Advection fog formed when warm, moist air moves over a cooler sea surface, reducing visibility to less than 1,000 metres.
Sea fog is a form of advection fog that forms when relatively warm, moisture-laden air flows horizontally over a sea surface that is significantly cooler, chilling the air mass to or below its dew point and causing water vapour to condense into minute droplets. Unlike radiation fog, which forms overnight over land and typically disperses rapidly after sunrise, sea fog can persist throughout the day and can be remarkably dense and extensive. Classic sea fog regions include the Grand Banks off Newfoundland, the North Sea, the California coast, and the cold-current upwelling zones off South America and southern Africa.
For the watchkeeping officer, sea fog triggers mandatory obligations under COLREGS. Rule 19 (Conduct of Vessels in Restricted Visibility) requires a vessel to proceed at a safe speed adapted to the prevailing circumstances, with engines ready for immediate manoeuvre. Rule 35 requires appropriate fog signals at prescribed intervals. A proper lookout by all available means — radar, AIS, echo sounder, VHF — must be maintained at all times, and the master is typically called when visibility falls below a threshold set in the SMS.
Fog patches can form and thicken rapidly, giving little warning. Officers should monitor the relationship between air temperature and sea surface temperature as a precursor: when the two values converge, the risk of fog formation increases significantly. NAVTEX broadcasts include fog warnings for coastal waters, and routing services can advise on the expected extent and duration of fog areas on a planned passage.