Definition
In-flight weather advisories issued by the National Weather Service to alert pilots to potentially hazardous weather. SIGMETs warn of severe conditions affecting all aircraft, such as severe icing, severe or extreme turbulence, widespread duststorms, sandstorms, or volcanic ash. Convective SIGMETs cover thunderstorm-related hazards. AIRMETs warn of less severe but still significant conditions primarily affecting smaller aircraft, including moderate icing, moderate turbulence, sustained surface winds of 30 knots or greater, widespread reduced visibility below 3 statute miles, ceilings below 1,000 feet, and mountain obscuration. On an MFD, these advisories appear as overlays showing the geographic areas they cover.
Plain English
Official weather warnings sent to pilots while they fly. SIGMETs are the more serious warnings — bad icing, bad turbulence, thunderstorms, volcanic ash. AIRMETs are the lower-level warnings — moderate icing, moderate turbulence, strong surface winds, low clouds, and poor visibility. The MFD can display the areas these warnings cover so the pilot can see if their route is affected.
Context Anchor
Seen as a weather layer or label on an electronic flight display or multi-function display, often shown as SIG/AIR.
Derivation
SIGMET combines 'significant' and 'meteorological.' AIRMET combines 'airmen' and 'meteorological.' Knowing this helps a pilot remember that SIGMETs flag the more significant (severe) hazards, while AIRMETs are aimed at all airmen for less severe but still important conditions.
Why Pilots Care
These advisories directly affect route selection, altitude choice, and go/no-go decisions to avoid weather that can damage the aircraft or reduce pilot control.
Intuition Check
Do not treat SIG/AIR as just a general weather picture. It marks official areas of concern, and a SIGMET is more serious than an AIRMET.
Example Sentence 1
Before departure, the pilot checked the MFD and saw a SIGMET for severe turbulence along the planned route, so the flight was delayed.
Example Sentence 2
An updated AIRMET for turbulence caused the pilot to request a lower altitude from air traffic control.