How is visibility measured?

Dear Tom,

We are studying weather at school How is visibility measured?

Thanks,
Isabella D.
Scullen Middle School
Naperville

Dear Isabella,

Before the era of automated weather observations, visibility was determined by trained observers who noted the most distant object that could be seen at a known distance, such as a building, tower or geographical landmark. At night, the observers relied on unfocused lights, as the darkness made most daytime markers unusable. Today, automatic observing systems use complex algorithms and data based on returns from a laser beam transmitter and a receiver to compute the visibility. Observers can augment the computer readouts during rapidly changing conditions or when the computer measurements have been deemed inaccurate. Ten miles is the maximum visibility reported by today’s system.

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