Why is the Sky Blue?
Again we have it explained by our brilliant favorite electrical engineering researcher from the University of Washington Bill Beaty:
The typical answer will immediatly delve into physics. But at the start that’s not really necessary. Detailed physics will distract us from the simple answer. The true answer isn’t very complicated. There is no sky. The sky is an illusory surface. “The sky” isn’t colored blue, since that blue surface up there, it doesn’t actually exist.
Think about it: during overcast days, is our sky colored gray …or is it really just clouds which are gray? During bright morning fog, is “the sky” white, or is it the morning sun shining on white fog? OK then, whenever there’s no fog and no clouds, what up there is colored blue?
Air is blue.
A thick layer of air is a blue-colored substance. And the only thing up there is the air.
When we look upwards, we’re seeing air, we’re looking at brightly sunlit nitrogen and oxygen. There’s nothing up there but bright blue air, seen against the black of outer space. (Air isn’t perfectly transparent as everyone thinks.) When light shines on air from the side, air is colored bright blue.
On the other hand, air isn’t blue like dyes are blue. Instead, air is blue like opals, like blue iridescent soap bubbles, bluejay feathers, blue aerogel, Morpho butterfly wings, and blue-colored human eyes. The blue color of sunlit air is a “structural color,” not an absorption color like paints and dyes. Air is blue because, when light shines upon it, the red-orange part of the light goes straight through, and the blue/violet part is scattered outwards. The same thing happens with aerogels and opal minerals (look through an opal and you see orange.) The same happens with bluejay feathers: the feathers themselves are black, but with a clear opalescent coating, where the coating passes orange light into the black feather below, but bounces blue light back out. (Bluejays are crow family, with black feathers!) A thick layer of air works the same, but the black feather is the black of outer space.
Also, the blue of the air is very weak, a dilute effect, and in order to see it, first we need a very thick layer of air. Ten or twenty miles thick, that’d be good. So, when you look at distant dark forested hills during a clear sunny day, they don’t look green-black. They look foggy blue. You’re seeing the bright-lit blue air in front of them. When you look up, you should be seeing the black of outer space. But instead you’re seeing the foggy blue layer of air that’s in front of the black.
Above famous NASA photo, showing the blue atmosphere glowing in the sunlight. Imagine diving down into that stuff. It would make outer space appear blue instead of black! You’d still see the moon of course, but it would look foggy blue, against a blue background.
Here’s an odd thought: when we look upwards in the late afternoon, we’re actually looking at the horizontal sun-rays passing overhead; rays which fly far to the east. Off in the east they become the red sunset colors of someone’s evening. The sun-rays look bright blue from the side. Our blue “sky” above us is actually the portion of blue light that’s missing from someone else’s sunset.
Another odd thought: if you were a giant, a thousand miles tall, standing on the ground, you could bend down and scoop up a handful of Earth’s atmosphere. Hold it out in space, and examine it closely. Against the black of space, it would look like sky-blue smoke! When held in sunlight, it would look like a bright blue gas. But if you poured it out in front of the sun, it would appear bright red-orange. And, if you held your hand 100KM above the earth, its shadow would block the sunlight from shining on the blue atmosphere. People down on the ground wouldn’t see the blue sky there. Instead they’d see a giant black hand-shape, like a hole punched in the blue “sky surface” above them.
Why does air act like this? Why colored blue, when light shines on it, but orange when light shines through it? Why does aerogel do the same? And also cigar smoke? Also, why isn’t the air white, since it scatters light similar to fog or a dust cloud? Now you’re ready for the physics. (But you don’t really need it anymore, since you already know why the sky is blue. Air is blue-colored substance, and there is no sky up there, just a thick blue blanket of sunlit air against the dark of outer space.)
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