Chances are you’ve asked your parents, “Why is the sky blue?” And chances are, they gave you an answer that kind of resembled something like, “Because it reflects the blue from water on Earth,” or “Because… uh, don’t you have homework to do?”
To be fair to your parents, this isn’t usually something that’s taught in school, unless you are getting into Earth science or physics. And believe it or not, sometimes even high school teachers don’t explain this correctly. So what is the answer, then?
No, I’m not going to say, “it’s complicated.” It’s actually not, as long as you know a few basic concepts.
First, let’s look at the parts of the equation. What’s involved in you seeing the sky as being blue?
Well, first, there’s you and your eyes. Your eyes are awesome pieces of equipment that have evolved to see what we call “visible light.” Visible light is, as you might suspect, the light you can see—namely, the colors red, orange, yellow, green, blue, indigo, and violet. (ROY G. BIV is a nice way to remember the order.) That is the list of the visible spectrum of light we can see with our human eyes, in order of wavelengths from long to short. (I’ll get into what a wavelength is in a moment.) So the first part of the answer is, your human eyes can see blue light. That’s why the sky looks blue.
But where does this light come from? That’s the second part of the equation. There is one source of light that really affects us. That is the Sun. The Sun emits (or gives off) light, also known as electromagnetic radiation, of all different kinds. Some we can see, like all the colors we can think of, and some we can’t, like gamma radiation or x-rays. The Sun is the source of all colors, including the blue we see in the sky. At night, the makeup of the sky hasn’t changed—it just looks black because there is no sunlight. When the sunlight is present, the sky looks blue. OK, so maybe you’re thinking, if the sun gives off all the colors of the rainbow, why do we just see blue in the sky rather than other colors, too? That takes us to the next part of the equation.
Before the light from the Sun reaches us, it has to go through the last part of the equation, the Earth’s atmosphere. The atmosphere is a pretty busy place. There are all kinds of gas molecules, particles, etc floating around up there. And while when we wave our hands in air, nothing much happens other than we feel a bit of pressure from it, and we might feel how the air flows around our hand and fills in where we waved some away, but when tiny things like air particles interact with tiny things like photons, things start to get interesting.
See, light is a pretty interesting and amazing thing, and we still don’t know everything there is to know about it. We consider light to behave like both particles (photons) and waves. There have been all kinds of experimentation into this, and if you are curious to learn more, you can look up “wave-particle theory of light.” There are a lot of great resources out there about this. But looking at the wave aspect of light, we know that light travels in waves. Waves have a structure, with parts, like this:
Each different type of light or radiation has a different wavelength which makes it different—that’s how we identify different types of radiation or light, by the length of its waves. Relatively speaking, red light has the longest wavelength of visible light, and blue/violet lights have the shortest. There are also many other types of light that we can’t see, like gamma rays (short wavelengths) and radio waves (long wavelengths). You can learn more about all of this by looking up “electromagnetic spectrum.”
So the light comes from the Sun, enters Earth’s atmosphere, and starts interacting with the air molecules that are in the atmosphere. It just so happens that most air particles are of a certain size where when the light of the Sun hits them, the blue wavelengths of light are of the right size to bounce off them. Other wavelengths are too long, others are too short.
As you can imagine, there are a lot of air molecules up there in the atmosphere, so there is a lot of blue light bouncing around. When light bounces off things, it’s called scattering, and when this particular type of scattering happens (when the diameter of particles, in this case, air molecules, are less than 1/10 the wavelength of the light interacting with it, in this case, the blue light), it’s called Rayleigh scattering, after the English physicist who discovered the relationship. Because there is so much scattering of blue light going on, that’s what our eyes see. Other colors are either absorbed by other things in the atmosphere so we can’t see them, or pass through and reflect off of other things on the ground, etc, and let us see those things as a color (a red wheelbarrow reflects red light, green leaves reflect green light, etc).
Blue light just happens to have the perfect size of wavelength to reflect off of air molecules in the atmosphere, and those reflections get scattered all over the place until they reach our eyes. That’s why the sky is blue.
Here is a simplified visual way of thinking of it:
Now, because you are human, and humans naturally have curious and observant minds, you might also have seen red sky, white sky, even green sky, though that usually means some severe weather is coming and you should seek shelter. ALL of those colors of sky mean that there are some kinds of particles up there that are causing the scattering of different kinds of light. Lots of water vapor, dust, aerosols, pollution, all these things can make the sky appear to be a different color. And that’s all due to the same concept—scattering.
Want to learn even more about the colors of the sky? Check out this article by GeekMom Patricia Vollmer, “Why is the Sky Blue Here, Not There?”