Why do ski goggles fog up and how do I prevent it?

Fogging happens when warm, moist air from your face hits the cold inner lens. Anti-fog coatings, dual-pane lenses and adequate venting all help, and most goggles here advertise anti-fog treatment. Reviewers note that fogging is worst when goggles are pushed up on the helmet or when vents are blocked, so keep them on your face while riding and avoid wiping the inner coating.