Not to be rude or sound like an ass here, but why are you posting this in WIP? THis should be in Off Topic for comment.
As for the atmosphere, that is typically added in post production through use of Premiere or Combustion, etc. Yes it can be acheived through lighting, but not the full effect.
When you say that most battles are not fought on clear, sunny days, I would tend to think that that is false. A battle is fought when it needs to be fought. Not when the weather is crappy. You are basing that statement on what Hollywood movies and TV have typically shown to add to the over all atmosphere and feeling that they want to portray in the movie or show. If you ask some real veterans or current day soldiers, thay will tell you that you fight when you need to fight. Could be raining like a bastard, snowing, or perfectly sunny, swimming day weather. The war doesn’t wait.