Any seasoned camper will tell you the key to staying warm in cold weather is not a big coat or thick socks, but rather a series of thinner layers built on top of each other to trap in warm air. But stacking layers isn't just for campers; anyone weathering cold winter temperatures throughout the world will benefit from a layering system. If you're one of those people, start with the most important layer, the base layer.
As the layer of clothing closest to your body, the base layer is less about warmth and more about the regulation of moisture. During outdoor activity --even just walking through town -- the body sweats just as it does in warmer weather. However, we are less likely to realize it than we are in warm weather, so it is much easier to forget that moisture against the skin in cold weather will make you colder. Therefore, the base layer is designed to wick moisture away from the skin, keeping skin drier and warmer.
A base layer can be made of many different materials, but the most common is polypropylene. Also known as polypro, this base layer is a synthetic material that is extremely effective in pulling moisture away from the skin. Polypro can come in the form of shirts and pants and performs its function well, but it is typically thin and therefore not very warm in itself. Warmth created by the body can escape the base layer, but an effective layering method includes a second layer to trap that warm air.
The number of layers you must wear depends on the intensity of the cold outside. Heat that escapes through the base layer will be kept close to the body by the next layer of clothing. The second layer keeps trapped air warm, which in turn keeps your body warm. You may wear as many layers as you need to keep warm, and you can top off your layering with a thick, water resistant coat.
One of the most useful aspects of the layering system is the ability to peel off or add layers as the temperature fluctuates. Wearing one large, bulky coat limits you to either extremely warm or extremely cold, whereas many layers allow you to adjust how warm or cool you are at any given time. As you become more active during exercise, your body will become warmer and it may become necessary to peel off a layer. Conversely, when you finish exercise or if you are simply standing still --as a spectator at a sporting event, for example -- you may need to add layers to further insulate your body. The layering system is far more adaptable than one bulky coat, thereby allowing you maximum comfort in a variety of cold weather situations.