Are the clouds purely cosmetic, or do they have additional metadata attached [that could lead to gameplay effects]?

Technically, there's some stuff attached to it. For example, in the Pyro demo, you could see the lightning storms. And those are all piggybacking off some data that's coming with the cloud density and the type that's there. And once other teams hook into this and use it more, they can make use of the clouds and have them have gameplay implications for the players and the player ship.