Definition
The greatest horizontal visibility equaled or exceeded throughout at least half of the horizon circle, which need not necessarily be continuous. It is the visibility value reported in standard aviation weather observations such as METARs.
Plain English
Look around the full circle of the horizon. Find the visibility distance that is matched or beaten across at least half of that circle. That distance is the prevailing visibility, and it's the number reported in the weather.
Context Anchor
You will see prevailing visibility in airport weather reports, recorded weather broadcasts, and preflight weather briefings.
Derivation
Prevailing' comes from the Latin 'praevalere', meaning 'to be stronger' or 'to have greater force'. In weather use, it points to the visibility distance that 'wins out' across the majority of the horizon, rather than the best or worst patch.
Why Pilots Care
It determines whether visibility meets legal minimums for VFR flight, takeoff, and landing.
Analogy
Imagine standing in the middle of a circle and looking all the way around. If you can see 5 miles in enough directions to cover at least half the circle, the prevailing visibility can be 5 miles even if one smaller area is worse.
Grounding Statement
At an airport, fog may reduce visibility in one direction while the view is clearer in most other directions; prevailing visibility describes the larger overall condition.
Intuition Check
Do not assume prevailing visibility means the best visibility anywhere around the airport. It means the greatest visibility that applies across at least half of the horizon around the observer.
Example Sentence 1
The METAR reported a prevailing visibility of three statute miles, which was just enough to begin the approach.
Example Sentence 2
Even with lower visibility in one quadrant from showers, the prevailing visibility remained ten miles.