Definition
Under ICAO standards, visibility for aeronautical purposes is the greater of: (a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background; or (b) the greatest distance at which lights in the vicinity of 1,000 candelas can be seen and identified against an unlit background. These two distances may differ in a given mass of air, and the larger of the two is reported as the visibility.
Plain English
How far you can see, measured by ICAO using two tests: how far away a dark object stands out against a bright background in daylight, or how far away a light of about 1,000 candelas can be picked out at night. Whichever distance is greater is the reported visibility.
Context Anchor
Seen in international aviation weather reports, METARs, SPECIs, and discussions of ground visibility.
Derivation
From Latin visibilis, meaning 'able to be seen.' The ICAO tag indicates this is the definition adopted by the International Civil Aviation Organization, which may differ slightly from the FAA definition used domestically in the United States.
Why Pilots Care
It decides whether visual flight rules apply and affects every decision about takeoff, landing, and seeing other traffic.
Grounding Statement
If haze, rain, fog, smoke, or darkness keeps you from recognizing objects or identifying lights beyond a certain distance, that distance is the visibility being reported.
Intuition Check
Do not read visibility as simply “the view looks clear.” In aviation, visibility means a reported distance based on what can be seen and recognized or what lights can be identified.
Example Sentence 1
The METAR from the European destination reported a visibility of 8 kilometers, measured to ICAO standards.
Example Sentence 2
Fog dropped visibility below minimums, so we requested an instrument approach.