A bivy sack is something of a riddle – both tent and sleeping bag, but not truly either of those items. A bivy is an ultra-portable personal shelter for backcountry hikers, climber or skiers who are more concerned with their objective than they are with spending relaxed time in a tent. A bivy is slightly larger than a sleeping bag and uses tent poles to create a minimalist shelter – an outer shell for your bag to repel nasty weather. It’s the shelter of choice on alpine climbs, big-wall expeditions, solo through-hikes and other weight-conscious adventures.
The top of the heap, the 37-ounce Advanced Bivy can survive the onslaught of a big-mountain storm. Like an expedition tent, it’s fully seam-sealed and the floor is coated with thick, waterproof nylon. It’s more than a dry bag for your sleeping bag, it’s an actual mini tent, complete with two poles and a foot vent.
The Alpine Bivy weighs 30 ounces (seven ounces less than the Advanced). But you lose some features when you trade off for the lighter weight and more packable design, since it comes with a single pole and does not have a foot vent.
If you’re looking for the most packable, minimalist shelter for a fast-and-light objective, the light (22-ounce) Aurora Bivy offers the same GORE-TEX® Respiration Positive™ fabric as the above models, but it has no poles at all, and is thus essentially a waterproof cover for your sleeping bag.