Properly layering your clothing is essential to staying warm and comfortable in the cold. A simple mnemonic device can help you remember the basics of dressing for winter activities outdoors.
First you’ll want a (non-cotton!) next-to-skin base layer that is both warm and effective at moving body-chilling moisture (a.k.a. your sweat) away from your skin and out of your clothing system. This layer should be as form-fitting as possible without being constricting in any way.
Next comes your insulating layer, typically a fleece or down jacket, which provides the majority of the warmth. As a general rule, the thicker the insulating layer, the warmer it will be. Keep in mind that you may not need this layer during periods of heavy exertion.
This is your outer layer, which protects you from wind and precipitation. Depending on conditions, it might be an ultralight windbreaker, a breathable and water-resistant soft shell, or a fully waterproof but less breathable hard shell featuring Gore-Tex or an equivalent.
It’s smart to carry an extra layer in the event of changing weather, an emergency, or an unexpected night out. This can range from a down vest for your core to a puffy go-over-everything down or synthetic-fill jacket.