Which type of weather phenomenon does SIGMET specifically address?

SIGMET, or Significant Meteorological Information, is issued for weather phenomena that may affect the safety of flight, particularly in relation to en route and terminal flight operations.

The correct answer points to volcanic eruptions, as SIGMETs are specifically designed to alert pilots to significant weather hazards including the presence of volcanic ash and activity that could pose severe risks to aviation. When volcanic eruptions occur, the ash clouds can extend over large areas and can significantly affect aircraft engines as well as visibility, thus classifying them under significant meteorological hazards.

While the other options represent various severe weather conditions, they are usually covered under different advisories or bulletins. For example, thunderstorms would typically be addressed by a type of advisory known as a Convective SIGMET, which is a subset of SIGMETs focusing on severe convective activity. Surface wind phenomena may be conveyed through different advisories like Wind Advisories or Special Weather Statements. Fog, while it can significantly affect visibility during approaches and landings, is often addressed through different means like terminal forecasts and flight advisories rather than through SIGMETs.