The key to staying warm in cool or cold weather is staying dry. When sweat and moisture stay trapped against the skin, they lower your body temperature and heighten your risk of developing hypothermia or weakening your immune system. This is why cold weather base layers are so important-they provide a permeable barrier that rests close to your skin and helps draw moisture away from your body, keeping you warm and dry.
But how do moisture wicking fabrics actually work? The answer is a matter of chemistry, and starts with the molecular properties of liquids. Moisture wicking is actually a product of capillary action-a natural property of fluids that allows a liquid to flow through tight spaces, even against gravity. The tightly woven fibers of most fabrics create the type of narrow pathways that allow for some degree of capillary action to take place. The fabrics with an exceptional ability to transport water from one side of a barrier (near your skin) to the other are said to be moisture wicking. These textiles behave like a candlewick, using their natural construction to draw sweat through the cloth.
Some fabrics like cotton actually absorb moisture, quickly becoming saturated and incapable of keeping you dry. This is why a cotton t-shirt makes a poor base layer in cool weather. It quickly becomes too wet to wick additional moisture away. Unlike the short, coarse fibers of cotton, silk is spun from long, fibrous strands that easily channel moisture away from your skin, even when the fabric is damp.