Human skin, (of the white kind), is a good accumulative UV dose meter. It turns red, blisters, and curls up, in that order.

The sunlight that reaches ground level contains a percentage of UV radiation divided arbitrarily into UVA and UVB. The shorter UVB is responsible for most of the damage. The percentage of UVB is determined by the elevation of the Sun, the clarity of the air, and the state of the ozone layer in the upper atmosphere.