Why the Sky is Blue

blue-skyToday I found out why the sky is blue.

For the short answer, when light from the sun enters our atmosphere it collides with molecules in the air.  The blue part of the light gets scattered more than the other parts during these collisions and thus makes the sky appear to our eyes as blue.  If the light from the sun took a straight path down to our eyes with no scattering or absorption in the atmosphere, the sky would in fact look much as it does at night in the day time, which would be kind of awesome in my opinion.

So a little background.  White light waves from the sun are in fact mixtures of all colors of the light spectrum.  Anyone who’s ever had a prism knows that when white light shines through it, the light gets separated and you get a rainbow spectrum showing up on the other side.  Humans can only see a portion of the total light spectrum of energy;  we see from Violet, which has a wavelength of about 380 nanometers, to red which has a wavelength of about 720 nanometers.  Descending from red to violet, we get orange, yellow, green, blue, and indigo between the two.

Another needed piece of information is that the atmosphere of the Earth is made up of almost all nitrogen and oxygen.

So without getting too technical, what is happening here is that oxygen, which makes up 21% of the Earth’s atmosphere, has a diameter such that it fairly effectively scatters  radiation that has a wavelength of around blue-ish light, while the reds, oranges, and others more or less passing straight through the atmosphere without being scattered much at all.  So when you look up at the sky, everywhere you look looks blue as the blue portion of the light spectrum bounces off an oxygen molecule and into your eye.

It actually turns out that the Earth’s atmosphere is also scattering quite a bit of violet light as well.  So why doesn’t the sky look more like a blueish/violet?   This is for a few reasons.  Most important is that our eyes are most sensitive to blue, red, and green.  So our eyes are naturally more inclined to respond to blue over violet.  Also important is that the sun doesn’t put out light at the same intensity at all wavelengths of the spectrum.  In addition to this less amount, some of the violet also gets absorbed in the upper atmosphere; so not as much of it gets to our eyes.  So it turns out more or less a combination of these three things makes it so we see the sky as blue instead of violet or blueish/violet.   It is interesting to note however, that our eyes are picking up some of the violet and indigo being scattered.  Without this, we would in fact see the sky as more of a blueish green.  This is because the green cones in our eyes respond to the small amounts of scattered yellow light wavelengths to some extent; meanwhile our eye’s red cones respond to the indigo/violet somewhat which balances things out a bit and so we only perceive the blue being scattered.

So you might now be asking, “Why is the sky red or orange when the sun is setting?”  What’s going on here is that as the sun is setting, the light you are seeing from it is having to travel through a lot more atmosphere given your angle to the sun.  Light at lower wavelengths gets scattered more than at the higher wavelengths.  So less of the blue light gets to you as it’s being scattered more than, for instance, yellow, orange, and red.  In this case, if it is a relatively clear day, the sky will appear more yellow than blue as you watch the sunset because the blue is being scattered so much it never reaches you or at least, not as much of it, but the yellow isn’t scattered nearly as much but still scattered enough to make the sky look yellow.  If it is dusty or there are a lot of other types of particles in the air along the path to where you are looking at the sun, the sky will appear more red.  If there is a lot of salt in the air, such as at sea, it will appear more orange.

So, if there was no scattering or absorption, the sky would appear black in the day time.  If there was more absorption or scattering going on than there is, then the sky might appear to us to be yellow, orange, or red all day long.

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  • Wow, so that finally explains (I think, right), why although the “sky” is black when you’re on the Moon or the space station that it’s still bright. It’s because of the lack of the elements you mentioned above that causes Earth’s sky to appear blue, right??

  • Uh, is there any possibility that the reason the sky is blue in the daytime but black at night is albedo? That the blue in the sky is not being “scattered” by the oxygen on the way in, but rather reflected by the oxygen on the way back out?

    As the Moon Missions demonstrated, that “big blue marble” is awfully bright (from reflection) in the daytime….

  • I don’t buy it. Why? Because the other day part of the sky was covered with pure white high altitude cirrus clouds. The part of the sky that was not covered was a beautiful pure blue. Seeing a high altitude aircraft passing under the cirrus clouds I realized they were very high, perhaps 45,000 feet. In other words, something like 80-90% of the air (and oxygen) of the atmosphere was *under* those pure white cirrus clouds. But in the light that was between me and those cirrus clouds (through which the sun was passing) there was not a trace of blue – the clouds behind that air were pure white. So whatever makes the sky blue was plainly *above* those cirrus clouds, in the rarified high altitude atmosphere where there is very little oxygen or anything else, no more than about 10% of the content of the atmosphere. So why was that rarified atmosphere making blue light, while the bulk of the lower, more dense atmosphere contributed no colour to the clouds?

  • ‘Canadian’ above does not realize that whatever blue you see through any smallest gap between the clouds comprises of light that travelled from the sun all the way to your eye through the gap. It does not matter whether the clouds had blanketed the entire planet and left just that gap which you saw. FYI between you and that gap was plenty of oxygen which scattered the light. You speak as if you were observing the sky above those clouds. Light scattering as an explanation to blue sky is not a theory, it’s a fact. The sky is not a blue wall up thea!

  • From the text:
    “because the wavelength of blue light is about the same size of an atom of oxygen,”

    The blue to violet part of the spectrum covers wavelengths form 460 to 400 nm. The diameter of an Oxygen or Nitrogen molecule (not atom) is about 0.3 nm. These are not very close to each other.

    • Daven Hiskey

      @Mark: Oxygen and Nitrogen are elements. So “atom” is more appropriate when describing them, not molecule. However, yes, as describing our atmosphere, I should have said “molecule of dioxygen” (O2). And, looking back, I worded that part poorly in a few different ways. I should have perhaps been more clear, but I don’t like to get too technical if I can help it, as most readers appreciate more plain speak, which also aids in understanding. Particularly in this case, most who’d understand the technical speak already know why the sky is blue from a technical standpoint. :-) But to correct my poor wording above, it’s more that the molecules are an appropriate size to increase scattering in the blue-ish part of the visible spectrum. More technically, the rate of scattering varies based on the ratio of the particle’s diameter to the wavelength of the radiation. When this ratio is less than about 1/10th, scattering (called Rayleigh scattering) happens such that the scattering coefficient varies inversely to the 4th power of the wavelength (and yes, I had to get out my old college Physics textbook for those specifics. I knew I kept it around for a reason!) So that’s basically just a fancy way of saying why oxygen and nitrogen, which are about the same size, scatter certain wavelengths of light better than other wavelengths. Thanks for pointing out the crappily worded bit! I’ve fixed it slightly now in the article. :-)

  • Close but no cigar. Just wanted to mention it is oxygen that is responsible for the sky being blue but its not the O2 that we breath that makes it blue. Its the layer of O3 high up in the atmosphere which is also known as the ozone layer. This also answers Canadian’s uncertainty because the ozone layer is higher than most planes fly.

  • The sky’s oxygen content is large in part, due to the arrival of algae in the earths ocean. Some Scientists speculate that the earth use to have a much higher methane content before this emergence, and also believe that the sky was much more orange as a result of the gas ratio.

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