When will an AIRMET-S be upgraded to a SIGMET?

The correct interpretation of when an AIRMET-S (a type of advisory for non-convective weather phenomena) will be upgraded to a SIGMET (significant meteorological information) revolves around the criteria defined for significant weather conditions.

In this context, the distinction leading to the upgrade from an AIRMET-S to a SIGMET is primarily focused on the extent and impact of weather phenomena. When conditions cover an area greater than 3,000 square miles, it signals that the weather conditions are extensive enough to warrant greater urgency and increased attention from pilots and air traffic controllers. This is crucial because a SIGMET indicates significant weather that could seriously affect the safety of aircraft operations, such as severe turbulence, thunderstorms, or icing, especially when they occur over large areas.

While conditions like clear icing forecasts or low ceilings are important, the defining factor that necessitates an upgrade to a SIGMET, in this case, is the size of the affected area. This is why option C is the correct point of focus, reflecting the critical nature of the coverage area when it comes to aviation safety considerations.