Comment by mixedbit
In winter, even on a sunny day, only tiny fraction of your skin is exposed to sun. 10-30 min of sun when you are wearing tshirt and shorts is much different from 10-30 min of sun when you are wearing long sleeves, gloves, and a scarf.
It's not really the exposed skin that's the issue. At higher latitudes the ultraviolet (UVB) gets scattered by the longer path through the atmosphere and so even if you were naked you still wouldn't be getting enough.