Typically there are three layers in a cold-weather clothing system. The base or wicking layer lies against the skin and is responsible for pulling moisture away so you remain relatively dry. This minimizes wet conductive heat loss. Commonly used fabrics for the base layer include polypropylene, polyester, Capilene, Thermax, wool and, more recently, carbon fiber. Cotton is the worst base-layer material. Cotton holds moisture against the skin and feels cold, wet and heavy. In my experience, silk isn’t much better.
Second, we have the insulating layer. Its prime function is to trap warm air and continue taking moisture away from the body. Synthetic fibers work great for this because they retain their insulating properties even when wet, and they do it without bulk. Fleece is the “in” fabric because it’s light and has a high insulative value. It also dries quickly and is easy to care for.
The third layer is the jacket or shell you wear on top of the base and insulating layers. It must resist wind, rain and snow, and hold in warmth while allowing enough body heat to escape to prevent overheating. It needs to let in cooling air through zippered openings and vents. Open the front on climbs and in tailwinds, close it on descents or in headwinds. Beware of outer layers made of nylon or plastic that keep rain and wind out but hold in body heat that makes you wet inside. High-tech fabrics such as Gore-Tex are pricey but they perform well in harsh winter conditions and hold up through years of use.
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