WeatherFX is the new Shader/modul, which will deliver all functionality to use the new features like 24h cycle, moon, stars, new sky Shader, sky color gradients, new clouds Shader and many more. Ilja will do a basic implementation, which is like a template to see all functions. Sol is just an advanced usage of WeatherFX with its own special functions like dynamic weather change and easy custom weather creation. If WeatherFX is out, every programmer can use it to create own weather systems...
So Sol will be the successor of the whole gbw stuff.