Sky, atmosphere and weather

The sky is computed, not textured. Scattering LUTs feed a full day and night cycle: sun and moon positions, lunar phase, sky colour, ambient irradiance and aerial perspective all derive from one phase value.

What the atmosphere pass produces

SkyAtmosphereEffect runs in the FrameStart compute phase and publishes five textures. Everything downstream — the sky itself, fog over distant geometry, ambient light on your models — samples those, which is why they cannot drift out of agreement.

• Transmittance LUT — how much light survives a path through the air.
• Multiple-scattering LUT — the second-order term that makes a blue sky blue rather than black with a bright band.
• Sky-view LUT — the radiance of the sky in every direction, at 256×128 by default.
• Cloud noise — a 512×512 noise texture the cloud layers sample.
• Aerial-perspective LUT — a 3D volume of in-scattering against distance.

Types to read: Rendering/Atmosphere.cs, DayNightCycle.cs, SkyState.cs, Effects/SkyAtmosphere.cs.

Sky atmosphere

A real planet, in kilometres

The atmosphere is parameterised physically rather than artistically. Defaults describe Earth: a 6360 km ground radius, a 6460 km atmosphere shell, a Rayleigh scale height of 8 km, and a viewer 200 m above the ground.

Atmosphere is a static class, and that is a deliberate narrowing too: there is one sky per application, not one per camera or per scene. It keeps the LUTs to a single set.

Sun, moon and stars

Both bodies are real discs with real angular sizes, which is what makes their shadows and their specular highlights the right size without tuning.

The day and night cycle

DayNightCycle turns one phase value into everything that depends on time of day. Evaluate is the entry point; the rest of the class is the conversions you need around it.

// A day takes 100 seconds at the default speed of 0.01.
WorldSettings.DefaultDayNightSpeed = 0.004f;   // slower
WorldSettings.DefaultStartHour = 5.5f;         // just before sunrise

// Anywhere afterwards:
float phase = DayNightCycle.PhaseFromHour(18f); // dusk
float hour = DayNightCycle.HourFromPhase(phase);

Speed and start hour live on WorldSettings, which reads from the application's settings object and therefore survives a restart if you persist settings.

Weather: four presets and a lerp

SkyState is a struct holding cloud layers plus the handful of scalars that make clouds read as weather rather than as texture. Four presets ship, and Lerp between them is how a front rolls in.

// Hold a settled state:
Atmosphere.Clouds = SkyState.Overcast;

// Or move between two over a minute:
float t = MathF.Min(1f, elapsed / 60f);
Atmosphere.Clouds = SkyState.Lerp(SkyState.Fair, SkyState.Storm, t);

Each layer is a SkyCloudLayer: AltitudeKm, ThicknessKm, TileKm, Coverage, Density, Detail and WindKmPerSec. Wind accumulates into Atmosphere.CloudWindOffsetKm, so clouds drift without any per-frame work from you.

Aerial perspective

Distant geometry picks up the colour of the air in front of it, sampled from the aerial LUT. This is the single cheapest thing you can enable to make a scene feel large.

AerialIntensity above 1 is not a bug fix, it is a concession to scale. A scene 200 m across gets almost no physical in-scattering; if you want it to read as a landscape anyway, exaggerate the weight and accept that it is a lie.

Known gaps

• Partial Volumetric clouds. Cloud layers are shaded from a noise texture against a layer model, not ray-marched as a volume. They read well from the ground and do not survive flying through them.
• Absent Precipitation, wind on foliage, wet surfaces. Weather affects light and cloud cover, not the ground.
• Absent Volumetric fog and light shafts. Aerial perspective is per-pixel in-scattering, not a participating medium the sun can shaft through.