Tents have mostly won the ultralight argument, but there's one corner of the outdoors where a bivy sack still makes sense: deep winter and storm-battered alpine camps. Backpacker's latest deal spotlight makes the case with the Rab Storm Bivy, a tough little nylon shell currently priced under $150.
The pitch is simple. At 1 lb 3.3 oz (roughly 545 grams), the Storm Bivy weighs a fraction of any four-season tent yet is built for exactly the conditions that destroy lightweight three-season shelters. Backpacker's testers have ridden out blizzards on Rocky Mountain ski tours in bivvies like this, conditions that would push most ultralight tents to their limits.
The construction explains why. The upper is made from 40D polyurethane-coated nylon ripstop, which blocks wind and precipitation, while a heavier 70D floor keeps ground moisture where it belongs. The 19.8-square-foot floor area won't win any space awards, but it's enough to accommodate a lofty winter sleeping bag. Condensation, the classic bivy problem, is managed by a mesh panel with a hook-and-loop flap that lets air move without letting weather in.
If you want to shave even more weight, Rab's Alpine Bivy sits about $50 higher and drops roughly 3 ounces by swapping in lighter materials, including a breathable 15D Pertex Shield Air upper and a 40D bathtub floor.
Who's this actually for? Not anyone who likes to sit up and read at night. A bivy is a sleeve for you and your sleeping bag, nothing more. But for ski tourers, winter alpinists, fastpackers and anyone bivouacking below a big route, the maths is hard to argue with: a shelter that survives genuine winter storms at a quarter of the weight of a four-season tent, for less than the price of a mid-range sleeping bag. It also doubles as an emergency backup for ultralight summer trips where the forecast looks doubtful. If you're not claustrophobic and your itineraries lean stormy, it's worth a look.
