[Graham Aldridge - Lead Rendering Programmer (Guest Appearance)] The way the weather works in the game I think is quite unique: At any time, there are thousands (often tens of thousands) of clouds moving through the world. Each cloud defines a number of states about not just how the cloud looks (density, shape, etc) but also the weather conditions below the cloud: Be it rain, snowfall, mist, wind, etc. This is how the weather feels so dynamic, as it moves through the world the weather below the clouds gets accumulated dynamically through the world (it's possible to have one part of the world wet and at the same time another snowy, etc – it all depends on the clouds and where they are). We have systems to control how these accumulations blend and combine in to the world as the clouds move through it, this also controls visual and gameplay transitions too: e.g. as snow melts it will typically puddle then turn mud. Sun shining on damp ground is more likely to produce evaporation fog, the motorbike splashes through puddles, etc.
Creating the visual look of a snowy day is quite challenging because it's not a single effect – it's a combination of multiple interacting systems. If it doesn't look right, it's often because there is a missing element in the simulation (Only applying a snow layering effect to surfaces wouldn't be enough). We decided early on that trying to eyeball and emulate these effects wouldn't feel right and we'd struggle to make it as dynamic as we wanted - so we would always try and figure out what combination of physical systems produced the look of snow in the real world and try and emulate those as best we could. We didn't want to fake it.
As examples, it meant making sure how light scatters through the volumetric fog was accurate. Having world lighting react accordingly too was really important (e.g. effective bounce light off the ground changes a lot to reflect the predominantly white snow). A really subtle system we have always running in the game is an automatic white balance system – just like a camera, this makes sure the image colour balance looks right and is incredibly important to the look of the game because it helps to keep more extreme circumstances (such as snow, sunsets, etc) from feeling out of balance.