Why do my mittens get damp inside even when they are waterproof?

Internal dampness is usually caused by sweat, not external moisture. Mittens are so warm that hands sweat during active skiing, and the moisture has limited ventilation to escape. This is why breathability matters in mittens, and why removable liners are valuable — they can be dried separately. Using a moisture-wicking liner glove inside the mitten can also help manage sweat. The dampness becomes a problem when you stop moving and the moisture cools against your skin, which is why proper insulation that retains warmth even when slightly damp (synthetic) is important.