What causes skin to darken in the sun?

The increase in pigmentation or coloring in the skin is a
combination of the Ultra Violet (UV) rays that come from the sunlight and the amount of
melanin within our skin. The sunlight will hit the cells of our skin. Immediately after
that, when it hits our skin, it causes for our melanin (or the amount of coloring
already within our skins) to increase its production. Each individual reaction and
sensitivity to UV/UVA rays from the son will eventually determine to what extent the
pigmentation of the skin will darken. Some people become really dark while others just
turn out reddish instead of dark. Eventually everything will depend on your individual
response to the rays.  Do not forget also that excessive exposure to UVA rays is
extremely dangerous and may cause that same pigmentation process to turn against itself
and produce cancerous cells.