Temperature has nothing to do with it. Skiers get sunburned. It doesn't even need to be very sunny. As pidgeon92 pointed out, a person can easily get sunburned on a coudy or overcast day. Anytime there is light enough to see, there is the possibility of getting sunburned, although that risk is far lower in the north during the wintertime when the sun's rays are barely strong enough for a person to produce enough vitamin D.