Horizon distance
From Glossary of Meteorology
horizon distance
The distance, at any given azimuth, to the point on the earth's surface constituting the horizon for some specified observer.
In the particularly simple case of an observer at sea, the horizon distance s in miles corresponding to an observer whose eye is at a height h feet above sea level, is given by the relation which holds to a good degree of approximation as long as both s and h are small compared to the earth's radius.
