Fog

Suspended microscopic water droplets or ice crystals near the surface that reduce horizontal visibility below 1,000 metres.

Fog forms through several processes, including cooling moist air to saturation, moving warm moist air over colder water, radiation cooling and evaporation into cold air. Mist is the corresponding obscuration when visibility remains at least 1,000 metres under the meteorological definition.

At sea, visibility can vary rapidly and differ with observer height and position. Navigation requires an appropriate safe speed, effective lookout by sight and hearing, prescribed sound signals and careful use of radar and other aids; AIS and GNSS do not detect every vessel or obstruction.