bivy

A bivy, short for bivouac sack, is a small, lightweight weatherproof cover placed over a sleeping bag to provide protection from rain, wind, and ground moisture. It is used in camping and mountaineering as a minimal shelter option when carrying a tent is impractical, or in emergency shelter scenarios.

Most bivies are made from waterproof and breathable fabrics such as nylon or polyester with a DWR coating. They are typically sleeved around the sleeping bag, with a single entry and usually a hood or peak to shed water. Some models include a mesh panel to keep insects out, and a vestibule area for gear storage.

Variations range from simple one-person sacks to more elaborate pod-style designs. They are commonly used by ultralight backpackers, climbers, and rescue teams. They can be used in combination with a tarp or micro-tent in poor weather and are often included in emergency survival kits.

Advantages include very low weight, compact pack size, and rapid deployment. Disadvantages include limited interior space, potential condensation buildup between the bivy and sleeping bag, reduced ventilation, and minimal protection in severe weather. They require a suitable sleeping bag and careful selection for the climate.

Etymology and history: The term bivouac derives from French, referring to an improvised shelter; the bivouac sack is a modern adaptation used since the 20th century by outdoor enthusiasts and climbers as a compact alternative to tents.