The Patagonia R1 Ultralight 1/2-Zip Jacket goes on at the trailhead in the cold dark before sunrise, when our breath still shows, and the climb ahead hasn't started warming us up yet. The slim fit rides close enough to layer cleanly under a shell without adding bulk, and the collar zip offers a quick way to dump heat the second the pace picks up. By the time we're booting up the couloir or grinding out mile ten, it's already doing its job in the background, no adjustments needed. What Our Gearhead(R) Experts Are Saying:"Most midlayers I own would've had me stripping down within twenty minutes of hard climbing, but the flat-faced double knit let heat escape as fast as I built it. I hit the top warm, not soaked, so the transition to descending was comfortable instead of clammy."