If you asked in a game, I’d say “that’s a good question! You should ask an NPC about that”
Which as we all know is code for “hell if I know, I’ll figure it out later”
The way I’ve played it so far they act like normal fog, but they seem to be connected to a lot more monsters hanging around. Trolls love it, of course - though I did have a player debate a troll until the sun came up and stoned him, mostly because it was cool. I suppose, since he was playing a paladin and obviously an Inca-inspired setting necessitates sun worship that that was divine providence
In all seriousness, the clouds/fog act mostly as a divider between the realm of men and the realm of the untamed and unknown. What’s below the sea of clouds is hidden from the life-giving sun, and thus partway into the land of the dead. The bones of ancient civilisations may be the remains of a time when the sun was stronger and the clouds lower - or it may be that they are the abandoned kingdoms of the dead. They also lend themselves to easy descriptions, as everything below can be describes as slick with condensation, covered in mold or infested with fungi!