snow or sweat turns into water vapor and is trapped under your goggles, if there is ventilation it is because the water vapor condenses on the colder lens. anti fog treatment creates a non-polar surface so the polar water molecules don't wanna stick (oil n' water).
use toothpaste on the lenses, or rub antipersperant all over your face and eyelids. girls, close your eyes when applying 'Secret'. Guys, don't smoke while doing this with the 'Right Guard'