Why Is The Sky Blue?

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Podcast Transcript

Children are highly inquisitive and are always asking questions.

One of the most common questions that children have is, “why is the sky blue?” 

It is a simple and reasonable question, yet the answer is rather complicated, and most adults really aren’t sure why the sky is blue.

Learn why the sky is blue and how to answer a child’s most common question on this episode of Everything Everywhere Daily.

The fact that the sky is blue is one of the most fundamental facts of the world we live in. If you wanted to use an example of something which is an obvious truth, you might say, “the sky is blue.” 

As simple and obvious a fact as it is, the reason why the sky is blue is a bit involved, and the truth is that people didn’t really know why until the 19th century. 

One of the explanations that people traditionally gave was that the sky was blue because it was displaying the reflected color of the water in the oceans. 

This, of course, makes no sense as it is blue everywhere on Earth, even if there is no water to be found anywhere nearby. 

On the same note, this was also the reason people gave as to why water was blue because it reflected the color of the sky. 

The real reason why the sky is blue is a bit involved, but it isn’t too hard to understand. To understand it, you need to know a bit about the nature of light, our sun, our atmosphere, and even the human eye.

Let’s start with light. 

As you probably know, white light is made up of different colors. You might remember the mnemonic device to remember the colors ROY G BIV, which are the first letters of red, orange, yellow, green, blue, indigo, and violet.

Each of these colors has a different wavelength of light. Colors near the red end of the spectrum have a longer wavelength, and colors towards the blue end of the spectrum have a shorter wavelength.

The entire spectrum of visible light lies in a range of wavelengths from about 700 to 400 nanometers. 

Next, the thing you need to know about is our atmosphere. 

By far, the two most common elements in our atmosphere are nitrogen and oxygen. Together they constitute 99 percent of the atmosphere. 

Both nitrogen and oxygen are usually found as molecules where they are bound to another nitrogen or oxygen atom. 

Both molecules are very similar in size. An oxygen molecule is 292 picometers in diameter, and a nitrogen molecule is just slightly larger at 300 picometers.

The next fact you need to know is how the light from the sun is distributed. The wavelengths of light emitted by the sun are spread over a much wider spectrum than just visible light, including ultraviolet and infrared light. 

It is distributed in a bell-shaped curve, with the peak of the curve located around where green and blue light meet. 

So, blue isn’t the most dominant color emitted by the sun, but it is close. 

With all of these basic facts about light, the atmosphere, and the sun, I can now put them all together.

The first inkling that something in the atmosphere was causing blue light was discovered by the Irish physicist John Tyndall in 1869. He was conducting experiments with gasses that replicated the atmosphere.

He enclosed the gasses in a long tube with a bright light at one end to replicate the sun. 

He would introduce smoke into the tube and notice something peculiar. When he looked at the glass tube from the side, the light had a bluish tint. However, when he looked at the light from the opposite end of the tube, it had a reddish tint. 

Tyndall proposed that the particles of smoke were reflecting the blue light, and as such, the blue in the sky must be due to particulate matter in the air. 

Tyndall’s explanation was wrong, but he was on the right track.

The answer was to be found in Rayleigh Scattering.

Rayleigh Scattering was discovered by the British physicist Lord Rayleigh. He was awarded the Nobel Prize in physics in 1904 for his discovery of argon and his study of atmospheric gasses.

Lord Rayleigh determined that it wasn’t particles in the atmosphere but the atmosphere itself. 

He determined that something happened when a certain light hit molecules that were much smaller than the wavelength of light. 

In particular, if the molecule’s size is near the wavelength’s negative fourth power, you get Rayleigh Scattering. The light will bounce in random directions. 

It just so happens that given the size of oxygen and nitrogen molecules, the wavelengths of light which are scattered the most are the shorter wavelengths near the blue end of the spectrum. 

Indigo and violet are actually scattered slightly more than blue is, however, the sun produces much more blue light than it does indigo or violet. 

Technically, all visible light can be scattered, but about 20 percent of blue light is scattered versus only five percent of red light. 

So, to summarize, the oxygen and nitrogen molecules that makeup almost all of our atmosphere are tuned to scatter shorter wavelengths near the blue part of the spectrum. 

The reason why the sky is blue instead of indigo or violet is that the sun produces more blue light. 

There is one other contributing factor as well, our eyes. 

The human eye is more sensitive to blue than it is to indigo or violet. We can see indigo and violet, but blue is more pronounced. 

So that is why the sky is blue. It is a unique combination of our sun, atmosphere, and eyes. 

At this point, you might be saying, “but Gary, the sky isn’t always blue. At sunrise or sunset, the sky can be red, yellow, or orange.”

That is correct, and the reason has to do once again with Rayleigh Scattering. 

The amount of atmosphere between you and space is shortest when you go straight up. When you look at the horizon, there is much more atmosphere that light has to go through. 

There is so much atmosphere that most of the short wavelength colors on the blue end of the spectrum are almost totally scattered, leaving only the colors in the longer wavelength part of the spectrum to reach your eye.

Although it is rare, it is possible for the sky to turn other colors. There is an old legend that the sky turns green before a tornado. 

While a green sky doesn’t necessarily mean a tornado, it sometimes happens when there are storm clouds just before sunset, and before sunset is the most common time when tornados occur. 

I’ve seen it myself several times, and it is very eerie. 

This is due to the blue part of the spectrum being scattered away just like you would at a normal sunset. However, the green part of the spectrum is scattered by the water molecules, and because most blue light has been scattered, the clouds appear green. 

Now some of you might further be wondering if the sky is blue on Earth, what color is the sky on another planet like Mars?

The Martian atmosphere is very different than Earth’s. For starters, the pressure is less than 1% of ours, so there just isn’t much of it. 

Second, the largest component of the Martian atmosphere is carbon dioxide at 95%. A carbon dioxide molecule is much smaller than a nitrogen or oxygen molecule at 232 picometers, so it isn’t going to scatter visible light as well. 

Much of the Martian sky’s color comes from dust in the atmosphere. The iron-rich dust tends to absorb blue light, giving the sky what is often called a butterscotch color. 

However, that being said, rovers on Mars have detected a bluish halo around the sun near sunset or sunrise when atmospheric conditions are right. 

What about some other planet? Could there be different colored skies on a planet around another star? The answer is yes. It is entirely possible, given a different atmosphere and a different light profile from a different star, you could get a different color sky. 

Granted, we probably couldn’t live on such a planet, but it could have a different color. 

So the next time a child asks you why the sky is blue, just tell them that it is due to blue light waves bouncing off molecules in the atmosphere. If that doesn’t satisfy them, just wait till they’re older.