The Arc'teryx Men's Alpha IS Jacket is key when you only have enough room for a single layer because your backpack is overflowing with ice climbing gear and mountaineering equipment. It's Arc'teryx's single-layer solution for having fun in severe alpine environments; Gore-Tex gives it reliable waterproof, breathable protection to fend off snow, water, and ice. Arc'teryx treated the synthetic insulation with a DWR to prevent water absorption so you don't feel weighed down halfway through a long climb, and e3D patterning keeps the jacket close for thermal protection and still allows for full mobility on the mountain.