Why do I tan more in winter than in summer?

To be honest, I haven't experienced this but it's probably because I don't sunbathe much. I don't know if the winter tan is also related to the skiing/mountain activities. If this is indeed the case, I think it has two causes. The first of these is that moist air scatters UV radiation more (that is, when travelling through the atmosphere, UV radiation rays are scattered in different directions and do not hit a given surface as intensely). Due to higher temperatures in the summer, there is more evaporation, the air contains more moisture and therefore less UV rays fall on us, which causes a weaker production of melanin, which turns the skin brown.

The second effect (especially in the mountains) is that white snow reflects light and therefore UV radiation well. Our skin is therefore exposed to UV rays both directly from the Sun and reflected from the snow.

And quite possibly, we don't pay as much attention to a possible tan in the winter as we do in the summer — we typically forget to pack sunscreen in the winter.