Heat Haze

Heat shimmer over the far desert by day in the enhanced outdoors: the distant land and the sky waver under a sunny sky, half as much under a cloudy one, fading in after sunrise and out before sunset.

Heat Haze is an entry in the Built-in mods database, listed under On.

Several of these change core rules, so the guide pages describe the game with them on. 'Fixed by the room' means online everyone plays with the mod at its shipped settings, whatever they choose offline.