Why is it an advantage for a broad leaf to have most of its stomata on the shaded lower surface rather than on the sunlit upper one?

Explanation

The top of a leaf is hotter and drier in sunshine, so pores placed there would let far more water vapour escape than pores tucked underneath. The idea that gases fall downwards out of the underside treats them as though they were heavy liquids, whereas gases diffuse in whatever direction their concentrations dictate.