Why are there random lines in the sky for Minecraft?

It's probably the game's version of clouds but I'm not really sure. It could also be a glitch. For better answer results I would suggest putting this question in the gaming category too. Hope this helps.

I think it might be clouds. Try disabling them in your settings.

It's a possibility those are clouds with that Texture/Resource Pack you're using. Check where you got that Texture/Resource Pack to see if there's an updated version of it.