Wick, Wick, Wick
Understanding the art of layering your clothes for cold-weather running is more important than each individual piece of gear itself. No layer is more important than the other—they all work together to provide you with the most efficient warmth.
The Base Layer is the first layer to touch the skin, so its purpose is to manage moisture, wick away sweat and regulate your body temperature. This is essential. If your skin stays moist in freezing temperatures, you won't be able to warm up completely and you put yourself at risk for becoming hypothermic.
Pro Tip: Stay away from cotton as a base layer in winter weather. Cotton soaks up moisture and dries slowly instead of evaporating or wicking it away, which can make you colder. Clothes made of merino wool and polyester are ideal to wear when running in cold weather, and some brands have even created their own proprietary synthetic blends, such as Mizuno’s Breath Thermo.