Smartwool's heaviest line, Midweight, is perfect for forays into alpine environments where temperatures - and schedules - fluctuate. Comfortable next to the skin, and offering the world’s most efficient moisture-management system, the Smartwool merino wool fibers keep you cool or warm, depending on whether you’re at the crux of a climb or belaying from the bottom. It also makes a great secondary layer for when temperatures make an unexpected drop.