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 footboxthe bag's bottom end swoops upward like a ski tipwon'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 meas well as several eager-to-try-it friendstoasty 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.