Thank you for this! Some very good info in this article, direct from Simon Barlow, Design Director at Evolution:
"The only way we could support weather was by building a full atmospheric simulation. Weather in most video games tends to be an afterthought, or it tends to be a fullscreen effect. Yeah, you could put a shader on the road, and you could have the road get wet and you could put raindrops on the windshield and all the rest of it, but to truly capture the thing that youve just described, to make sure that you cant see the seams, you have to start with the simulation.
This isnt just an engine for DriveClub, its the engine for our whole lifecycle of PlayStation 4 so it had to be extensible. When we wanted to add to it, weather was the next logical step really, and as trivial as Im making it sound, it was almost as simple as just adding precipitation to that system. If youre simulating everything else, and then you add precipitation to it, it interacts with all the other systems.
For example with the rivers in the game, if you put it on heavy rain and you let it rain constantly throughout the race, they will rise because everything interacts. Everything is a simulation.
I was playing around with Photo Mode the other day and I didnt even know we did this until I paused it and I was looking right at one of the tyres in the rain and theres kick-up from the wheel, but looking closer, there was pools of water in the grooves of the tyre and I thought, why have we done this, were insane! Surely were insane, but I think it was just a by-product of the simulation, it just, it blew my mind."
Take a minute and read the full article, it's very good.