Achieved using Advanced Reshade presets (like screen-space reflections) to fake wet surfaces.
Select the rendering API (usually DirectX 9 for the original, or DirectX 11 if using certain wrappers).
If you're interested, I can walk you through the specific installation steps for the Asylum Reborn pack mentioned earlier.
Rain streaks playing across the rusted iron gates, cracked stone walls, and decaying brickwork of the asylum intensify the feeling of isolation and dread.
If you want to experience Arkham Island under a torrential downpour, you will need the PC version of the game (either the standard or Game of the Year edition).
Modding older games is generally highly accessible on modern hardware, but adding high-density alpha textures (like rain) and screen-space reflections can still impact performance.