The detailed characteristics of fog over South Korea were analyzed using the three-years quality controlled (QC) 237 visibility meter data operated by Korea Meteorological Administration. The fog (dense fog: DFog) frequency varies greatly with season and geographic location. The fog frequency at inland is highest in autumn, but at the West Coast in spring and summer. Fog occurs frequently from spring to autumn in the mountainous regions. Unlike the fog, the DFog is mostly prevalent in summer at land, mountain, and coastal regions. The large coefficients of variation of fog and DFog at the three regions and four seasons indicate that the locality of fog over South Korea is very high. The formation and dissipation (FaD) of fog show strong diurnal variations irrespective of geographic location and season, strongest at inland and weakest at sea. Fog usually occurs from night to sunrise and dissipates from early morning to late morning. The maximum FaD time of fog show seasonal variation with the seasonal change in solar elevation angle. The frequency of fog is inversely proportional to the duration time, mostly less than 3 hours regardless of season and geographic location. Also, the duration of DFog is mostly within 1-3 hours.

This article aims at suggesting a practicla auide to navigation under restricted visibility by a study of an English court. This case os a good example of setting out safe speed. close-quarters situation, safe passing distance as the above factors are not enumerated in Collision Regulations.