Next to the Skin
Keep your first layer light and comfortable.
When choosing your baselayer, keep in mind you’re going to be wearing it all the time. You’ll want something close fitting but not restrictive. When in doubt choose a thinner layer, because it’s much easier to add a layer than to change into a lighter base layer during the middle of the day. The main function of the base layer is moisture management. You need a fabric that will wick moisture well. Wool is best, but there are good synthetic options out there too. However, wool is vastly superior when it comes to odor management as well (something your tent mates will thank you for). Your baselayer is the foundation of comfort for any cold weather activity so don’t skimp on it. You’ll likely be wearing it for days at a time and be sleeping in it too.
Filling in the gaps
Insulating layers are less fickle. There are all kinds of layers that will work here; Synthetic fleece, down puffy layers, wool sweaters. The only rule is STAY AWAY FROM COTTON. For high output activities, like ski touring, moisture wicking is still crucial. A thinner grid-fleece layer like the Wintergreen Portage Top or Patagonia R1 works great. When you really need to hold in the heat, there is nothing like big puffy jacket.
The weight of your insulative layers will depend on the type of activity you’re doing and the temperature outside.
For example: While skiing to the North or South Pole, while moving you need fairly little in the way of insulation because of the heat you’re producing through exertion. However, when stopping for a break, the first thing you do is to throw on a puffy parka to trap that heat until you get moving again. If you try to ski with too many layers, you start sweating and end up getting very cold once stopped.
PRO TIP - Puffy vests are a very versatile layer that can add that extra bit of warmth to your core without making you overheat, and they’re so packable that there’s no excuse not to bring one.
The layer that protects you from the brunt of the elements. Your outdoor armor.
We won’t get into fabric technology, only the functions of a shell. Wind and rain/snow are the main elements to guard against. Without a windproof layer, even a light breeze will steal precious heat. The function of a waterproof shell speaks for itself.
Waterproof shells are also windproof but generally don’t breathe very well, so it will keep all of the moisture from your body trapped inside. If it’s snowing hard, the tradeoff is worth it, but here is where ventilation comes into play. Many waterproof shells have vent pockets and zippers under the shoulder (pit zips) to help vent without compromising waterproofness. During high output activities, venting heat and moisture can become a tough chore. If your climate doesn’t call for waterproofness, a windproof anorak or softshell jacket will cut the wind without sacrificing breathability.
DON’T GET SWEATY!
Do whatever you have to. Remember that moisture is the enemy. If you’re starting to get too warm, open up your jacket or take your hat off for a few minutes; It’s easy to make small adjustments to stay dry, but hard to get dry AND harder to stay warm once you’ve soaked your base layers through with sweat.
Examples of Layer Combinations and Their Uses
Base Layer + Windproof shell (cross country skiing on a breezy day)
Base layer + Midlayer + Windproof Shell (A normal day skiing to the North Pole)
Baselayer + Midlayer + Puffy jacket + Insulated Shell = Very cold, low output activity (watching other people ice skate, sitting on polar bear watch)
Underwear + Puffy Jacket = (Letting the dogs out on a chilly morning)
PSA on Microplastics
The next time you're shopping for a layer, consider natural fiber options like wool. Synthetic fabrics degrade over time, releasing fibers that do not biodegrade. These "microplastics" infiltrate the entirety of the global water system and their impacts are not yet fully known.
For additional info about fabrics, microplastics, and best practices, check out this great summary by Patagonia.