When it's sunny, you do not need sunscreen.
False! Very wrong!
We are inclined to think that clouds create protection and block UV rays (responsible for sunburn, tanning, DNA damage, etc.), but this is not entirely true. What you need to know is that some types of clouds block some of these rays, but others accentuate their effects.
Scientists still do not explain why 100%, but there seem to be two main reasons. The first is that the rays reflect on the sides of very dense clouds and are redirected towards the surface of the Earth. The second is that they are amplified and directed towards us when they pass through a thin layer of cloud. Some even say that clouds let 80% of the rays, which is huge.
It must be remembered that UV rays are reflected on different surfaces, so even in winter we are likely to get sunburn. The snow and the water act like a mirror for the rays and send them back to us directly ... Remember also that the higher you are at altitude (eg mountain skiing), the rays are fierce.
So be careful, use sun protection even when the sky is overcast, summer and winter. ;)
Hockberger, PE (2002). " A history of ultraviolet photobiology for humans, animals and microorganisms ". Photobiol. 76 (6): 561-579