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INSIGHTS / WEATHER PRODUCTS

AIRMET vs. SIGMET

Both are in-flight weather advisories issued for hazards to aircraft. The difference is severity, scale and who they are primarily written for. This guide summarizes the FAA framework (AIM 7-1-6 and AC 00-45) for planning context.

The short answer

A SIGMET warns of weather potentially hazardous to all aircraft. An AIRMET warns of weather that is primarily significant to light aircraft and smaller operators, at intensities below SIGMET criteria. For a single-engine piston pilot, both matter — an AIRMET can describe conditions already beyond your aircraft or personal minimums.

Non-convective SIGMETs

Issued for widespread phenomena not associated with thunderstorms. In the contiguous U.S. they are valid for up to 4 hours (6 hours for volcanic ash or hurricane-related products) and are identified by a phonetic name (e.g. NOVEMBER 1).

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

Convective SIGMETs

Issued hourly (at 55 minutes past) for the contiguous U.S., valid for 2 hours, and divided into East, Central and West regions (e.g. 12C). Special bulletins may be issued between scheduled times. Criteria include:

  • An area or line of thunderstorms covering at least 40% of 3,000 sq mi, or a line at least 60 mi long with 40% coverage
  • Embedded or severe thunderstorms expected for 30 minutes or more
  • Tornadoes, hail of 3/4 inch or greater, or surface wind gusts of 50 kt or greater

A convective SIGMET implies severe or greater turbulence, severe icing and low-level wind shear — these are not restated in the bulletin.

AIRMETs and G-AIRMETs

Issued every 6 hours with amendments as needed. The graphical G-AIRMET provides snapshots at 3-hour intervals and is the primary product; text AIRMETs are derived from it.

Sierra
IFR conditions (ceilings below 1,000 ft and/or visibility below 3 SM) affecting over 50% of an area, and extensive mountain obscuration.
Tango
Moderate turbulence, sustained surface winds of 30 kt or more, and non-convective low-level wind shear.
Zulu
Moderate icing and freezing levels.

Side-by-side comparison

ProductIntended forValidityTypical hazards
Convective SIGMETAll aircraft2 h, issued hourlyThunderstorms, hail ≥ 3/4 in, gusts ≥ 50 kt, tornadoes
SIGMETAll aircraftUp to 4 hSevere turbulence / icing, dust, volcanic ash
AIRMET / G-AIRMETLight aircraft primarily6 h cycleIFR, obscuration, moderate turbulence / icing, strong surface wind

Reading them against a route

The key questions are geometric and temporal: does your planned line pass through the area, at an altitude inside the stated levels, during the validity window? SIGMET.si tests exactly that — sampling the route every 0.5 NM and comparing modeled passage times and cruise altitude against each product. See the methodology for the exact calculation.

Then add aircraft context: an AIRMET Zulu over your route is a different decision in an aircraft without approved ice protection, and an AIRMET Sierra matters differently to a non-instrument-current pilot.

What an advisory does not tell you

  • The absence of a product does not mean the absence of a hazard.
  • Polygons describe areas, not exact cell positions; conditions evolve inside the validity time.
  • Pilot reports, METARs, TAFs, radar and NOTAMs complete the picture.

This guide is educational. Obtain an official briefing; the Pilot in Command retains final authority under 14 CFR § 91.3. Questions? See Help & FAQs.