blender-weather-fx

Weather FX

The execution model is in blender-core. Sky, sun/moon, fog and mood come from blender-time-of-day, so run scene_mood.py first, then add weather. Rendering settings are in blender-rendering.

One call each

scripts/precipitation.py -- --type rain --intensity heavy --ground Street Roofs --wind 2 0
scripts/surface_weather.py -- --objects Street Car Walls --wetness 0.7 --puddles 0.4 --ripples 1 --streaks 0.5
scripts/lightning.py -- --flashes 2 --strokes 3
scripts/precipitation.py -- --type snow --intensity moderate --ground Terrain
scripts/surface_weather.py -- --objects Terrain Roofs --snow 0.8 --snow-depth 0.06
scripts/surface_weather.py -- --objects Street Car Walls --remove     # exact restore

A rainy night: scene_mood.py --time night --fog-visibility 250 → rain → wetness + puddles → lightning. Rain and puddles at night are what sell a wet scene: wet surfaces reflect every practical light.

precipitation.py (rain, snow, dust, ash, embers)

Builds a GN modifier on ENV_Weather_<Type>. Its box covers what the camera sees across the frame range: world-fixed, so moving cameras don't drag the rain. With no camera, it covers a 18 m box. The floor is the lowest top of --ground.
Seamless loop with no simulation: each particle lives N whole times per loop (Lives), so the last frame + 1 equals the first frame exactly. It renders any frame on its own and works headless.
Real speeds: rain 7 m/s (9 at heavy/storm), snow 1, ash 0.6, dust 0.15, embers rise at 1.5, combined with --wind X Y (m/s).
Ground: a raycast against the --ground objects stops particles at the first hit (roofs, awnings, streets). Rain spawns splash crowns lasting 80 ms at each hit, aligned to the surface normal.
Streaks, not motion blur: rain is a 3-sided streak of length = speed × ½ frame (a 180° shutter), aligned to the travel direction. Motion blur off is cheaper and looks the same.
Particles closer than 1.5 m to the camera are hidden (Near Clear), because huge blobs ruin shots. The viewport shows 10% (Viewport Fraction), renders show 100%.
Density per intensity (particles/m³): rain 5/15/40/80, snow 10/30/80/160. Override with --density. The count is capped by --max-count (400k), and the result warns when it caps.
All values remain modifier inputs (Count, Speed, Wind, Lives, Particle Size, Streak Length, Sway, Ground, Splash...), so tweak them there instead of re-running.

surface_weather.py (wet, puddles, ripples, streaks, snow)

Puts one ENV_Weather shader group in front of each Principled BSDF and records the originals on the material. --remove restores links and values exactly.
Wet (Lagarde model): porous albedo darkens (up to 55% × --porosity), roughness drops, and a water film is added as Coat. Use porosity 0.6–0.9 for asphalt, concrete, wood and fabric, and 0.1–0.2 for metal, glass and plastic.
Puddles: domain-warped world-space noise on up-facing faces. Coverage ≈ --puddles, feature size --scale (m, default 3). Inside a puddle: full coat, mirror coat roughness 0.02, and a flattened base normal, because water fills the cracks.
Ripples: expanding rings, one per ~17 cm cell, driven by a frame driver (a simple expression, so it works without auto-run scripts). They only show inside puddles.
Streaks: vertical noise on walls adds coat runs.
Snow: up-facing coverage with a noisy edge, albedo 0.9, roughness 0.6 and a fine bump. --snow-depth adds real thickness with the ENV_SnowCover GN modifier: up-facing faces are subdivided and extruded upward by a noise height that is never zero, so there's no z-fighting. It uses the ENV_M_Snow material with subsurface.
--vertex-mask writes the ENV_Weather color attribute (R puddles, G snow, B walls) for engine shaders.

lightning.py

A strike is 3 return strokes ~50 ms apart (at least 2 frames apart, so the flicker reads at 24 fps), each a peak followed by a 2-frame decay. --strokes, --flashes or --frames set the timing. The flash is a cool ENV_Storm_Flash sun (12° soft) keyed against the scene's key light (×2.5).
ENV_Storm_Bolt is a branching emissive curve (midpoint displacement plus 4 branches), keyed with the flash. By default it is 300 m away, in frame and off-center. The world background strength flashes too (--no-sky to skip).
Keys use LINEAR interpolation (flashes must snap, not ease).

Rules

Rain is visible only when backlit or side-lit against a darker background: put a practical or bright sky behind it. Front-lit rain disappears.
Combine layers: fog (atmosphere) + rain streaks (mid) + splashes/ripples (ground) + wet reflections. Any one alone looks fake.
Real scale is mandatory: speeds, densities and puddle sizes are in meters (blender-scene-setup).
Check with a render at the final resolution. Rain density and streak visibility don't show in the viewport (10% count, no fog).
Run blender-asset-qa on the snow geometry if the asset will be exported.

Games and real-time

Weather is an engine system (Unreal Niagara, Unity VFX Graph, Roblox ParticleEmitter). From Blender, export the masks: the vertex color attribute (--vertex-mask) or baked textures (blender-texture-baking self-bake of the wet material), plus snow cover meshes. Details are in references/realtime-engines.md. Physical values and the look library are in references/weather-values.md.

Gotchas

GN particles are instances, so object.data stays empty. Read results through the evaluated depsgraph (depsgraph.object_instances).
Background (-b) evaluation counts as the viewport for the Is Viewport node, so a headless test sees 10% of the count. Renders get 100%.
Ground objects are linked into the ENV_Weather_<Type>_Ground collection (linked, not moved).
surface_weather changes shared materials: every object using that material gets wet. Duplicate materials first to limit it.
Coat appears in both engines. EEVEE needs raytracing on (eevee.use_raytracing) for good puddle reflections.