Bill Milne writes from sunny Sykesville: "Tell me about the UV Index. ... How much stronger is a value of 9 than a value of 7?" The UV Index, forecast below, is not a direct measure of ultraviolet radiation. It combines predictions of the sun's height at solar noon, atmospheric ozone (which blocks UV rays), local elevation and cloud cover, and estimates the UV dose rates to the skin. The World Health Organization says 0-2 is "low;" 3-5 is "moderate;" 6-7 is "high;" 8-10 is "very high." Just use sunblock.