They could be triggered in appropriate situations, for example just as the sun starts to rise, it would be typical to have a light, rainy mist, or even, far less frequently, a dense fog.
Desert areas could be prone to "sandstorms", and some dungeon areas could be filled with a vile green, almost luminescent fog.
Some mockups of how it might look:

No effect

A cool mist

A dense fog

A sandstorm
Of course it would be rather difficult to play in a dense fog, or a sandstorm, but that's realism for you

They also have the added advantage of making the game look absolutely beautiful (imo)
