Mark Wiens

Behold the Gear of the Year: A 16-ounce, 900-fill-down wonder that's destined to be the new sleep standard for fastpackers and anyone else who appreciates gossamer weight in a three-season bag. Marmot's ultrastuffable Atom practically vanishes into your pack.

1. How feathery is the Atom, with its über-premium down and superlight Pertex fabric? It reminded me of a chunk of San Francisco fog I once spotted scudding along California Street. How compressible? Picture a cantaloupe.

2. The Atom's trapezoidal footbox—the bag's bottom end swoops upward like a ski tip—won't force you to splay your feet sideways.

3. Its 40-degree threshold might seem sketchy, but this nocturnal nest is all you need for most late-spring-through-early-autumn outings: It kept me—as well as several eager-to-try-it friends—toasty on a chilly September summit attempt yet stayed cool on summer car-camping trips to my local crags.

4. With its half-zip, the Atom won't be mistaken for a stateroom, but it slips on like silk pajamas and is roomy enough around the shoulders for sleepers who don't toss like a Maytag all night long.

5. Not many above-freezing bags sport a mummy hood, complete with an insulated face muff, and a cinch cord to batten down the hatches on brisk nights. Outfit yourself with a wool cap and some merino long underwear and push the Atom's rating down to the frost line.