What type of weather phenomenon is conveyed through SIGMETs?

The correct choice conveys that SIGMETs (Significant Meteorological Information) are specifically issued to alert pilots to severe weather events that may impact aviation safety. This includes phenomena such as severe turbulence, thunderstorms with severe or frequent lightning, wind shear, and volcanic ash. The purpose of a SIGMET is to inform pilots about hazardous weather conditions that are significant enough to require immediate attention and could affect flight operations.

In contrast, minor turbulence and general thunderstorm activity do not warrant a SIGMET, as these conditions are typically addressed in other weather advisories. Cloud formations, while potentially significant in another context, do not directly indicate severe weather conditions that a SIGMET would cover. Thus, the focus of a SIGMET is distinctly on serious weather events that pose a direct risk to flight safety, making it essential for pilots to stay informed about these hazards while flying under IFR.