Definition
The full 360-degree circle around an observer at eye level, used as the reference for measuring prevailing visibility. Prevailing visibility is the greatest horizontal distance an observer can see and identify prominent unlighted objects by day, or prominent lighted objects by night, throughout at least half of this circle — though the visible halves do not need to be continuous.
Plain English
Imagine standing in one spot and slowly turning all the way around. The complete ring you scan around you is the horizon circle. To report prevailing visibility, weather observers look at how far you can see across at least half of that ring.
Context Anchor
Seen in weather observation and prevailing visibility discussions, where visibility is judged around the entire observation point.
Derivation
Horizon comes from the Greek 'horizein,' meaning 'to bound' or 'to divide' — the line that bounds what you can see. Pairing it with 'circle' captures the idea that the horizon is not just the line in front of you, but the full ring surrounding you in every direction.
Why Pilots Care
Correct use of the horizon circle produces accurate prevailing visibility reports that determine legal flight minimums, approach options, and go/no-go decisions.
Grounding Statement
Stand in the middle of an airport ramp and turn a slow, complete circle — every direction you look, out to where the sky meets the ground, is your horizon circle.
Intuition Check
Do not read horizon circle as only the visible line straight ahead. Here it means the complete 360-degree view around the observation point.
Example Sentence 1
Because visibility was four miles or more across at least half of the horizon circle, the observer reported prevailing visibility as four statute miles.
Example Sentence 2
Prevailing visibility is the greatest distance equaled or exceeded over at least half the horizon circle.