So let's say you're driving along and all of a sudden you come across a patch of fog. Instantly, your visibility is diminished and your first instinct it to switch on your headlights to get a better view. Bravo! You'll need those headlights to see clearly. But then you push the headlight button just one click further and your high beams are on full blast, tearing light straight through that nasty fog like a pro, right? Wrong.
High beams will not help you see well in the fog. In fact, they're actually going to decrease your visibility. The light from high beams reflects off of the fog and comes back into your eyes, making it more difficult to see what's in front of you. Think of how difficult it is to see when the sun reflects the wrong way off of the car in front of you. You move your head, squint, or look away so the sun's not in your eyes. The fog acts similarly when your high beams are on, sending too much light back into your eyes.
One reporter from Dateline NBC took a test drive with a visibility expert from Virginia Tech (yeah, we didn't know that was an actual job either) and found that when he turned the high beams on he couldn't see a pedestrian that was standing near the car. When the high beams were turned down to low beams, the pedestrian magically appeared.
So in short, don't put the high beams on in fog. It may sound like a good idea but, like a lot things, sounding right just doesn't cut it.