That is a vague question, and it is something that should better be discussed in the forums with reference screenshots.
More speficcaly in cryengine there are many post process options that you can alter depending on the time of day this allowes you to make realistic looking environments for all times of day I’m sure there’s a way of doing this in unreal 4 I just don’t know how. Please forgive I’m new to ue4.
You should check out post process volume, atmospheric fog and light shafts in directional light properties.
https://docs.unrealengine.com/latest/INT/Engine/Actors/FogEffects/AtmosphericFog/index.html
Keep in mind that atmospheric fog may not be so useful if you have a sky sphere in the level.