Yes, on exposed skin: cold is not shade - NIH MedlinePlus notes UV and temperature are less connected than you think, and snow reflects up to 80% of UV back at you, which is why skiers burn under their chins.
Add altitude - UV climbs about 10% per thousand feet - and winter mountain days are serious sun events. The rule follows exposed skin, not the calendar: daily facial SPF year-round, SPF lip balm and goggles for snow sports. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Do I need sunscreen in winter?
Yes, on exposed skin - because UV and temperature are different things, and winter has a multiplier nobody feels. NIH MedlinePlus puts it plainly: UV levels drop in winter but temperature and UV are less connected than you think - a cold bright day still delivers radiation to your face, the one body part winter clothing never covers. Then the multiplier: snow reflects UV - the Skin Cancer Foundation notes fresh snow can bounce back up to 80% of the sun's rays, effectively doubling exposure - which is why skiers burn under their chins and noses, the underside surfaces no summer beach hits. Add altitude (UV climbs roughly 10% per thousand feet) and winter mountain days are serious sun events. The practical rule: daily facial sunscreen through winter is the dermatologist-standard habit - your face accumulates photoaging year-round; add lip balm with SPF and sunglasses or goggles for snow sports, because snow blindness (photokeratitis) is literally a corneal sunburn. The part you can relax: bundled body parts need nothing - the rule follows exposed skin, not the calendar.