Ask The Science Experts

One way would be for two people communicating wirelessly to start in a wide-open area and start walking until they could no longer see each other. Of course this is ridiculous. The way it’s usually done, by airports and such, is by looking at predetermined landmarks of known distances. Daytime visibility is defined as the distance in which is is possible to see and identify, with the naked eye, “a prominent dark object against the sky at the horizon.”Nighttime visibility is the distance at which it’s possible to see a known light source at “moderate density.” But a more accurate way airports measure visibility is by using a transmissiometer, which is a narrow light beam focused on a photoelectric cell 250 feet away. If the cell receives all of the light, visibility would be considered unlimited. If it receives none of the light, visibility would be considered “less than 250 feet.” Any variation in between would be calculated accordingly.

Source: The New York Times Book of Science Questions and Answers.

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