The worst thing that can happen to a pair of snow goggles is when they get fogged up and almost impossible to see out of. This happens when the difference in air temperature from the inside of the lens to the outside is high enough that the water vapor inside the goggle begins to condense on the inside of the lens. The colder it gets outside the easier and quicker a goggle will fog up making it impossible to ride safely.
To avoid fog snow goggles employ a dual lens that has an air pocket in between the lenses. This sealed air pocket prevents the inner lens from fogging because the outer lens can still be cold while the interior of the goggle can have plenty of water vapor, but it won’t condense because the inner lens is just as warm as the vapor. The sealed, dry, vapor-less air in between the cold and warm lens cannot condense because there is nothing to condense into droplets. If that seal is broken the anti-fog feature doesn’t work anymore and the lens should be replaced.
Snow goggles typically have a comfortable foam seal that keeps snow, ice and wind from entering. They are pulled tight to the face with a wide elastic strap that can be adjusted for many different head sizes and extend to fit around helmets. Fit and comfort are incredibly important factors, goggles are typically worn for hours at a time, along with a suitable lens for the conditions that day, either light, dark or mid level light range.