storms move according to the MOV FROM line, so a fast line will be outside its own polygon well before the valid time ends (26.2.4.2). Each bulletin ends with a 2 to 6 hour outlook of where issuances are expected next (26.2.4.2.5).

Sierra, Tango and Zulu are the AIRMET groups

An AIRMET describes specified en route weather phenomena at intensities lower than those that require a SIGMET, and it is intended to inform all pilots, especially VFR pilots and operators of sensitive aircraft (AIM 7-1-6). The handbook lists the criteria: ceiling below 1,000 feet and/or visibility below 3 statute miles, widespread mountain obscuration, moderate turbulence with top and bottom specified, sustained surface wind above 30 knots, moderate icing with its layer and the freezing levels, and non-convective low-level wind shear potential below 2,000 feet AGL (FAA-H-8083-28B, 26.3.1.3).

The three letters group those hazards. Sierra is IFR conditions and extensive mountain obscuration. Tango is moderate turbulence, sustained surface winds of 30 knots or greater and non-convective low-level wind shear. Zulu is moderate icing and the freezing level heights (FAA-H-8083-28B, 26.3.1.2). The letters are labels, not products. Both the handbook and the AIM define them under the text bulletins still issued for Alaska and Hawaii, and the AIM's only lettered examples are a Juneau and a Honolulu bulletin. Over the lower 48 the same hazards are drawn as separate layers on the graphical product.

A freezing-level line is not an advisory of icing. The AWC includes freezing-level information after AIRMETs for moderate icing and after statements that no significant icing is expected (AWC product reference). The icing is the hazard; the freezing level says where to start looking for it.

How the graphical snapshots work

Over the lower 48, AIRMETs are issued graphically and are valid at discrete times no more than 3 hours apart for up to 12 hours into the future, at 00, 03, 06, 09 and 12 hours, with extra snapshots sometimes inserted at 01, 02, 04 and 05 hours (AIM 7-1-6). The 00-hour snapshot is the initial condition. Each later one shows where the hazard is expected at that valid time, and the 00 through 06 hour snapshots correspond to the text AIRMET bulletin underneath. The handbook says these are "sometimes known as G-AIRMETs" (FAA-H-8083-28B, 26.3.1.1), and the AWC still uses that name.

The rule for the gaps between snapshots is the one to memorize, and the AIM states it as an instruction. If the 00-hour chart shows no hazard and the 03-hour chart shows one, assume the hazard starts immediately after the 00-hour time. If the 06-hour chart is clear again, assume it lasts until just before 06. The AIM's own example: clear at 00, hazard at 03, clear at 06 means the hazard is present from the 0001 hour to the 0559 hour (AIM 7-1-6). Snapshots are boundaries of a block, not instants of weather.

Issuance over the lower 48 is every 6 hours at 0245, 0845, 1445 and 2045 UTC (AIM TBL 7-1-6), so a 1445Z issuance has its 00-hour snapshot valid at 1500Z and its 03-hour snapshot at 1800Z.

A worked read along a route

Suppose you are planning a VFR flight from KRDU to KAVL, Asheville in the mountains of western North Carolina, departing at 1600Z with about two hours en route, and the 1445Z AIRMET issuance is current. The 00-hour snapshot (1500Z) shows nothing on your route. The 03-hour snapshot (1800Z) shows a Tango polygon for moderate turbulence below 8,000 feet over the mountains west of Charlotte, and a Sierra polygon for mountain obscuration over the same area. The 06-hour snapshot (2100Z) shows the Sierra polygon gone and the Tango polygon still there.

By the interpolation rule you treat the moderate turbulence as beginning right after 1500Z and lasting at least until 2100Z, which is your whole flight. You treat the mountain obscuration as present from just after 1500Z until just before 2100Z, which covers your arrival at Asheville around 1800Z.

The forecast does not say the ridges will be obscured at the minute you arrive. It says the forecaster expects widespread obscuration in that block, and a pilot who plans on threading the gap between snapshots is planning on luck.

Then check the other two products. The 1455Z Convective SIGMET bulletin for the eastern region says NONE, which means only that no storms met the criteria at that moment; the outlook at the end of the bulletin is where the afternoon's convection shows up first. That picture, read against the METARs and TAFs at KAVL and the airports on the way, is what you take into the decision.

Not required for VFR, and what that means

No regulation forbids VFR flight through an AIRMET, and the AIM describes the AIRMET as a product intended to inform pilots, not a restriction (AIM 7-1-6). The obligation is 14 CFR 91.103: for a flight not in the vicinity of an airport, the pilot in command must become familiar with weather reports and forecasts, and alternatives available if the planned flight cannot be completed. An AIRMET covering your route is a forecast you are required to know about. What you do with it is yours.

The fourth advisory, the Center Weather Advisory, is the odd one out: a nowcast from a Center Weather Service Unit for conditions meeting or approaching advisory criteria within the next two hours, and the AIM says plainly that it is not a flight planning product (AIM 7-1-6).

The AWC Graphical Forecasts for Aviation page shows the current G-AIRMET snapshots, SIGMETs and Convective SIGMETs on one map, with the text behind each polygon a click away. FlyFren's free airport weather decoder covers the METAR and TAF at each airport, from the NWS Aviation Weather Center feed, as supplemental information rather than an official briefing. Our post on how to read a PIREP covers the reports that confirm or contradict an advisory while it is in effect.

This article is educational and is not a substitute for the current regulations, the AIM, official weather briefings, or instruction from a CFI. Check the current text at the linked sources before you rely on any detail.