The primary cause of restricted visibility is fog, heavy rain, or
very rough seas. Haze or snow can also cause visibility to be restricted.
Sailing in restricted visibility presents hazards, such as collision with another vessel or object, and navigational errors, such as getting off course and running aground.
Fog is quite difficult to predict and is likely to take you by surprise.
Still, there are some warning signs that can help you to predict fog is coming; for example, a fogbank on the horizon, cold and moist air, general mistiness and progressively reduced visibility as the fog approaches, also misty and diffused navigation lights at night.