BACKPACKER first reviewed the Atom LT when it debuted a decade ago, and it’s become a go-to midlayer for many hikers since then. But we haven’t given it a look since that initial writeup, which is why I’ve been testing the latest version. Arc’teryx updated the synthetic fill this year to make it more packable, changed the hardface (yet breathable) nylon shell, and tweaked the cut, and the result is a piece that I don’t want to take off.
This jacket has helped me run the temperature gauntlet: It kept me warm in below-freezing weather while I hiked snow-dusted trails near in the foothills of Colorado’s San Juans, and I didn’t overheat on a 40°F ski tour up 12,162-foot St. Vrain Mountain outside Rocky Mountain National Park until the skin track got steep at the very top. I’ve even worn the Atom LT on sunny, 60°F days, with the full zipper and underarm fleece panels helping with temp regulation.
Fit-wise, this thing is dialed: It hugs my body without feeling too tight. (Note: I'm on the slim side.) And the whole jacket scrunches down to the sized of a 1-liter Nalgene, an improvement over the original’s football-like packability. The only ding I’ve noticed is that the face fabric is conducive to stains more than other midlayers I’ve worn; be sure to treat it nicely. But that’s a small price to pay for a jacket that I’ll wear year-round in the mountains.