Unfortunately, since there's currently no quality option for clouds for DBP, I've decided to port over a vastly oversimplified version of my atmosphere shader from C++. I made a poor example here, and a better version would use better colour choices for the sky and horizon, as well as fixing my formula for positioning the sun. Nonetheless, here's an easy-to-use shader and set of functions ready for use in your game.
-Supports day to night transitions
-Supports clouds & stars
-Shader based - fast to load, fast on fill rate
You can easily add special colors for dawn, dusk, midnight, noon, etc by setting them yourself and using the vector linear interpolation commands to adjust them, but I didn't spend the time to do it.
EDIT: For even less transparent clouds, change in the shader
Color=lerp(Color,CloudColor,lerp(0,CloudColor.a,saturate(-IN.Normal.y*.333)));
to
Color=lerp(Color,CloudColor,lerp(0,CloudColor.a,saturate(-IN.Normal.y*.9)));
