It's more than just "lighter" in the background, though: It is less contrast and a decrease in certain specific hues as the background recedes. The human eye loses the ability to 'see' yellow hues first, over distance, then greens and orange, then red, then violets- but it keeps blues until whatever 'thing' is there is simply too far for a human eyeball to see.
You can see this for yourself if you have that kind of vista to look at. It's why we have "purple mountains majesty"- and why those mountains you KNOW are covered in pines and firs, don't look green: Given the distances of their heights, you can no longer see the yellows which inform the greens, you only get the blues.
Courtesy of science, and Deborah Christensen-Secor.
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