ICAO附件Annex 3 – Meteorological Service for International Air Navigation：

定义：

Visibility： Visibility for aeronautical purposes is the greater of:
a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background;（这个应该是指白天）
b) the greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit background.（这个应该是指晚上）

Prevailing visibility：The greatest visibility value, observed in accordance with the definition of “visibility”, which is reached within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors.（至少一半以上的地平线或机场表面，连续或不连续。所以这个能见度对落地的飞机没什么用处，关键要看RVR）