Wait, what? That? It’s my vid. My clouds. Pretty simple to tell the truth. I can’t imagine no one else did that yet 
It’s got quite heavy impact on the FPS but that’s because it uses the NOISE 3D material shader. If you use simple textures it should be fine but won’t look that good.
And I was thinking to put that on the Marketplace for a buck or something. But one simple shader with one simple particle system? Really? I gotta figure something more to that.
Here are the clouds in game.