One of my favorite things about hiking in Colorado is the assorted colors I encounter: the trees are beautiful greens (or red and yellow in the fall!), the rocks are covered in red and browns, and the sky is a mix of different blues and grays. The sky looks extra beautiful on my hikes, and one thing that strikes me is how different the colors are when you look in different directions. Now, I’m an atmospheric chemist and I know a lot about the sky, but one thing I did not understand was why the sky is so many different colors. Why is the sky light blue in some places, dark blue in others, and sometimes gray? Why isn’t the sky ever green? What about sunrises, sunsets, and rainbows?

I spent some time looking into these questions. And so, without further ado, Science Buffs presents: Colors of the Sky, part 4. (Click here for part 1, here for part 2. and here for part 3.)

Aroob Abdelhamid

Part 4: All of the colors the sky is NOT

In the final part of this series, let’s discuss all the colors we do not see. We’ve talked about almost all of the colors in the rainbow in the sky, red, blue, yellow… why not green? And what happens at night?

What about the night sky? 


Figure 1: The night sky is not black

What color is the sky at night? You might be tempted to say black, but have a second look at Figure 1. If the sky was black, why would the trees look so much darker and ‘blacker’? They are actually black because nothing is illuminating them.

The color of the night sky is actually a mix of blues and greens and reds caused by a phenomenon called airglow. Remember that the sun gives off other kinds of light besides visible, like ultraviolet light. In fact, there are rays coming from other things in our universe besides the sun, called galactic cosmic rays. All of those rays will come and sometimes they will interact with molecules in our atmosphere so strongly that they excite electrons in those molecules. Well, the excited electrons eventually relax, and when electrons relax, they give off energy. It just so happens that sometimes the energy is in the form of visible light, and that gives our night sky many, many colors (Figure 2).


Figure 2: The Airglow of the sky happens because of galactic cosmic rays exciting our atmosphere

In some regions of the Earth, you have different interactions that cause the sky to look different colors at night. Auroras are one such example of this. Auroras happen when charged particles coming from our sun (yes, the sun will just throw everything at us) collide with atoms in our atmosphere.


Figure 3: Charged particles from the sun can also excite our atmosphere

When charged particles from the sun collide with atoms in the atmosphere, a concentrated band of colors appears. The reason we only see this on the poles is because the magnetic field of the Earth (remember those are concentrated in the North and South pole) actually attracts and then stops most of the charged particles from making it to our atmosphere and only a select few escape the magnetic field and create these beautiful auroras (Figure 3).

Why isn’t the daytime sky ever green?

In the daytime, the sky is blue, white, grey, red, orange, yellow… why isn’t it ever green? In fact, during sunsets, the sky will skip over green when it goes from blue to red. Why? The sky actually gets around to being green all of the time. It’s just that we often don’t notice it because of how much our brain like to mix green with other colors  and label them as other colors (we tend to define green-blue as more ‘blue’) or with red (making what we would end up calling ‘yellow’).


Figure 4: There is green in the atmosphere during a sunset, it just tends to mix really well with the other colors. Listen to the Radiolab episode “Colors” for a really great talk about this.

In some sunsets you can see a thin band of green (Figure 4) but our eyes will register the other mixes as turquoise and yellow. So we think that the sky skips green, but it doesn’t. Our eyes just do. This in and of itself is a very fascinating topic about how humans ‘name’ colors and how they’re socialized to view colors a certain way. There is a wonderful Radiolab podcast about this called “Colors” that talks more about the sociological aspect of this phenomenon.

What if we didn’t have an atmosphere?

 The one main idea you should have gotten from this discussion is that our atmosphere causes all of the colors of the sky. If there were no atmosphere, what would we see? If we don’t have an atmosphere to scatter or absorb, then if we look at the sun we would see white (before we damage our eyes), but if you look anywhere else, you would see black because there is nothing for the light to interact with, which makes for a really boring sky!

What do we know now?

A lot. We know a lot. This week we learned about what it means when we don’t see colors in the sky and why the color green doesn’t make a huge splash in our painted sky. A lot of the absence is our eyes and brains playing tricks on us, telling us we see one things when we really see another.

The atmosphere is a beautiful place and while researching my own questions, I came upon so many other phenomena that I just didn’t have the time to learn about or elaborate on (yet). If you are curious to learn more, here are a few phenomena I stumbled onto: green sky before tornadoes, auroras (we barely discussed it here, you can write so much about them!), fogbows and cloudbows, the ‘green flash’, the sky during a volcano eruption and how a volcano can affect the temperature of the Earth, the ‘blue’ moon, and how the sky looks in very pristine atmospheres during sunsets/sunrises, rainbows, and even twilight. This is just scratching the surface, so go start searching for answers (and images, both real and on the computer) of the sky!

By Aroob Abdelhamid

Posted by Science Buffs

A CU Boulder STEM Blog

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