Fog signals are sounds produced to warn ships of danger in restricted visibility. They are usually horns, bells or sirens, and can be either manually operated or automated.
In manual operation, the signal is sounded by someone on board the ship who sees the danger. Automated fog signals are usually triggered by radar or other electronic means.
Fog signals can be very effective in warning ships of danger, but they can also be a nuisance to nearby residents. For this reason, their use is regulated by maritime authorities.