Prevailing visibility requires that the visibility be equal to or exceeded for what portion of the horizon?

• A. One quarter
• B. One half
• C. Three quarters
• D. The entire horizon circle

Answer: (B)

Prevailing visibility is defined as the greatest distance at which objects can be seen through at least half the horizon around you. In practice, a pilot needs a measure that reflects what can be seen in most directions, not just in one spot. So the requirement is that visibility be equal to or exceed that distance for at least half of the horizon circle. That’s why half the horizon is the standard: it provides a representative, practical sense of visibility for flight operations.

If visibility only meets the distance in a smaller portion of the horizon (like a quarter), it wouldn’t reliably represent what you’d experience while scanning around. Requiring the entire horizon would be too strict for typical weather variations, and the 3/4 option isn’t the defined threshold.