The lightest, fastest, and most breathable jacket, great for high-intensity rides or staying at the right temperature in cool conditions. Membraneless, with Polartec Alpha insulation. Nano FlexCastelli take their thermal fleecy stretch Thermoflex fabric and coat it with millions of tiny nanofilaments to create the most water-repellent fabric finish ever. Drops of water stay as spherical balls on top of the fabric or bounce off the fabric without leaving a trace. Polartec Alpha ActivePolartec Alpha was invented for the U.S. special forces. They required a superior insulation system for extreme temperature variations and the start-stop conditions of combat. Alpha active insulation is engineered for consistent warmth, while allowing greater breathability during activity. This air exchange technology creates sustained comfort without shedding layers, which isn’t an option during a mission.