Castelli can officially only rate these warmers at UPF 50+, but in their laboratory tests they actually blocked 99.8 percent of damaging UV rays. Using a polyester base they found this it is not only more effective than nylon at blocking UV rays but is also much better at keeping you dry, which means these skins are better in both cooler and warmer conditions. Castelli have kept the construction very clean to eliminate seams that can rub, and used double-sided silicone elastic to make sure they stay in place. They fold up extremely small, so they’re great to stick in your pockets for your next big mountain day.