General Question

Patty_Melt's avatar

Why is the sky blue?

Asked by Patty_Melt (11116points) October 9th, 2017

So, I know about different colors of light, refraction, and stuff like that, BUT, what would have to change so that instead of blue, on clear days the sky would be purple?
Could it happen if there were a change of gas content in the atmosphere? Atmospheric depth?
Would it require a different sort of sun?

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32 Answers

Mimishu1995's avatar

I remember reading somewhere that it has something to do with the sunlight. There are 7 colors in the sun ray, making it white. When the sun ray hits the Earth’s atmosphere, all the colors go straight ahead to Earth except for blue. It gets scattered around. That’s why the sky is blue.

canidmajor's avatar

@Mimishu1995, read the details.

@Patty_Melt, Maybe go to either Neil deGrasse Tyson’s or Michigan Kaku’s Facebook pages and ask? You might get an interesting response!

Lightlyseared's avatar

The effect you’re describing is Raleigh scattering. Longer wave lengths ie red get scattered more by air molecules than shorter wave lengths ie violet and blue.

There is more violet light than blue getting through but our eyes are more sensitive to blue light so see it better. So the sky appears blue.

Response moderated (Unhelpful)
Soubresaut's avatar

I thought the question in the details was: What would it take to change the color of the sky? (And now I’m curious about it, too!)

@Patty_Melt it almost sounds like a question you could submit to xkcd’s “What If?”

Muad_Dib's avatar

@Lightlyseared beat me to it – Our eyes just aren’t particularly sensitive to purple, which is why we see the sky as blue. The purple sky is there, we just can’t see it.

Response moderated (Unhelpful)
Patty_Melt's avatar

Not a WN fan, but that song is so pretty…

Thanks all.
So I asked my question the way I did, because at first I had a long winded Q, and I thought all that could be covered in the details. So, I went for super ultra simple, but still related.
Whatever thing, or combinations of things make the sky blue, those things must be alterable, even if only in theory, to make it purple (my personal favorite) or something else.
Everyone knows a blue sky is indicative of a clear weather day, yet when people are down, they say they are blue. If clear days the sky were purple, then the day would be rich, full if depth.
I would love to see a purple sky every day, or some days.

Does anyone know what the sky looks like from other planets in our solar system? There are voyager pics, aren’t there?

kritiper's avatar

The sky isn’t blue. It just seems that way because it is always looking down.

Patty_Melt's avatar

That doesn’t answer my question at all.

kritiper's avatar

I guess it just depends on how you interpret the question…

Patty_Melt's avatar

I believe I was pretty clear, thanks.
I did readdress my question.
For the fun of my own imagination, I want to know what it would take for the sky to be (appear) purple on typical good weather days, instead of blue.
As a side note, I queried whether anyone knew what the sky looks like elsewhere in our solar system. (Blue? Yellow? Black?)

kritiper's avatar

Yes, you were clear. And clear open for a joke.

Patty_Melt's avatar

Notice you are joking on a general thread.

kritiper's avatar

They’ll delete it, if they think it has no relevance. I took a chance…

Mimishu1995's avatar

Sorry @Patty_Melt. I didn’t understand your details at first.

Patty_Melt's avatar

That’s okay.
You gave it an honest go.
Any real attempt is a good thing.
If I can get an answer which lays it down to an absolute, I will be tickled pink…
or possibly green.

Rarebear's avatar

Reyleigh scattering.

edit. Sorry. I see it was already answered.

Nessafly's avatar

I believe it is because of the reflection. BTW, I believe it would be strange if the sky is red, yellow, green or else.

ragingloli's avatar

The sky has no colour.
Neither does anything else.
Your brain merely interprets the bioelectric signals produced by the interaction of certain wavelenghts of light with your eyes’ photo receptor cells.

ragingloli's avatar

What it would take for the sky to appear a different colour?
Brain damage, or mind altering drugs.

Rarebear's avatar

@Patty_Melt @ragingloli is correct. I am an astrophotographer and I image in color. But not really. What I do is put wavelength filters in front of the camera. The output is grey. Then, later, in software, I assign the color and assemble the color image.

Strauss's avatar

Our perception is subjective; I don’t know what blue (or any other color, for that matter) really looks like to anyone else. We all agree it is blue, so that makes it an objective fact.

Patty_Melt's avatar

Whatever our PERCEPTION is, there is still the fact that we all see A color when we look at a clear sky.
There are physical aspects involved in causing which particular color we all see.
My question is not, “why do we perceive the color blue as we do?” My question is, “What would have to be changed for the sky to appear purple to us, instead of blue.”
If it were possible to change the aspects which cause the sky to appear blue to instead cause it to appear purple, what is it we would have to change?
I am not asking about changing the color perceptions of every human.
Here is my thought on the subject.
Perhaps the depth of the atmosphere is at least one component.
When the sun sets, and rises, we see a variety of colors.
That is also when the sunlight is traveling through the atmosphere at the longest distances. So, perhaps if the atmosphere were, say, triple its current depth, would sky color appear different?
If not, what other changes would have to take place?
When sunlight filters through a prism, it breaks into a rainbow of colors.
Sunlight through rain causes the same effect in the sky.
Cameras pick up this effect. The sky looks blue in photos.
So, the sky has a color, a color we call blue.
What about our sun, atmosphere, distance, or other physical properties cause the appearance of blue, and how would any of those determining factors have to change, to make the sky take on the appearance of some other color?

Muad_Dib's avatar

Well, we could find a genie, and wish that a Violet-reactive cone could be added to human eyes.

Rarebear's avatar

@Patty_Melt From Wikipedia:

“It is this scattered light that gives the surrounding sky its brightness and its color. As previously stated, Rayleigh scattering is inversely proportional to the fourth power of wavelength, so that shorter wavelength violet and blue light will scatter more than the longer wavelengths (yellow and especially red light).”

Here is a pretty good picture of it

Muad_Dib's avatar

@Rarebear – That makes me sad to read. It means that the sky on Darkover can’t be reddish-pink due to the red sun, without it also being so dusty the planet would be uninhabitable. Which would make the snow not particularly pretty.


Patty_Melt's avatar

So, by that, would air molecules have to be bigger, or smaller?
I am extra confused today. Rough day, and fibro fog rolled in a few hours ago.

Muad_Dib's avatar

If I’m reading this correctly, if the atmosphere were like ridiculously high in ozone, and really low in O2 and H2O, enough of the red/orange/yellow/green/blue would be filtered out that MAYBE enough of the violet would be visible.

But you wouldn’t be able to breathe and everything would die. So there’s that.

Patty_Melt's avatar


Rarebear's avatar

I don’t pretend to understand the quantum nature of it. I just know that it scatters that wavelength of it preferentially.

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