Non-Uniform Visibility
A localized visibility which varies from that reported in the body of the report.
Prevailing Visibility
The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Sector Visibility
The visibility in a specific direction that represents at least a 45º arc of a horizontal circle.
The distance at which a given standard object can be seen and identified with the unaided eye
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.

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