While most arm coverings are designed to keep you warm, the UPF 50 Light Arm Sleeves from Castelli can actually keep you cool too. They are rated at UPF 50, however, Castelli claims that in laboratory tests, they actually blocked 99. 8 percent of damaging UV rays. It uses a polyester base that's more effective than typical nylon at blocking UV rays while keeping you dry. Minimal seam construction prevents rubbing and double-sided silicone elastic at the openings ensure they stay in place throughout the ride. They also pack up extremely small so you can stash them into your jersey pocket when they're no longer needed for the day.