Driving and fog: A science demonstration by Derek Kevra

Fog can be a dangerous thing, especially when you're talking about driving in it.

Essentially fog is just a cloud on the surface of the earth, and clouds are made up of water particles. So fog is water.

When you're using your headlights, the floating water acts like tiny mirrors that reflect the lights from your headlights back at you -- and back at other drivers. Using high beams can be bad and just about the worst thing you can do. 

Low beams aim the light down at the ground, allowing you to see the road and limit the amount of light bouncing around. 

So basically: low beams > high beams in the fog.