Which vitamin does your skin make from sunlight?

Vitamin D is not really a vitamin. A vitamin is by definition something you must eat because your body cannot make it, and your skin makes this one perfectly well given ultraviolet light - it is closer to a hormone. It got the name because it was discovered while looking for a dietary cure for rickets.

The wavelength that does the work is UVB, and it does not pass through glass or much cloud, and it arrives at a steep enough angle only when the sun is reasonably high. Above about fifty degrees of latitude - Britain, most of Canada, northern Europe - there is a stretch of winter during which you can stand outside all day and make essentially none.

This is one of the leading explanations for why human skin lost most of its pigment as populations moved north. Melanin protects against UV damage, but it also blocks the UVB you need, and at high latitudes with little sun there is not enough to go round. The trade-off runs the other way at the equator. It is one of the tidier examples of a visible human difference having a boring biochemical cause.