What would make a sky appear purple during the day?
I'm thinking of making a world where the sky is purple, or at least perceived as purple by the native species taking centre stage in my story.
What gases could cause this colour? Could the colour have anything to do with Rayleigh scattering?
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Sky Colour
The sky looks blue to us because of two major factors:
- Atmospheric composition
- Light hue and intensity
There are also a number of other factors including atmospheric temperature and pressure, but they have a lesser effect - they only inflict small changes on the other major effects.
Atmospheric Composition
The composition of the atmosphere affects the colour of the sky by refracting, reflecting, and deflecting the Sun's light. Each chemical compound in the atmosphere refracts light differently; on Earth it is just the case that blue light gets refracted the most. The more a colour of light gets refracted, the more of it there appears to be in the sky (because it's spread out), so it becomes the color we see in the sky.
Let us be clear on this point: the colours of the compounds themselves has no effect. Unless there is such a huge amount of a compound as to be clearly visible to the human eye, the colour of any molecule doesn't affect the colour of the sky. There is chlorine gas in our atmosphere; the sky doesn't appear green.
Light
For the sky to have any colour, there has to be some light. If there is no light, the atmospheric compounds cannot refract it and create colour. This is clearly demonstrated on Earth: at night, the sky is dark - because there is no light coming through the atmosphere to get refracted.
The hue of the light also has an effect. If, for example, the Sun didn't output any blue light, the sky would not be blue because there would be no blue light to refract. The hues of the refracted light mix together to create the colour of the sky - if red and yellow are refracted most, you will end up with an orange sky.
Changing Sky Colour
To change the colour of the sky on your world, simply change the light that's hitting the atmosphere, or change the atmospheric composition. The atmosphere's total refractive index is what determines what light is refracted most. The Earth's refractive index is close to 1: if it increased, the sky would tend towards the red end of the spectrum; if it decreased we'd tend towards ultraviolet.
See Also
Atmospheric Refraction
Atmosphere of Earth
List of refractive indices
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