in Thoreau’s Journal:

The sky has gradually become overcast & now it is just beginning to snow—looking against a dark roof—I detect a single flake from time to time—but when I look at the dark side of the woods 2 miles off in the horizon there already is seen a slight thickness or mistiness in the air—In this way, perhaps, may it first be detected…..Pines & oaks seen at a distance—say 2 miles off—are considerably blended & make one harmonious impression—the former if you attend—are seen to be of a blue or misty black—and the latter form commonly a reddish brown ground, out of which the former rise—These colors are no longer in strong contrast with each other—