Whatever are we to make of the UV ratings bandied about on some radio and TV stations? As excess ultraviolet exposure can lead to all sorts of unpleasant health effects, from severe erythema (red skin, dude) to cancer, one curious reader wants to know what exactly these ratings mean. Unfortunately, they have something to do with atmospheric ozone and Dobson units and the coefficient of … well, we’ll just let Umbra explain — in Ask Umbra, today on the Grist Magazine website.