From AMS Glossary
In meteorological usage, contrast C is defined by
where L0 is the luminance of a target object and Ls is that of its immediate visual surroundings.
Both luminances may include airlight or reflected glare, and Ls is always assumed to be greater than zero. Because detection outdoors often depends on luminance rather than chromaticity differences, C is usually calculated from spectrally integrated luminances. Contrast for a perfectly black target is -1 but, in principle, C has no upper limit for a self-luminous target. If
is less than a variable threshold contrast, the target is visually indistinguishable from its surroundings.