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I'm designing a few planets and the life that could inhabit them, and was wondering if an uncommon Earth phenomenon like iridescent clouds might be more common on a planet under different conditions. Are there any atmospheric properties that would make iridescent clouds a more common occurrence? I think it might be cool to include this as a feature on one of my planets.

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The key to having strong iridescence is the uniformity in the distribution of the droplet or ice crystal size and that the they are relatively small. A strong light helps make the phenomenon more visible and the cloud can not be optically thick or you won't get a strong effect. By relatively small, most cloud droplets are between 1 and 10 um but iridescent clouds the droplets are probably less than 1 um or around 1 um. If the particle size is changing you may see a shift in color.

So for your world a bright sun and perhaps strong upper atmospheric winds to shear the clouds so they don't get too thick. The effects would be most strong when looking in directions near the sun.

To keep the droplets more uniform, I think not having as many vertical convection currents within the clouds might help to keep the particles small. Of course you have several different cloud types and you still may want to have thunderstorms.

Since the droplets in clouds often nucleate around a small particle which needs to be less than 1 um, it might be that you have some special way to introduce a special uniform particulate size. Maybe some kind of air-born microbe, or dust particle from some dessert where for some reason the dust is especially fine, or a volcano emitting something very fine.

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  • $\begingroup$ Thanks!! I have a couple more questions now if you still wanna help: what would cause fewer vertical convection currents within the cloud? And how do you think the thickness of the atmosphere/scale height and air density might affect the thickness of the clouds? If I had a lower g planet with a less dense atmosphere, would that create thinner clouds and enhance the effect or would the clouds being less dense make them more optically thick with the droplets being more spread out (I also guess I don't know what optically thick means exactly lol)? $\endgroup$
    – Elhammo
    Aug 26 at 8:06
  • $\begingroup$ Optically thick is just a fancy way to say that the light has to make it through the cloud for you to see it. So wispier, thinner clouds or the edges of thicker clouds are more likely to have the effect. I think temperature differences and availability of water moisture is more important than gravity, or the density of the atmosphere, unless you really push it. This website weather.gov/source/zhu/ZHU_Training_Page/clouds/… explains cloud formation is pretty good. $\endgroup$
    – UVphoton
    Aug 26 at 12:23
  • $\begingroup$ Not sure about how to limit thunderstorms, except by limiting humidity, but even then water vapor gets pushed from humid areas to drier areas some times. I think you can think some about the geography and geology. Mountain ranges are wetter on the ocean side, and dryer on other side etc. But really it is your story, if you want shimmering clouds on the top of a mountain range, or a certain time year the iridescent clouds are likely to appear I think you are o.k. $\endgroup$
    – UVphoton
    Aug 26 at 12:28

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