8.26 Visibility and Fog

8.26.2 Define prevailing visibility.

Visibility refers to how far an observer can see through the atmosphere.

Meteorological visibility is measured in the horizontal plane and is based on how far an observer on the ground can correctly identify distant objects of suitable size against the horizon sky.

Prevailing visibility is the greatest horizontal visibility value that is reached or exceeded throughout at least half of the horizon circle, either continuously or in separate sectors.

8.26.4 Explain why illumination from the sun or moon has no effect on prevailing visibility.

Visibility is a measure of the transparency of the atmosphere, not simply how bright the light source is.

Atmospheric transparency depends on the amount of particles suspended in the air, such as:

• water droplets
• ice crystals
• rain
• drizzle
• fog
• haze
• smoke
• dust
• sea salt

These particles scatter and absorb light, reducing visibility.

During the day, natural light from the sun allows visibility to be assessed. At night, lights of known intensity are used. The same visibility criteria apply, because the measurement is still based on how transparent the air is, not on the amount of moonlight or sunlight available.