Owen Shepherd, Rockstar’s vice president of art for lighting and rendering, described Grand Theft Auto VI’s weather in detail on 30 September, seven weeks out from the 19 November release. Hurricanes got the headlines, and they will photograph well. The two adjectives in his description are where the work is: Rockstar, he says, “re-engineered our weather system to make everything much more localized and persistent.”
Both of those words cost money.
One number for the whole world
Weather in an open world of this lineage has historically been a single global value. The game holds a state — clear, overcast, rain, thunder — and interpolates between authored presets on a timer. Every system that cares about weather reads that one value. The sky dome takes its colours from it. The wet-surface parameter in the road material is driven by it. Ambient audio crossfades on it. Particle emitters switch on and off against it.
It is cheap, it is trivial to debug, and it has one property that becomes obvious the moment you look for it: when it rains in Vice City, it is raining on every square metre of Leonida at the same intensity. Red Dead Redemption 2 did beautiful things inside that constraint. Its storms are still some of the best weather art shipped in a game. They were also, everywhere on the map at once, the same storm.
Make weather localized and that single value becomes a field — something that varies across space, which means something has to store it, sample it and interpolate it. In practice that is a coarse grid over the world, updated on a slow tick, with a handful of weather cells drifting across it. Low resolution is fine. The expense is not the grid.
The expense is that every consumer of weather now needs a position before it can ask a question. The road shader can no longer read a global uniform for how wet it is; it needs to sample a mask. Audio needs to know whether the rain is on this block or four blocks east. Traffic and pedestrian density, if they respond to weather at all, respond differently in different districts. Shepherd’s wind description gives the shape of it — offshore breezes after sunset, warm air rising, buildings blocking the flow, debris swirling on rooftops. All of that describes a wind volume with the city’s own geometry feeding into it, and anything reading wind has to read it per-position.
Persistence is the part I would worry about
Rockstar’s own example is wet ground and puddles that mark where rain has recently fallen. Which means the world carries a memory of weather that has already finished.
Surface wetness stops being a value derived from the current sky and becomes accumulated state. It fills while the rain is over a region and drains at some rate afterwards, and that rate probably varies by material, because asphalt and grass and sand do not shed water alike. That state has to live somewhere. Somewhere that survives the player driving two kilometres away and the region streaming out, and survives a save and a reload, and resolves without a visible line where one region meets its drier neighbour.
Seams at region boundaries are the failure mode here, and they are the kind of problem that lands on an environment artist’s desk as “can you hide this”.
A rainbow, by contrast, is nearly free once the rest exists. You need the sun angle and the position and extent of a rain cell, and a localized system already holds both. Rainbows appearing at all is a decent signal that the weather data is physical rather than authored — nobody hand-places a rainbow in a world this size.
What it does to the budget
Volumetric clouds that are advected across the sky and occlude the sun are a per-frame cost that scales with screen coverage, and they are the one item on this list I would expect to be measurable on a frame graph. Shepherd’s department owning them — lighting and rendering, not VFX — tells you how they are being treated. Clouds that cast real shadows across a city are a lighting system.
That matters for the argument everyone was having in August. When Rockstar put 27 minutes of footage out, nobody at the studio confirmed a frame rate, and the reasoning behind the 30fps deduction was always the CPU rather than the GPU. Localized weather pushes in the same direction. A global weather value is one float read by everything; a weather field is spatial queries, per-region state, and simulation ticks that keep running whether or not the player is looking. Those are CPU costs. The clouds are the GPU half.
It is worth saying what the opposite choice looks like. Insomniac got ray tracing running at 60fps on a base PS5 by being ruthless about where the budget went. Rockstar appears to be spending its on systems that are mostly invisible in any given five seconds and only legible over hours. Both are defensible. They produce very different games.
And there is a real hole in what we know. Rockstar declined to say how often hurricanes occur, or whether they affect the story at all. Until somebody plays it, a hurricane could be an emergent event the weather field occasionally produces, or it could be a scripted sequence that fires once in act two with a simulation-shaped announcement attached. The puddles are the thing to watch. If wetness is still draining off the pavement twenty minutes after a storm has moved inland, the field is real.