Definition
A SIGMET is an in-flight weather advisory issued by the National Weather Service warning of weather hazards considered significant to all aircraft, regardless of size or type. SIGMETs cover non-convective hazards including severe or extreme turbulence not associated with thunderstorms, severe icing not associated with thunderstorms, and widespread dust storms, sandstorms, or volcanic ash that lower surface or in-flight visibilities to below three miles. In the contiguous United States, SIGMETs are issued for four-hour periods (six hours for volcanic ash) and identified by an alphabetic designator from November through Yankee, excluding Sierra and Tango.
Plain English
A SIGMET is an official weather warning sent out to pilots in flight about serious weather that could be dangerous for any aircraft, such as severe turbulence, severe icing, dust storms, or volcanic ash.
Context Anchor
Seen in preflight weather briefings, aviation weather products, and instrument flight planning when checking for hazardous weather along a route.
Derivation
Built from SIGnificant METeorological information. 'Meteorological' comes from the Greek meteoron, meaning 'thing high in the air' — the same root that gives us 'meteor.' It reminds you this is weather information serious enough to be flagged for every pilot.
Why Pilots Care
Pilots must review SIGMETs to avoid hazardous weather that could lead to loss of control or structural damage.
Grounding Statement
If a large area of air contains weather that could seriously threaten an aircraft, a SIGMET is one of the ways that warning is officially passed to pilots.
Intuition Check
“Significant” does not mean simply noticeable or interesting here. It means serious enough to matter to flight safety for any aircraft in the affected area.
Example Sentence 1
During the preflight briefing, the pilot noted a SIGMET for severe icing along the planned route above 12,000 feet and chose a lower cruise altitude.
Example Sentence 2
During the flight, an updated SIGMET warned of icing conditions along the route.