Glossary: Visibility

Visibility

Visibility is defined as the greatest distance an object can be seen and identified. It is usually a visual estimate and is reported as prevailing visibility -- the maximum visibility common to one half or more of the horizon circle. When visibility is uniform in all directions, prevailing visibility and visibility are the same. When visibility is not uniform in all directions, you can determine prevailing visibility by dividing the horizon circle into sectors of visibility. Estimate the highest visibility common to one half or more of the horizon circle.

The maximum visibility on a clear day is 11 kilometres (7 miles), after which a flat ground horizon will fall away to a point that surface conditions cannot be observed. In order to report visibility beyond this distance visibility markers are used. These markers are measured and could include mountains/hills, tree lines and microwave towers. The visibility is determined by the weather observer at an airport or weather station. The observer reports how far away objects can be identified without obstruction.