Mike Kerekes, age 11, of Daly City, California, for his question:
Why do we get more tan from summer sunshine?
In summer the sun marches high over the sky. In winter its daily path is lower. We get the best tan in summer, especially in the middle of the day, because then the sun's rays come down from high overhead. On the way, they must pierce through hundreds of miles of the earth's atmosphere. When the sun is low in the sky, its slanting beams must come through a much thicker layer of air. And it is the gaseous molecules of the air that regulate the sun's tanning ability.
The different wave lengths of sunlight create the rainbow colors of the spectrum. Solar radiation also includes invisible longer and shorter wave lengths. Ultraviolet is shorter than the shortest blue rays. And air molecules steal more of the short wave lengths of radiation. The ultraviolet rays do the most to tan our skins. More of them reach us in summer because they travel a shorter distance through the atmosphere.