Visibility is a measure of the distance at which an object or light can be clearly discerned. Visibility may vary according to the direction and angle of view, and the height of the observer. Visibility is affected by the presence of fog, cloud, haze and precipitation.
Visibility for aeronautical purposes is the greater of:
the greatest safe distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background;
the greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit black background.
