Well i sent a more nuanced answer from my blackberry but it never showed up so here goes.

 

 

1.) the atmosphere editor is very easy to understand and is very well documented in the manuals.

2). More important is your eye. Look at this picture and try to determine where the sun is in the sky. Play with pitch and azimuth in an empty scene until you get the sun there. What kind of clouds are these. You may want to do some research on cloud types, or just remember what you learned from high school. How dense are they? do they let light through everywhere or just at the edges? How high is the top and how low is the bottom. Are there more than 1 type, are they in layers, how far away are they. What about fog and haze, how low, how high, how dense, what color. That is how you do this atm.