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The definition of Prevailing Visibility is:

"Prevailing visibility is the horizontal distance over which objects or bright lights can be seen and identified over at least half of the horizon circle."

My question is simple: What does "half of the horizon circle" means?

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Definition

From WMO Technical Regulations, volume II, Meteorological Service for International Air Navigation:

Prevailing visibility. The greatest visibility value, observed in accordance with the definition of “visibility”, which is reached within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors.

Note: This value may be assessed by human observation and/or instrumented systems. When instruments are installed, they are used to obtain the best estimate of the prevailing visibility.

Explanation

For someone standing on the ground, the horizon is the limit between sky and ground, and when the ground is "flat", is a circle which the person is the center.

objects or bright lights can be seen and identified over at least half of the horizon circle.

Half of the horizon circle means 180°. So visibility is the distance objects can be seen at least on 180°. This angle has not to be in a single sector, this can be 3 sectors of 60°, 20 sectors of 9° or any other combination.

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