Interesting. I believe wiper rain sensors project infrared onto the windscreen screen glass.The short distance is expected. If there is no rain most is reflected back and read by a detector . If there are rain drops on the glass the beam is disrupted to a lesser or greater extent.
I am not 100% sure but I believe some of the other camera based driver aids will be projecting beams through the glass and reflecting back from distant targets such as white lines, solid objects etc. If the windscreen is dirty or has heavy condensation etc the beam is interrupted much too soon y and will register as an anomaly . The systems will be considered unreliable and shut down.
Also I think the computer switches on headlights without reference to sensors if the wipers are running at speed for a certain length of time. It assumes lights will be needed in these circumstances.
Plus there is a light intensity sensor.
If I am right about the above I can imagine that fog would be quite difficult to detect . It wouldnt disrupt beams and detectors enough to be reliable. Light intensity is not necessarily reduced. Just my theories and I may be repeating what has already been said. Feel free to disagree
