[[File:BAScoldweatherclothing.jpg|thumb|right|upright|Antarctic clothing [ECW] used by the British Antarctic Survey in Antarctica]]
In arctic or mountainous areas, the primary function of extreme cold weather clothing is to trap air as an insulator to prevent heat loss from the wearer's body. Secondary and necessary is to conduct water vapor away from the body to keep the insulating layers dry. A shell keeps the wind from disturbing the still air in the insulating layers. In warmer conditions, the shell protects from water intrusion.
The U.S. National Weather Service defines extreme cold as with winds less than . In these conditions, the unprotected skin of a healthy adult will develop frostbite in ten to twenty minutes. The Canadian standard includes even lower temperatures.
Principles
A vacuum is the best insulator, but its use in clothing is impractical. Dry air is a practical insulator. Extreme cold weather clothing uses still dry air to insulate the body, trapping air via layers of material. The inner layers should conduct moisture away from the body. Outer layers should be windproof as well as suitable to the harsh terrain.<!-- not controversial, but a good reference would be useful -->
Materials
The original cold weather clothing was made of furs. The fibers of the fur trapped insulating air, lanolin on the fur repelled water. Knitted wool is an effective insulator when dry, but ineffective when wet. Goose down is the lightest insulator, and still used today. Its quality, called loft is a measure of its low density. It is ineffective when wet.
Artificial fibers have good loft, and do not lose it when wet. One effective fiber is Hollofil a hollow fiber of polyolefin. Outer garments are often made of nylon, which is strong and abrasion resistant. The nylon is often bonded to a layer of polytetrafluoroethylene (trade name Teflon) in a form that has holes small enough for moisture to escape, but not allow liquid water to intrude. This material is trade named Gore-Tex.
Best practices
The U.S. Army describes cold weather best practices by the mnemonic acronym COLD.