What is the relationship between UV radiation and skin color?​

The erythema response of skin to ultraviolet radiation (UVR), i.e., the sunburn reaction, has long been known to correlate with skin color. Dark-skinned individuals are able to tolerate a longer sun exposure than light-skinned individuals. Skin color in humans is mainly determined by the quantity, quality, and distribution of the complex biopolymer, mel~ni1_1, and its organized form, the melanosome (1,2}; melanm IS thought to protect skin from the deleterious effects of UVR.

Gravitational force i believe