What type of weather phenomena are outlined in a SIGMET?

A SIGMET, or Significant Meteorological Information, is specifically designed to highlight weather phenomena that could significantly affect the safety of flight operations. These include severe thunderstorms, tornadoes, icing, turbulence, and volcanic ash clouds, among others. The key feature of a SIGMET is that it addresses weather phenomena that are significant enough to pose a hazard to aircraft in flight. Therefore, the correct answer centers around the nature and thresholds of weather phenomena that qualify for a SIGMET.

The option that states "weather impacting flight but below SIGMET thresholds" does not accurately capture the essence of what a SIGMET entails. SIGMETs are issued for significant weather events exceeding certain thresholds that have the potential for widespread effects on aviation. Weather phenomena that fall below these thresholds are typically communicated through other advisories, such as AIRMETs, rather than SIGMETs. Therefore, the chosen answer fails to recognize that SIGMETs are intended for serious weather conditions that warrant immediate attention from pilots and air traffic control.