System	Driven by	Category on UDW
Rain Particles	Rain value	Rain Particles
Snow Particles	Snow value	Snow Particles
Dust Particles	Dust value	Dust Particles
Shared particle settings	All particle systems	Shared Particle Settings
Lightning Flashes	Thunder/Lightning value	Lightning
Obscured Lightning	Thunder/Lightning value	Lightning
Wind Direction	`Wind Direction` Basic Control	Wind Direction
Wind Gusts	Wind Intensity value + perlin noise	Wind Gusts
Wind Debris	Wind Intensity value	Wind Debris
Wind Directional Source	Wind Intensity + Direction	Wind Directional Source
Wind Physics Force	Wind state per actor	(component class)
Wind Camera Shake	Wind Intensity	Wind Camera Shake
Weather Sound Effects	Wind / Rain / Thunder values	Sound Effects
Sound Occlusion	Player Occlusion (on UDS)	Sound Occlusion
Environment Sounds	Time / Weather / Wind (via metasound)	Sound Effects

When to use this skill

Rain or snow not visible, or visible where it shouldn't be.
Choosing between particle collision modes for perf vs flexibility.
Lightning bolts striking specific actors as game logic.
Adding wind force to physics objects or making chaos cloth react.
Weather sounds going quiet at wrong times.
Adding ambient outdoor audio (birds, insects, city) that changes with time/weather.
Diagnosing VR splash particles only rendering in one eye.

Rain / Snow / Dust Particles

Three per-particle categories on UDW: Rain Particles, Snow Particles, Dust Particles. Each is controlled by the corresponding weather state value.

Category	Specific extras
Rain Particles	Splash particles (raindrop impacts on collision surfaces); color + lighting + max count
Snow Particles	Particle scale + max count + ambient light intensities
Dust Particles	Particle scale + color + alpha + max count

For shared behaviors (collision and spawning), see Shared Particle Settings below.

Splash Particles in VR

Symptom	Cause	Workaround
Splashes render only in one eye	Instanced Stereo doesn't support GPU sprites with deferred decal materials (the default splash setup)	Set Splash Particles Rendering Mode to Translucent. Renders in both eyes; subject to translucent lighting limitations.

Weather Particle Collision

Set with Particle Collision Mode.

Mode	Cost	Flexibility	Best for
Simple Collision	CPU cost (significant)	High — uses level simple collision	Default; need precise blocking
Distance Field Collision	Cheap on CPU (pure GPU)	Limited — can't separate from distance-field lighting	Open worlds with distance fields
No Collision	Cheapest	None	Diagnosing "particles aren't visible"

Simple Collision details

Line traces against simple collision geometry of the level. The particles trace on one channel — Weather Particle Collision Channel (default: Visibility).

Niagara particle traces are queries — meshes need queries enabled on their collision response.

GPU particles take a small number of valid particle paths and reuse them — still can be a significant CPU cost.

Particles not blocked when they should be (Simple Collision):

Object lacks simple collision geometry. Traces only check simple collision.
Collision profile isn't set to block the channel selected on UDW (default Visibility), with collision response Collision Enabled or Query Only (Niagara uses queries).
Particles coming through a high ceiling: raise Ceiling Check Height on UDW.

Distance Field Collision details

Pure GPU emitters querying the global distance field.

Constraint	Detail
Requires project to generate mesh distance fields	Project Settings
Distance field collision and lighting can't be separated	Surface that blocks particles also affects distance-field lighting; vice versa
Only works while global distance field exists	Engine scalability can disable it

Particles not colliding (Distance Field):

Project not generating mesh distance fields.
View Show → Visualize → Global Distance Field to see what particles actually see.
For meshes that should block, enable Affect Distance Field Lighting.

Blocking particles without a mesh

UDS Occlusion Volume actor at Blueprints/Occlusion/ — enable its setting to block weather particles. Only available with Simple Collision mode.

Latency-free per-sphere blocking

For a moving space where particle collision latency is a problem (e.g. a player vehicle interior):

Component	Effect
`Weather Particles Kill Sphere`	Kill any weather particles inside the sphere with zero latency

Only one Kill Sphere can be active at a time globally. Manage activation/deactivation manually for multiple.

Shared Particle Settings (Spawning)

In Shared Particle Settings. Particles spawn only in a volume around the camera; the illusion is they spawn everywhere.

Setting	Effect
`Max Spawn Distance`	How far from camera particles spawn (X/Y)
`Spawn Distance Distribution`	How particles distribute within that range
`Spawn Box Height`	How far above camera particles can spawn
`World Spawn Offset`, `Camera Forward Spawn Offset`	Relocate the center of the spawn area
`Custom Weather Particle Camera Transform` (advanced dropdown)	Spawn around a manual location instead of the camera

Particles Not Visible

Cause	Fix
Particles blocked by an invisible collision source	Switch Particle Collision Mode → No Collision to test
Editor viewport Show flags hiding sprites	Show dropdown → Use Defaults
Rare engine bug: Niagara compilation failed on first load	Open system asset in `Particles/` (Rain/Snow/Dust) → Compile dropdown → Full Rebuild
Running in Simulate play mode	Limitation: it renders through editor camera and neither the blueprint or Niagara can sample the editor camera for the particle spawn area. Use a different play mode.

Lightning

Two types:

Type	What	Setting
Lightning Flashes	Big bolts that cast light + crash of thunder	Interval + light source enable/disable + colors
Obscured Lightning	Idle flashes hidden in cloud layer	Frequency

Both scale with the Thunder/Lightning value of current weather state.

Thunder sound settings live in the Sound Effects category — see below.

Manual lightning flash

UDW.FlashLightning(customLocation, customTargetLocation)
// customLocation: optional; should be at cloud level (root of bolt)
// customTargetLocation: if nonzero, the bolt ends there

Lightning striking specific actors

1. Add the UDW_Lightning Strikable Actor Interface to your actors
2. Implement "Test for Potential Lightning Strike":
   outputs: can-be-struck-now (bool) + world location where strike hits
3. On UDW → Lightning category:
   Enable "Enable Lightning Flashes Striking Actors"
4. Set "Fraction Of Lightning Flashes Which Can Strike Actors"
   (how many potential flashes go to actors when any are in range)
5. Implement the interface's "Struck by Lightning" event
   to spawn extra effects or run game logic

Lightning in Sequencer

1. Add UDW to the sequence
2. Create Event → Trigger track on UDW
3. Add a keyframe; right-click for properties
4. Endpoint → Quick Bind → Flash Lightning
5. Configure custom location/seed in keyframe properties
6. Check Call in Editor to preview in the sequence editor

Wind

Wind Direction

Controlled separately from weather state via Wind Direction in Basic Controls + the Wind Direction category. Direction drives weather particles and cloud movement on UDS.

Setting	Effect
Wind Direction Variation	Random noise variation at runtime, centered on Basic Controls Wind Direction

WOVs can override wind direction. If Apply Wind Direction is true on a WOV, the volume's wind direction wins as the player enters.

Wind Gusts

Wind intensity has a perlin-noise variation applied to simulate gusts. Settings in Wind Gusts: scale of variation, speed of the perlin noise.

Wind Debris

Small debris particles flying through the air when Wind Intensity is high. Category: Wind Debris.

Setting	Effect
Amount, scale, texture sheet	Debris appearance (texture must be 4×4 with alpha)
`Wind Debris Particle Spawn Count`	Live-updates at runtime
Spawning/collision	Uses Shared Particle Settings

Wind Directional Source

A Wind Directional Source is an engine component used by chaos cloth and SpeedTree assets. UDW makes and controls one with current wind by default.

Category	Setting	Effect
Wind Directional Source	Scale Wind Speed	How high Wind Speed is set on the source
Wind Directional Source	Disable when in interior	No wind speed when inside an interior

Wind Physics Force

Apply wind force to physics-simulating objects. Add the Wind Physics Force component as a child of any primitive component simulating physics. At runtime, the component queries UDW wind state and line-traces to test exposure.

Wind Camera Shake

Enable in Wind Camera Shake to shake the player camera at max wind speeds.

Setting	Effect
Scale of shake	Magnitude
Range of Wind Intensity that triggers shake	Threshold band
Player Occlusion masking (automatic)	No shake when player is in an interior. Controlled by Player Occlusion on UDS — see `uds-modifiers-configs-state`.

Weather Sound Effects

Enabled by default for wind, rain, thunder. Category: Sound Effects.

Setting	Effect
Enable/disable globally	Master toggle
Per-effect Volume	Scales with weather state and these settings
`Close Thunder Delay Per KM`	How much thunder is delayed from the visual flash. Raise for realistic speed-of-sound delay.

Runtime control

UDW.SetEnableWeatherSoundEffects(bool)   // toggle all
// Or set the per-sound Volume variables directly

All UDW sounds use the UDS_Weather sound class. Add it as a child of your project's Sound Effects class so weather sounds inherit your sound mix.

Sound Occlusion

Default: a simple system periodically traces from the camera and adjusts sound volume + low-pass filter (muffling) if the player is in an enclosed space. Toggle/adjust in Sound Occlusion category.

The actual occlusion calculation lives in Player Occlusion on UDS — see uds-modifiers-configs-state.

Forced-occluded zones

Place a UDS Occlusion Volume actor (Blueprints/Occlusion/) to always-occlude a space. The Occlusion Sampling Mode in Player Occlusion decides if volumes are sampled.

Sound portals

UDS Occlusion Portal component. If instances exist around the occlusion sampling location, they're traced against and can negate occlusion when the trace is unobstructed. Use case: an open door/window letting outside weather sound naturally into the space.

Your own outdoor sounds following weather occlusion

Set your own sound sources to use the sound class UDS Outdoor Sound — they get occluded by the same volume attenuation as weather sounds.

Environment Sounds

System for general ambient outdoor audio (birds, insects, city ambience) that responds to time/weather and the sound occlusion. Sound Effects category on UDW selects the environment sound asset; a Forest ambience example ships.

Authoring an Environment Sound asset

1. Make a new data asset of type UDS_Environment_Sound
2. Point at a metasound source asset (the actual sound played)
3. Tune volume + parameter overrides

Metasound source format

For sound occlusion and four-direction stereo panning, output format must be 5.1:

Channel	Use
Front Left (0)	Directional audio from -X
Front Right (1)	Directional audio from +X
Front Center (2)	Directional audio from -Y
Low Frequency (3)	Directional audio from +Y
Side Left (4)	Global audio left channel
Side Right (5)	Global audio right channel

Directional audio is spatialized based on listener orientation and occluded directionally. Global audio is stereo, occluded by global occlusion.

Audio bus setup

Source Settings:
   Enable Bus Sends: true
   Enable Base Submix: false
   + Post-Effect Bus Send → audio bus "UDS_Weather_AudioBus"

Reacting to Time/Weather/Wind

Five input parameters UDW sets on your metasound:

Param	Type
`Time`, `Weather`, `Wind`	Integers
`Weather Interp`, `Time Interp`	Time parameters

Included metasound nodes use these:

Node	What it does
`UDS_TimeWeather_VolumeManager`	Outputs a changing volume using array inputs for what the volume should be in different times / weathers / wind levels
`UDS_DayNight_VolumeManager`	Simpler day/night version

Reference: included Forest Example metasound source at Sound/MetaSounds/.

Runtime control

UDW.ChangeEnvironmentSound(asset)  // change at runtime
UDW.ChangeEnvironmentSound(null)   // stop environment sounds

Gotchas

Splash particles only in one eye in VR — Splash Particles Rendering Mode = Translucent.
Particles not visible at all — invisible collision source blocking; or Show flags hiding sprites (Use Defaults); or Niagara compile fail (Full Rebuild on the asset).
Particles ignore an interior space (Simple Collision) — ceiling too tall; raise Ceiling Check Height. Or use Weather Particles Kill Sphere for a moving interior.
UDS Occlusion Volume doesn't block particles — Particle Collision Mode is Distance Field. Switch to Simple Collision (volume blocking is Simple-only).
Lightning never strikes the player despite the interface — Enable Lightning Flashes Striking Actors is off, or Fraction Of Lightning Flashes Which Can Strike Actors is 0.
Sounds go quiet at incorrect times — Sound Occlusion traces hitting surfaces they shouldn't. Adjust Player Occlusion on UDS (trace count, channel, update period).
No sound from open door/window — add a UDS Occlusion Portal component to the door/window. Make sure traces can reach it.
Wind doesn't affect chaos cloth / SpeedTree — Wind Directional Source category on UDW isn't enabled, or Disable when in interior is on and the actor is occluded.
Wind Camera Shake doesn't trigger — Wind Intensity below the threshold band, or player is occluded.
Environment Sound metasound playing but no panning/occlusion — output format isn't 5.1, or Post-Effect Bus Send to UDS_Weather_AudioBus is missing.
Custom outdoor sound not occluded with weather — assign it the UDS Outdoor Sound class.

References & source material

Docs (Ultra Dynamic Weather 9.5):

Rain Particles
Snow Particles
Dust Particles
Weather Particle Collision (Simple / Distance Field / Kill Sphere / Occlusion Volume)
Weather Particle Spawning
Lightning (Flashes, Obscured, Flash Lightning, Strikable Interface)
Weather Sound Effects
Sound Occlusion (Portal, UDS Outdoor Sound)
Environment Sound (metasound format)
Wind Direction
Wind Debris
Wind Gusts
Wind Directional Source
Wind Physics Force
Wind Camera Shake
Common Issues — Weather (incl. VR splash, Simulate mode)

Plugin asset paths:

Particles/ — Rain, Snow, Dust, Wind Debris niagara systems
Particles/ — Lightning Strike + Obscured Lightning systems
Sound/MetaSounds/ — included Forest Example environment sound metasound source
Blueprints/Occlusion/ — UDS Occlusion Volume actor; UDS Occlusion Portal component
Materials → UDS_Weather sound class; UDS_Weather_AudioBus audio bus
Component classes: Weather Particles Kill Sphere; Wind Physics Force

Related skills: udw-setup-and-state, udw-spatial-weather (WOV wind override; UDS Occlusion Volume), uds-modifiers-configs-state (Player Occlusion calculates occlusion; AmbientSound — Time and Weather Controlled is the simpler sound option), udw-material-and-screen-effects (material wetness/snow accumulates from particle landing context).

udw-particles-lightning-wind-sounds

UDW particles, lightning, wind, sounds