I doubt for the cotton stuff I wear it's that big a deal but technical fabrics can be pretty transparent. Cosmetics companies have made some hay touting the SPF, why not clothes too?
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Skipjacks you are indeed more likely to get sunburned when it's hot for the same reason it's hot, the sunlight is more intense because of its higher (astronomical) altitude and the sun is up longer in summer. You get an extra shot of sunlight when you are skiing because of the higher (above sea level) altitude giving less attenuation of the light, and the sun reflected off the snow