Visual-range formula

From Glossary of Meteorology

visual-range formula

The formula for the daytime visual range V,
where there is a contrast C between the horizon target and its background viewed through an atmosphere of extinction coefficient σ by an observer whose momentary threshold contrast is ε.

In practice, an important application of this formula is to the case of a black target (for which C = 1); hence
In calculations of the meteorological range, the threshold contrast is also assigned a constant value.