Visibility (meteorological)

Meteorological visibility is the greatest horizontal distance at which a suitable dark object can be seen and recognized against the horizon sky by day, or a known moderate light identified by night. It is the meteorological optical range reported in weather observations, reduced by fog, mist, haze, precipitation, and spray. The mariner separates it from the geographic and luminous range of a light: visibility is the state of the atmosphere, while the range a light is actually seen depends also on its height, the height of eye, and its intensity. Visibility sets the safe speed required by COLREGs Rule 6 and triggers the restricted-visibility conduct of Rule 19 and the fog signals of Rule 35.