Is there a visibility standard for fog?

Atmospheric visibility is an index reflecting atmospheric transparency. It is generally defined as the maximum horizontal distance on the ground where people with normal vision can see the outline of the target clearly under the weather conditions at that time. There is also a definition of the minimum distance at which the last feature of the target disappears. Generally speaking, for the same goal, the visibility determined by these two definitions is different, and the latter is greater than the former.

Visibility is a meteorological factor that has an important influence on aviation, navigation, land transportation and military activities. In the aviation field, visibility defined by the former is generally used.

Fog yellow warning signal:

Standard: Fog with visibility less than 500m may appear within 12 hours, or fog with visibility less than 500m and greater than 200m has appeared and will continue.

Fog orange warning signal:

Standard: Fog with visibility less than 200m may appear within 6 hours, or fog with visibility less than 200m and greater than or equal to 50m has appeared and will continue.

Fog red warning signal:

Standard: Fog with visibility less than 50 meters may appear within 2 hours, or fog with visibility less than 50 meters has appeared and will continue.