Grinding up a steep skin track in the cold builds heat fast, and the Marmot Aerothermal 1/2-Zip Pullover is a lightweight insulated midlayer that keeps pace with the climb. PrimaLoft Active Evolve synthetic insulation delivers soft, fleece-like warmth without the bulk and helps regulate temperature as effort changes. A recycled polyester ripstop shell adds durability, and raglan sleeves with a touch of stretch keep every stride and reach moving freely. What Our Gearhead(R) Experts Are Saying:"After a few cold-morning trail runs, I noticed the pullover never felt swampy once the pace picked up, and I kept it on from the first warm-up mile to the last hard push."