The Nike ACG Lava Loft Therma-FIT Jacket pairs 750-fill down insulation with AeroLoft venting, which places perforations between the baffles specifically where the body runs hottest during effort. That combination holds real warmth on cold trail miles without trapping heat once the pace picks up. Stretch panels move with the body through a full stride, and a 2-way zipper plus a bungee hem let a runner dial in fit without losing venting access. At 9. 9 ounces, the whole jacket packs into its own internal pocket when the temperature climbs. What Our Gearhead(R) Experts Are Saying:"I start a climb bundled up and unzip the two-way front to vent heat through the perforated panels without ever pulling the jacket off, so I never lose the down warmth once the trail flattens back out. Packing the whole thing into its own pocket means I'm not carrying dead weight for the descent."