Fog Signals

Some lighthouses and navigation marks have fog signals — sound signals activated in poor visibility. These include diaphones (two-tone), sirens, horns, and bells. The character of the fog signal (e.g., 'Horn(2) 30s' = 2 blasts every 30 seconds) is shown on the chart.

Sound travels unpredictably over water — it can be reflected, refracted, and absorbed, making it unreliable for estimating distance or direction. Fog signals confirm you are near a mark but should not be relied upon for precise positioning.