Built as a minimalist prototype for our weight-and-performance-obsessed alpinists, the original Nano grew into a cult classic and spawned offspring. This pullover remains sleek and pared down, using the best available synthetic insulation and designed with a deep-venting half-zip to avoid harness-line bulk and minimize weight, while maximizing trapped heat. It's weather-resistant enough to wear as an insulated shell in a wet snowstorm, trim enough to pull on for chilly rock pitches (and still see your footholds), and warm enough for light belay parka duty on quick alpine dashes. The wind-blocking, moisture-shedding 100% recycled polyester shell also glides effortlessly, whether worn as a mid-or-outer-layer. Low-bulk, hydrophobic, highly compressible PrimaLoft® One (60-g) traps heat with remarkable efficiency, even when wet. It packs down to practically nothing and stuffs into a single chest pocket that has a carabiner clip-in loop. Elasticized cuffs and hem seal out wind and trap warmth.