In-Flight and Reported Visibility
To measure the prevailing visibility at an airport, the horizontal ground visibility at a height of 1.8 m is estimated in all directions. The maximum visibility that applies for at least half the horizon is considered the prevailing visibility. This usually, although not always, gives a clear indication of conditions affecting take-off and landing.
During flight or while landing, however, it is the slant visual range that is most important, that is in-flight visibility from the aircraft. The reference used is the distance to the farthest identifiable point on the ground. Weather forecasts and reports give only an indirect indication of the slant visual range.