What is visibility in weather?
According to the World Meteorological Organization (WMO) observation guidelines, atmospheric visibility is a physical quantity of atmospheric transparency (or turbidity) described by distance from the point of view of whether the target object can be seen and identified. Meteorological Visibility by Day refers to the maximum distance that a suitable scale black object placed near the ground can be seen and identified when observed against scattered light background such as fog and sky.
It must be emphasized that the criterion used is to identify the target, not just to see the target but not to recognize what it is.
Meteorological Visibility at Night refers to the maximum distance from which an appropriate scale of black object can be seen and recognized, or the maximum distance from which a medium intensity luminous object can be seen and recognized, assuming that the overall illumination increases to the normal daytime level. The latter definition is more in line with the actual operation of night visibility measurement.