Wicking
“Wicking” simply means that sweat is carried away from your skin, helping to keep you from getting too cold. Base layers provide the wicking layer, and are the primary level of insulation. They are the make-or-break foundation that can be the crucial difference between having a great time or a miserable experience. All undergarments, including socks, underwear, and bras are considered base layers, not just long underwear. Glove liners also fall into the category of a wicking layer. Choose synthetic fabrics, wool, or certain silks for your base layers. Combining preferred base layer fabrics with cotton undergarments defeats the purpose, so the base layers themselves should be worn as your primary undergarments instead. Some choose to wear short-sleeved and short-legged base layers under long underwear, and two pairs of socks just to provide another layer of insulation. This is fine, based on the individual’s preference, as long as the materials are appropriate and the wearer is prepared to take these extra layers on and off as needed.
Warmth
The “Warmth” layer is your secondary set of insulation. This includes shirts, jackets, hats, neck gaiters, ski masks, and other balaclava-style headwear. Preferred materials include wool, synthetic (non-cotton) fleece, synthetic fiberfill, or down. Down is lightweight and very warm, but dries very slowly if it gets wet. Some down fill is now being treated with hydrophobic chemicals to help remedy this effect, so that’s something to look for if you’re interested in going that route. Again, some choose to wear two insulating layers over their base layers, which is just another matter of preference. Not to beat a dead horse, but cotton is not a preferred insulation layer, as the moisture wicked away from your base layers will soak into it and chill you. As far as outdoor activities go, “cotton is best forgotten”.
Weather (Wind)
The “Weather” or “Wind” layer is also sometimes referred to as the shell layer. This is the layer that is at least water-resistant, preferably waterproof. It keeps the other layers under it dry, and keeps the wind from cutting through and carrying off all of the heat you’re trying to retain. Gaiters fall into this category, as do boots, jackets, and pants. Gloves and mittens tend to fall into a category that straddles both warmth and weather. There are a wide variety of fabrics available to meet this need, but try to select one that’s breathable so that condensation doesn’t collect inside. If it’s not breathable, go for a design like a rain poncho that is very loose-fitting to allow for air circulation. Depending on your level of exertion and weather conditions, you may find yourself stripping off warmth layers and wearing your weather layer directly over your wicking layer. This is precisely why the layering system was developed: to give you quick versatility and options as conditions change.
Pro tip: Go bold, start cold! It may seem like a good idea to put on all of your layers and take them off as you warm up, but this can be a hassle when you’re trying to set out doing whatever you had planned for the day. Instead, consider starting off with fewer layers and stash the rest in your pack. You’ll warm up fast from the exercise involved with your activity and the excitement of starting your adventure, and you can always pause to add a layer after 10 minutes, if needs be.