Visibility is less than 500 meters. What concept?

Indicates that the fog level has been reached. According to the inquiry of the paper. Com, the yellow warning signal standard of fog is: 12 hours, fog with visibility less than 500m appears, or fog with visibility less than 500m and greater than 200m has appeared and will continue, and fog with visibility of 500m is larger. Therefore, visibility less than 500 meters indicates that the fog level has been reached. Visibility means that under the weather conditions at that time, people with normal vision can distinguish the outline of the target (the smallest black recognizable target) from the sky background and determine the distance between the target and the observer in meters or kilometers.