Does weather or time of day affect combat in XCOM:EU?
Solution 1:
When you aim at a target, you can click "More info" to get a detailed breakdown of what modifiers affect your chance to hit the target. There is never a night/day/weather modifier to be seen.
Checking movement and visibility for hidden modifiers is more tricky, but I've not seen any evidence of that either, so I'd say no, there are no such modifiers.
Solution 2:
No the time of day and weather does not affect line of sight or gameplay in any way. It is just atmospheric visual. This was stated by the game designer Mr Solomon in a video interview.