You prompted “raining heavily throughout the scene” and the rain looks great for the first 2 seconds, then thins to a drizzle, then disappears entirely by second 5. Same problem happens with “neon flickering,” “smoke filling the room,” or “snow falling.” This is temporal prompt decay — the model attends strongly to the prompt at the start of a clip and progressively less as it generates later frames. Fix it with timestamped storyboard prompts on tools that support them (Veo, Sora storyboards), shorter clips chained with consistent prompts, or first-frame anchoring that makes the effect already present visually.
Common causes
Ordered by hit rate.
1. Single global prompt for a long clip
You wrote one prompt and asked for 10 seconds. The model attends to the prompt at frame 1 strongly and frame 240 weakly. Anything continuous (rain, fire, smoke) decays.
How to spot it: Plot the keyword effect frame by frame. Strong at start, fading toward end is the decay signature.
2. Keyword is not visible in the first frame
If you wrote “raining throughout” but the first generated frame has no visible rain, the model loses its anchor. The rain only ever appears as the prompt’s word, not as a visual pattern, and decays even faster.
How to spot it: Pause at frame 1. If you cannot see the effect there, the model is unlikely to reinforce it later.
3. Tool does not support timestamped prompts
Runway Gen-3 Alpha, Pika 2.0 base mode, and Kling 1.6 take a single prompt per clip. There is no native way to say “rain heavier at second 4.”
How to spot it: Check the UI. If there is no timeline / storyboard / per-segment prompt input, you are stuck with single-prompt decay.
4. Clip duration is at the model’s upper limit
5-second Runway clips are pretty stable. 10-second Runway clips lose continuous effects more often. Pika’s 10-second mode is more decay-prone than its 4-second mode.
How to spot it: Generate the same prompt at 4s and at 10s. If 4s holds the effect and 10s drops it, duration is the issue.
5. Prompt overloaded with competing details
Prompt mentions rain, neon, crowd, traffic, breath fog, and a specific character action. The model has limited attention budget; secondary effects decay first.
How to spot it: Strip the prompt to one effect plus one action. If decay disappears, overload was the cause.
Shortest path to fix
Step 1: Use timestamped storyboard prompts where supported
# Veo 3 timeline / storyboard mode
Frame 0s: "Heavy rain falling, neon-lit street, character walking forward."
Frame 4s: "Rain intensifies, neon flickers, character pulls hood tighter."
Frame 8s: "Heaviest rain, neon distorted through water on lens."
# Sora storyboards
Card 1 (0-3s): "Continuous heavy rain, soaked streets, steady visibility through downpour."
Card 2 (3-6s): "Same rain intensity, character pauses under awning, water sheeting off the edge."
Card 3 (6-9s): "Rain continues at full force, character steps back into it, water visible everywhere."
# Kling 2.0 Multi-Shot mode (where exposed)
- Shot 1, 0-3s: rain emphasis
- Shot 2, 3-6s: rain plus character motion
- Shot 3, 6-9s: rain at peakStep 2: Chain shorter clips at the model’s stable duration
If your model has no storyboard, do it manually:
# Generate three 4-second clips
- Clip A (0-4s): "Heavy rain, neon street, character walking, rain visibly soaking jacket"
- Clip B (4-8s): "Same heavy rain, character continues walking, water beading on hood"
- Clip C (8-12s): "Continuous rain, character reaches doorway, water dripping off edge"
# Stitch in Premiere / Resolve with 8-frame Cross Dissolve
# Each clip is short enough to avoid decayStep 3: Anchor the effect in the first frame
For image-to-video, build a reference frame that already shows the effect at full intensity:
# Reference image checklist
- Rain streaks visible in the air across the frame
- Wet surfaces, puddles, water on character's coat
- Lens flecked with water droplets
- Specular highlights on wet asphalt
# Prompt for the video pass
"Continuation of the depicted scene: same heavy rain, same wet textures,
maintain rain intensity throughout, no drying, no fading."If the first frame visually commits to the effect, decay is much weaker.
Step 4: Reduce prompt to one effect plus one action
# Bad (overloaded):
"A woman in a red coat walks through a rainy neon-lit street at night with
flickering signs, crowd around her, breath visible, taxi passing, holding
phone, looking up..."
# Good (focused):
"A woman in red coat walks through heavy rain on a neon-lit street.
Continuous heavy rain throughout. Steady forward motion."
# Then add complexity only after rain holdsStep 5: Add the effect in post if all else fails
Some effects are easier to add later than to keep alive in generation:
# Rain overlay in CapCut / Premiere
- Add a "rain stock footage" layer with Screen blend mode
- Adjust opacity to match generated rain
- Helps continuity even if generated rain decays
# Smoke / atmosphere
- ProductionCrate or ActionVFX have free overlays
- Blend mode Screen or Add
- Color-match in Lumetri / Color page
# Neon flicker
- Apply Strobe / Flicker effect in editor on a separate luminance layerPrevention
- Use storyboard / multi-shot mode by default for any clip over 5 seconds.
- Always anchor continuous effects in the first frame visually, not just in prose.
- Cap single-prompt clips at the model’s stable duration (4-5s for most current tools).
- Keep prompts focused: one effect plus one action.
- Pre-build atmosphere overlays (rain, smoke, flicker) for post-production fallback.