Fog is a cloud of tiny water droplets suspended at or near the sea surface, reducing visibility — by international convention, “fog” specifically refers to conditions reducing visibility below 1,000 meters.
Main Types Affecting Ships
- Advection fog — forms when warm, moist air moves over a colder sea surface, common where warm and cold ocean currents meet; the most significant fog type for shipping in many regions
- Radiation fog — forms over land or coastal waters on clear, calm nights as the surface cools; can drift offshore
- Sea smoke (steam fog) — forms when very cold air moves over relatively warmer water, common in polar and sub-polar regions
Operational Impact
Fog is one of the most operationally significant weather conditions at sea because it directly triggers specific COLREGs requirements:
- Sound signals — specific fog signals (prolonged blasts at set intervals) must be sounded
- Reduced speed — vessels must proceed at a safe speed appropriate to restricted visibility
- Increased reliance on radar — since visual lookout alone becomes unreliable, radar-based collision avoidance becomes primary
High-Risk Fog Areas
Certain regions are known for frequent, significant fog — the Grand Banks off Newfoundland and parts of the North Pacific are classic examples where advection fog regularly affects shipping.
Exam Relevance
Fog signals, safe speed in restricted visibility, and COLREGs Rule 19 (conduct in restricted visibility) are core Navigation and Meteorology topics in deck officer competency exams.