What is a defining characteristic of Sigmets?

A defining characteristic of Sigmets is that they are non-scheduled warnings issued to alert pilots about significant weather phenomena that can impact flight safety. These phenomena can include severe turbulence, thunderstorms, icing, and volcanic ash, among others. Unlike other weather reports that may be issued on a regular schedule or that provide routine updates, Sigmets are created in response to specific, significant weather events that require immediate attention from pilots.

The non-scheduled nature of Sigmets ensures that they are only issued when conditions warrant, thus providing focused and timely information to those in flight. This is critical for high-altitude and IFR operations since the safety of aircraft can be directly influenced by rapidly changing weather conditions. Such warnings are essential for situational awareness and making informed decisions in-flight, emphasizing the importance of their timely issuance when certain weather phenomena occur.