# How to predict the color of this sky?

How can I predict the color of the sky based on the information provided?

Atmospheric composition:

• | 2% Trace gases | 58% Nitrogen | 26% Oxygen | 11% Argon | 3% Carbon dioxide |

Average surface temperature:

• 36 degrees Celsius

Planetary rotation:

• 34 hours

Axial tilt:

• 0

Info regarding of altitude, pressure, and atmospheric density:

altitude    pressure    density
(meters)    (atm)       (kg/m^3)
0           17          10
1000        15.3        9
2000        13.8        8.1
3000        12.5        7.3
4000        11.3        6.6
5000        10.1        6
10000        6          3.6
15000        3.6        2.1
20000        2.2        1.3
30000        0.8        0.5
40000        0.3        0.2
50000        0.1        0.06


Keep in mind, the type of star this world is orbiting is an F9V class star, only slightly above a G1V.

• Perhaps this question is a better fit for Physics SE? – cobaltduck Nov 19 '18 at 12:43
• Good question regardless. It would be interesting to know the answer to this. – Neil Nov 19 '18 at 12:47
• For regular, normal earth human? – SZCZERZO KŁY Nov 19 '18 at 12:52

# Blue, but darker

Earth's sky is blue due to Rayleigh Scattering. Rayleigh scattering redirects at an angle a percentage of incident light through a gas. The percentage of light reflected is proportion to $$1/\lambda^4$$, where lambda is the wavelength. That is, the shorter the wavelength, the more light is reflected. Violet light is most reflected, percentage-wise, but the purple section of the visible spectrum is small. So Blue light is the next most reflected, and since that portion of the visible spectrum is large, the sky is blue.

For your atmosphere, the percentage of oxygen and argon are higher, but he percentage of nitrogen is lower. From Table 1 of Shardanand and Rao, 1977 here is a comparison of scattering cross section (in $$10^{-27}\text{ cm}^{-2}$$) for various atmospheric gasses at certain wavelengths (on the x-axis, in Angstroms):

Molecule       6328    5145    4880    4579    3638
O2             2.06    4.88    6.50    8.39   20.03
N2             2.24    5.61    7.26   10.38   23.82
Ar             2.08    5.46    7.24   10.13   23.00

CO2            7.28   17.25   23.00   29.60   70.70
CH4            5.26   12.44   16.59   21.40   51.10


Notice how similar oxygen, nitrogen, and argon are to each other, as compared to carbon dioxide or methane. The relative rate at which the different wavelengths of light are scattered by your atmosphere will be almost the same as on Earth, so the color of the sky will be almost the same as on Earth.

Your star will be putting out light in a spectrum very similar to that of our sun, so the highest magnitude wavelengths will be in the blue range, just as our Sun's are. See here for more details.

The final factor is the mass of the air column. The proportion of light both absorbed by the atmosphere and raleigh scattered is affected by the number of air molecules that a light wave encounters as it travels into the atmosphere. Without you giving a surface gravity, I can't exactly calculate the air column's mass, but from your pressure and density, the mass of the air column is something like 10 times as massive on Earth. Therefore, light will be that much more likely to be absorbed or scattered.

This will make the sky a much darker shade of blue. But note, due to the same emission spectrum of the sun and the same scattering factors, that doesn't mean that the shade of blue is changed, just the brightness of the blue. The blue color will be the same shade, but darker.

• Eh ill upvote though i wish you put more detail into describing the science behind the chart and other gases. Since thats the real crux of the question and what could help others figure out how to make different colored atmospheres. – anon Nov 19 '18 at 13:58
• @anon that is answered here: worldbuilding.stackexchange.com/questions/13279/…. The question in the title is a duplicate of : worldbuilding.stackexchange.com/questions/13279/…, but given the specifics I can give a specific answer. – kingledion Nov 19 '18 at 13:59
• "That is, the longer the wavelength, the more light is reflected." should be "That is, the shorter the wavelength, the more light is reflected. – GretchenV Nov 19 '18 at 14:07
• See i thought this was a duplicate i just couldnt find that. Even though this is specific it should be closed for much the same reason physics se closes reworded questions. The SE shouldnt be used as an on demand plug in chug resource. – anon Nov 19 '18 at 14:11
• @kingledion ok, deleted. Another factor which would possibly have small influence is absorption bands. Since the chemical makeup is very similar to Earth (and argon's absorption band is almost in the infrared), the largest factor will be the amount of humidity. Water vapor absorbs in the red portion, making low-horizon solar events (sunset/rise) have less red influence. If the humidity is the same as earth, then yeah, not much difference. – ColonelPanic Nov 19 '18 at 14:32

It will stay blue.

But what blue? As stated, rayleigh scattering will be most deciding factor for sky color. As your atmosphere has more volume and density, more carbon dioxide >> more scattering and more scattering for shorter wavelenghts.

Paper on color of the sky, it is more about our sky and how color changes as sun sets. But you can get some understanding from that.

So blue light wavelenght will be moved to around 450-480nm, that gives us blue to pale blue. How much light makes it to surface? Is up to you, but it too will influence how dark your blue is.

But it would be way more nicer if we make your density more in line with atmospheric pressure of 17 atm >> around twice your choise. That would give us pinkish sky or yellow with a lot of dust.