skyColorInfluence property
How much the fog color is taken from the sky instead of color (0 = flat color, 1 = fully the environment sampled in the view direction). Above 0, far geometry fades into the actual sky/horizon behind it (aerial perspective) rather than a flat wall of color. Sampled from the image-based-lighting environment, so it applies to lit materials; unlit materials always use the flat color.
Implementation
double skyColorInfluence = 0.0;