What is the visibility without a trace of fog?

The classification standard of fog is divided according to the horizontal visibility distance: ① Light fog is called when the horizontal visibility distance is between 1 ~ 10 km. ② The horizontal visibility distance below1km is called fog. (3) The distance between the horizontal visibility of 200-500m is called fog. (4) The horizontal visibility distance between 50 and 200 meters is called dense fog. ⑤ Fog with horizontal visibility less than 50m is called strong fog. Visibility is an indicator of atmospheric transparency. How many meters of visibility is defined as the maximum distance that a person with normal vision can see the outline of the target clearly under the weather conditions at that time. Visibility is closely related to the weather conditions at that time. When rainfall, fog, haze, sandstorm and other weather processes occur, the atmospheric transparency is low, so the visibility is poor. Atmospheric visibility can generally be measured by visual inspection, and can also be measured by measuring instruments such as atmospheric transmittance instrument and laser visibility automatic measuring instrument.