In conjunction with ceiling, it determines whether or not to close the airport.
Visibility is decisive for pilots and crews.
They must determine the nature and horizontal and vertical extent of any serious obstacle to visibility that may affect their route or destination, each time they take off. Most people define visibility as the greatest distance at which an object can normally be identified. As used in observations and weather forecasts, however, the word has a different meaning, in that it is prevailing visibility that is measured and forecast. 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.