A bright, brisk fall or spring day can turn drizzly in a flash and leave standard arm warmers damp, chilly, and certainly not doing their job to keep you warm. That is, unless you have a pair of Nano Flex+ Arm Warmers. Made from Castelli's proprietary, drizzle-blocking Nanoflex fabric, these warmers keep you comfy even when the weather goes south. Nano Flex starts its life as Castelli's super-warm and ultra-stretchy Thermoflex fabric, used in many of its winter pieces, and then gains and outer coating of tiny nanofilaments of silicone fiber. These fibers create a near waterproof barrier to moisture, and raindrops run off the fabric without soaking through. Despite its water-resistant finish, Nano Flex fabric remains highly breathable, keeping you comfortable in almost any conditions. Across the back, Castelli utilizes a panel of its lighter Nano Light fabric to increase stretchiness around your elbow for full freedom of motion and comfort no matter how many miles are on the schedule.