noun
fog signal
A signal used in foggy conditions, typically by ships or lighthouses, to warn of hazards or indicate position.
霧が濃くなると、船は霧中信号を鳴らし始めた。
When the fog thickened, the ship began sounding its fog signal.
Compound of 霧中 (むちゅう, 'in fog') and 信号 (しんごう, 'signal').