Blue Skies

Good astronomy: The sky is blue because it is scattering blue light from the sun.

[Note (Nov 11, 1998): there is a mistake in this page which I have corrected. I have put the correction in immediately following the offending statements!]

Here's a simple refutation of this Bad Idea: if the sky were blue due to reflection from the oceans, wouldn't the sky be less blue farther inland? It isn't, so this cannot be the correct explanation.

We actually need to start this explanation far from the Earth: we need to look at (figuratively, of course) the Sun. The path of your typical photon from the Sun to the surface of the Earth is a tortuous one. Created in the Sun's core, it takes up to one million years for the photon to make its way to the Sun's surface and then to race away. After all that time it only takes a little over 8 minutes to reach the Earth, but when it hits the top of the Earth's atmosphere, it still has one last dance to perform.

As a photon encounter particles (such as nitrogen and oxygen molecules) in the Earth's atmosphere, it scatters off these particles. In other words, it hits a molecule and rebounds off in some other direction. The amount of scattering depends on a lot of things, but mostly on the amount of stuff in the air and the wavelength, or color, of the light. The shorter (bluer) the wavelength of light, the more it is scattered. Ever notice how red brake lights on cars are easily visible through even a relatively dense fog? That's because red light scatters less. If brake lights were blue, you'd be having a lot more unexpected meetings with cars in front of you!

[Oops! I blew it here. The particles in fog-- tiny water droplets -- do not scatter any colors preferentially. If they did, then fog wouldn't look white, it would look blue! I put in that part because I have seen red lights better through fog than green. However, red light actually travels less through water than blue! This is why the oceans are blue; they absorb red light so that only the blue part gets out. I am not positive this also happens in fog, but I will do some research and find out. Until then, ignore the last part of the above paragraph! --The Bad Astronomer, 11/11/98]

Now sunlight may look white, but actually it is made up of many colors; in fact, it is a smooth blend of all the colors in the rainbow. That's why, when you pass light through a prism, you see a rainbow: the prism breaks up the white light into its component colors.

When a stream of photons of all different colors comes into the atmosphere, the red, orange, yellow, and even green ones tend to get through unimpeded. But the photons that are more blue tend to get scattered away. They fly off in some other direction, until they hit another particle and scatter again. When you look up in the sky at the Sun (of course, you should never ever look at the Sun without lots of special protection, but hey, this is a description, not an order), chances are you'll see very few blue photons coming directly from it; they have all been scattered away. Those scattered photons still get to the ground, and to your eyes, but they will come from some random direction (from wherever they were scattered last before they hit your eyes). So everywhere you look you see blue photons, except directly from the Sun, where you'll see all colors. This is why the sky is blue!

(Note that there are still lots of blue photons coming from the direction of the Sun: while blue photons scatter the most, not all the blue photons are scattered! Also, there will be some photons coming from the direction of the Sun because after the initial random scattering, the last scattering occurred directly in line with the Sun.)

Note also that the amount of scattering depends on how much air the light goes through. When the Sun is low in the sky, the light has to travel through a lot more air than when it is high in the sky. The top of the atmosphere follows the curvature of the Earth, so near the horizon the distance through the atmosphere is a lot longer than near the zenith (a great image of this effect is available at the NASA Langley Research Center website). By the time the Sun sets, the light pushes through so much air that even the green and yellow photons get scattered. This makes the Sun look redder at sunrise and sunset. The sky does not look blue near the horizon because more yellow photons (and even orange ones right at sunset) are coming from that direction as they get scattered away from the Sun.

Thanks to Keith Lynch, Tomasz Kwiatkowski and Frank E. Oldham for some helpful suggestions here!