The Icebreaker 200 MerinoFine Ace Long-Sleeve Half Zip is a midweight merino base layer built for cold days with changing effort, from a slow approach to a harder push uphill. Its 17. 5-micron MerinoFine fiber is spun using Icebreaker's OTW Omega Twist process, a spinning method that pulls moisture away from skin faster than a standard merino weave so the layer dries quickly instead of holding dampness against the body. What Our Gearhead(R) Experts Are Saying:"I drop the half zip the second my effort picks up on a climb, and the fabric catches up fast instead of staying damp. The gusset and thumb slits mean I never have to stop and adjust anything else."