Horizontal visibility is defined as the maximum distance up to which prominent objects on the earth’s surface can be seen and identified by day and prominent lighted objects of a given brightness, on the earth’s surface, can be seen by night.
Horizontal visibility may be estimated by observing suitable objects or lights at known distances.
The horizontal visibility value reported in meteorological reports – referred to as meteorological visibility – may be either the minimum visibility observed in any direction or the visibility which prevails over more than half the horizon (prevailing visibility). There is no common world-wide procedure in this respect.
Whenever conditions of restricted visibility exist, neither method meets the operational requirements, since, by virtue of the concepts of weather minima, what the pilot requires to know is whether adequate visual guidance will be obtained on the approach to the runway is use.
This is, however, not a matter of horizontal visibility but of ’slant visual range’ (SVR). Until such time that slant visual range observations are available ’runway visual range’ (RVR) observations are made in conditions of restricted visibility.
For reports for take-off the meteorological visibility observations must be representative of the take-off and climb-out area, and for reports for landing the observations must be representative of the approach and landing area. Visibility observations made for reports disseminated beyond the aerodrome must be representative of the aerodrome and its immediate vicinity; in such observations special attention should be given to significant directional variations.
In reports in abbreviated plain language, the name of the element should be given and the units used for visibility should be specified clearly. When the visibility is 5 kilometres or less, it should be expressed in increments of 100 metres, in the form ’VIS 2300 m’; when it is more than 5 kilometres, but less than
10 kilometres, the units should be kilometres, in the form ’VIS 7 km’; and when it is 10 kilometres or more, it should be given as 10 kilometres, except when the conditions for the use of CAVOK apply.
1. Horizontal visibility aspects
2. Visual range
3. Observing techniques of RVR
4. Observing of RVR
5. Availability of RVR observations at ATS units
6. Reporting procedures of RVR reports
7. Accuracy of RVR reports
8. Slant visual range
9. The visual segment
10. Variation in visual segment
11. Flight visibility and vision at high altitudes