Definition
An inflight weather advisory issued by the National Weather Service warning of weather phenomena that are potentially hazardous to all aircraft, regardless of size. SIGMETs cover severe icing not associated with thunderstorms, severe or extreme turbulence or clear air turbulence not associated with thunderstorms, dust storms or sandstorms lowering surface visibility below three miles, and volcanic ash.
Plain English
A SIGMET is a weather warning sent to pilots in flight about serious weather that could be dangerous to any aircraft, big or small. It alerts pilots to things like severe icing, severe turbulence, dust storms, or volcanic ash.
Context Anchor
Pilots may see SIGMETS during a weather briefing or hear them in hazardous in-flight weather broadcasts while already flying.
Derivation
From SIGnificant METeorological information. The name signals its purpose: a flagged weather report serious enough that every pilot needs to know about it.
Why Pilots Care
They mark areas pilots must avoid or prepare for to prevent encounters with flight-threatening weather.
Intuition Check
Do not treat a SIGMET as a routine weather note. A SIGMET means the reported or expected weather is hazardous enough to matter to all aircraft, not just small or lightly equipped ones.
Example Sentence 1
Flight Service advised us of a SIGMET for severe turbulence along our planned route, so we requested a lower altitude and a deviation south.
Example Sentence 2
An updated SIGMET warned of volcanic ash, so the crew rerouted the flight to stay clear.