In simple words, Visibility is the transparency of the atmosphere and for meteorological purposes visibility is defined as the greatest distance at which an object with specified characteristics can be seen and identified by the unaided eye in daylight.
At night it is assumed that the illumination of the object is raised to normal daylight level. Visibility is assessed by viewing the horizon through 360° and recording the shortest distance. Land observing stations use objects at known distances in daytime and a visibility meter at night, thus making it possible to provide accurate visibility ranges.
