How to explain the visibility in foggy days (daytime) in weather forecast?

It is an index reflecting the transparency of the atmosphere. The aviation industry defines it 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.

Visibility (visibility)

In meteorology, visibility is expressed by meteorological optical visible range. The visible range of meteorological optics refers to the optical path length through which the luminous flux of parallel beams emitted by incandescent lamps with a color temperature of 2700K weakens to 5% of the initial value in the atmosphere.

Daytime visibility refers to the maximum horizontal distance that people with normal vision (contrast threshold is 0.05) can see and identify the target (black, moderate size) from the sky background under the weather conditions at that time. In fact, it is also a meteorological optical range.