Go to Settings > Custom Shaders Patch > Weather FX . Set the Weather script to "Sol".
Sol requires CSP to function. Version 0.7r5 is very old, but modern CSP versions (1.76 or newer) are generally recommended for stability. SOL_0.7r5_Win64_.zip
Use a tool like 7-Zip to open SOL_0.7r5_Win64_.zip . Go to Settings > Custom Shaders Patch > Weather FX
After installation, you must activate the mod in Content Manager settings: SOL_0.7r5_Win64_.zip
In Settings > Assetto Corsa > Video , set your Filter to a Sol-compatible one (like "Sol" or "Sol__Extra") for the best visuals.