Castelli Flanders Wind Long Sleeve Base Layer Windstopper fabric on front is stretchy, windproof, and water-resistant Multi-denier polyester 3D structure rib-knit on back and main body for mechanical wicking and exceptional fit Multi-denier polyester 4-way stretch sleeves for great wicking and fit Perforated microfiber polyester mesh underarms for additional cooling Temperature Range: 10-18 C / 50-64 F