The Rainy Day pack replaces the old sounds for rain and thunder with much more realistic and atmospheric sounds. It also adds three extra thunder sounds, however, after 1.12 Mojang changed something and stopped these extra sounds from working. Finally, the texture of the rain has been changed, lowering the opacity to create a less in-your-face weather experience.
I give my permission for anyone creating modpacks to use this resource pack in their modpack.
The sounds I used to create this pack are all copyright free. Some have an attribution license, so these have been listed below:
115254__matucha__rain - CC BY 3.0 - https://www.freesound.org/people/matucha/sounds/115254/
140294__sean-townsend__rain - CC BY 3.0 - https://www.freesound.org/people/sean.townsend/sounds/140294/
198856__bone666138__rain-1 - CC BY 3.0 - https://www.freesound.org/people/bone666138/sounds/198856/
194849__aaronstar__nm-mejor-lado-thunder-01 - CC0 1.0 - https://www.freesound.org/people/aaronstar/sounds/194849/
195522__sarson__distant-thunder - CC0 1.0 - https://www.freesound.org/people/sarson/sounds/195522/
198857__bone666138__thunder-1 - CC BY 3.0 - https://www.freesound.org/people/bone666138/sounds/198857/
198858__bone666138__thunder-2 - CC BY 3.0 - https://www.freesound.org/people/bone666138/sounds/198858/
253953__yoyodaman234__thunder-1 - CC0 1.0 - https://www.freesound.org/people/Yoyodaman234/sounds/253953/
37774__swuing__donner1 - CC BY 3.0 - https://www.freesound.org/people/swuing/sounds/37774/
