This is a very simple attempt to make a procedural weather generator in Unity. I start with some basic icosphere shapes, then distort them, and change their opacity among other things to generate the clouds. I use a basic line-renderer to make the lightning every so often.

Controls: WASD to move | Left Shift to "Sprint" | "E" to toggle walking/noclip | Space bar to move up while in noclip | "P" to toggle mouse cursor