Tanning: Why does some parts of the body burn more easily.

Lots of tanning threads today - summer’s here.
And it got me thinking - If I spend time in the sun, why do some parts burn, while others don’t. At first, at thought it had to do with previous exposure, but that can’t be it: I burn the tip of my nose quite easily, but not the cheeks. Bot are outdoors, so to speak, all year. So is it the thickness of the skin? And how about the torso? The skin on my stomach doesn’t get burnt, but on my chest, it can really flare.

I’m a dedicated tanner (I know, BAD), and know from personal experience that some parts tan more deeply than others. I’m guessing that melanin isn’t evenly distributed throughout the body. Why that would be the case, I don’t know.