I was wondering if anyone can answer my question: I was wondering what conditions would be needed to perceive a yellow sky and how this sky would change colour as the sun(s) set. Its for a book I’m writing which is not set on Earth (as you may be able to guess)
The simplest method would be to just have a bunch of yellow particulates suspended in the air.
Title edited to give an idea of the subject (although I’m not sure that “perceive” is the word you want).
Colibri
General Questions Moderator
A planet with a fluorine atmosphere would have a yellow sky.
I looked into this a few years ago; as far as I can determine, a planet with a clear, transparent particulate-free atmosphere with a density of 40 x Earth’s atmosphere would be yellow at the zenith, but whiteish around the horizon. Sunsets would be dark red, I think.
Here’s a diagram I copied from Atmospheric Optics by Craig F Bohren, which shows the shift in sky colour with increasing density.
more details on this page (partly my own work, written for a SF scenario) with a number of references at the bottom that will give more details
http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds
Suspended particles needn’t be yellow themselves – they just have to preferentially scatter yellow light. This is, perversely, just the opposite of the situation we have here in normal Earth skies, where Rayleigh scattering preferentially scatters blue light (which is, ultimately, why the sun is perceived as yellow – but I don’t want to dredge that up again right now). You can get this circumstance where chemical compounds are present that absorb or scatter differently than the air molecules, dust particles, etc. are present in place of or in addition to what’s in the air now. Under those circumstances you can have Blue and Green Moons and Suns. Look up stuff on Blue Moons and what causes them, or have a look at van de Hulst’s Light Scattering by Small Particles. I know that one circumstance that can lead to this is after forest fires, when there’s a lot of plant residue – ash, fine sap, pollen, ertc. – in the air.
Also, after volcanic eruptions and meteor falls you can get a lot of dust in the atmosphere that will give you spectacular sunsets, including yellow skies near sunset. Look up “Noctilucent Clouds” After the Tunguska Meteorite, the noctilucent cloiuds lit up the European night sky so brightly that people could read by their light well into the night.
I’d also add that the sky over Mars is colored, mostly by iron oxide (I believe), giving it an orange to yellow cast.
So you could get a yellow sky by the addition of various dusts, chemical droplets, or other substances in the atmosphere, which need not be yellow themselves in bulk, but which preferentially scatter and/or absorb light to give you the color. Or you could have a lot of dust in the sky that “amplifies” the yellow twilight sky to be more brilliant and last longer.
It’s worth mentioning that in some rainy/overcast conditions the sky can take on a yellowish hue. And once in near-tornado conditions I saw an aquamarine sky.
That’s right. Recently I noticed a particular raincloud over the Yorkshire Wolds; below the cloud the sky was a pale straw-yellow, and above it the most beautiful blue. This is due to Mie scattering, I expect. However I can’t suggest a way to reproduce that colour in a cloudless sky on an alien planet using raindrops, so I think you’d have to imagine some other particulate component.