The main character in my novel-in-progress is an alien whose home planet has a yellow sky. This planet orbits Altair (an A7 main-sequence star) and has a nitrogen-oxygen atmosphere similar to Earth's, with a few notable differences:
- A much higher level of water vapor, resulting in the planet being mostly cloud-covered. (The clouds are not yellow, but clear sky, when it shows, is.)
- Higher barometric pressure. I don't have a numeric figure, but it isn't extreme. An inhabitant of this planet would likely suffer altitude sickness at the same elevation on Earth but would be unlikely to develop serious complications.
The air doesn't have to be human-breathable, but I need the reverse to be true. Toxic gas(es) would be pretty cool, but I don't know if it's plausible for the native oxygen-breathers to be unaffected. I've been told any particulate matter would settle when it rains, which isn't what I want; I'm looking for a permanently yellow sky. I've also considered the possibility of airborne microorganisms, possibly above the cloud layer (and thus not disrupted by rainfall), but I have no idea if that's plausible.
Is a yellow sky possible, given these constraints? How?