Visibility is defined as the greatest distance in a given direction at which it is just possible to see and identify with the unaided eye during daytime conditions, a prominent dark object against the sky at the horizon, and at night, a known, preferably unfocused, moderately intense light source that is visible over half or more of the horizon.
After visibilities have been determined around the entire horizon circle, they are resolved into a single value of prevailing visibility for reporting purposes. Daytime estimates of visibility are subjective evaluations of atmospheric attenuation of contrast, while night time estimates represent attempts to evaluate something quite different, namely, attenuation of flux density. Thus, visibility data must be regarded as falling into two distinct classes, those obtained by day, and those by night. Generally weather observing practice is the value as obtained and reported by an observer.
Meteorological visibility is expressed as “GOOD” or kilometres or metres. An aerodrome forecast express visibility that is greater than 10KM as a four figure group code “9999”.
“GOOD” or “9999” is used in the visibility section of low level area forecasts to indicate a visibility greater than 10KM over the entire area. When weather elements are forecast to reduce the visibility below 10KM, “GOOD” or “9999” is replaced by those elements and their associated visibilities. Note that the visibility remains greater than 10KM in parts of the area unaffected by those elements. (e.g. “GOOD” or “9999” or 8KM or 3000m)