Daytime visibility is defined as the greatest distance at which it is possible to see and identify, with the naked eye, a prominent dark object against the sky at the horizon.
Daytime visibility reports are estimates based on local landmarks and conditions.
Nighttime visibility is the greatest distance at which it is possible to see a known light source of moderate intensity.
For a pilot making a nighttime takeoff on a runway, visibility can be measured with equipment like a transmissiometer, a narrow light beam focused on a photoelectric cell 250 feet away.
The photo cell’s output depends on the amount of light it receives through the atmosphere.
The weakening, or attenuation, of the light beam is a measure of how well the atmosphere transmits light and is related to what the human eye, or the pilot’s eye can see.
If the light is not attenuated by atmospheric conditions, the visibility is said to be unlimited.
If the light is attenuated 100 percent, the visibility is less than 250 feet, the distance from light source to sensor.