The greatest distance at which a black object of suitable dimensions can be seen and recognised by day against the horizon sky, or, in the case of night observations, could be seen and recognised if the general illumination were raised to the normal daylight level. The term may express the visibility in a single direction or the prevailing visibility in all directions.
Reference: W.M.O. (modified)
Symbol: V Unit: kilometre (km); sea-mile (s.m.)
Note 1: It has been established that in such circumstances an object may be seen and recognised if a certain value of the contrast threshold C is reached (2-1-295 and 2-1-300). For practical observations, the value of C = 0.05 has been agreed by the Reference: W.M.O. In the case of a uniform atmosphere, the visibility so defined is equal to, where s is the atmospheric extinction coefficient (2-1-270, Note 2:).
In Germany, the visibility so defined has the Symbol: V0.05 and is often called the Praktische Sichtweite.
Note 2: Under ideal conditions (in daylight laboratory observation), however, C is equal to 0.02. A visibility defined by this value has been called the Meteorological Range (and, in German, Normsichtweite, with Symbol: VN)
In the case of a uniform atmosphere, the visibility so defined is equal to , where s is the atmospheric extinction coefficient (2-1-270, Note 2:).
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
axis of the centre of water plane
Celsius (previously Centigrade)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM