What type of weather phenomenon is indicated by a SIGMET?

A SIGMET, or Significant Meteorological Information, specifically refers to important weather phenomena that can have a considerable impact on the safety of aircraft during flight. This includes occurrences such as severe turbulence, thunderstorms with severe effects, volcanic ash events, and areas of icing.

SIGMETs are issued by meteorological authorities and are critical for informing pilots and flight crews about potentially hazardous conditions that may not be covered by routine weather reports. They are intended for immediate action and adjustment of flight paths to ensure safety. This is why the correct answer focuses on the significance of the weather information outlined in a SIGMET, emphasizing its impact on flight safety.

In contrast, a routine weather report does not convey the same level of urgency or specific threats and rather presents general weather conditions. A forecast for aircraft fuel efficiency does not relate to immediate weather threats. Lastly, a summary of past weather events serves a different purpose altogether, providing historical data rather than current safety-related alerts.