Definitions for "fog signal"
Keywords:  whistle, horn, bell, loud, siren
a loud low warning sound that can be heard by fog-bound ships; a foghorn.
A device used as an aid to navigation in fog or darkness. Some types of devices used are cannons, bells & whistles.
A bell, horn, whistle or other device that sounds an alarm, often automatically, near places of danger where visible signals would be hidden in rainy or foggy weather.