One thing that surprises people when they begin to hike in the winter is how much your need for insulation can fluctuate. Natalie and I found that out on our recent hike up Buford Mountain. It was something I should have remembered from my distance running days.
When Natalie and I arrived at the mountain it was several hours after dark and the temperature was in the mid 20s with a wind blowing. Stepping out of the car at the trailhead with my little girl I thought "what have I done?" We bundled up in our warmest coats, hats, and gloves (my fingers quickly became numb while trying to adjust my pack straps) and began walking. I was dreading the evening ahead of us. But within a few minutes we were unzipping the coats. As the climb became steep we were removing our hats and gloves. I would have taken the coat all the way off but it was inconvenient with the pack. So we were hiking with winds and below freezing temperatures and feeling hot.
You should think about your body the way you think about a car or a steam engine. It's hard to get going in the cold but once you get started moving your muscles produce a tremendous amount of heat. When I was running I learned that if I were comfortable at the beginning of a long run then I would be sweating 15 minutes later. You need to start by feeling a little cold.
The difference between hiking and running is that typically you only stop running at the end, when you are about to get into your car or house and go for a hot shower. But during a hike you may stop to rest many times. At these times you find out that your muscles quickly stop heating you up and you get cold in a hurry. If you were sweaty when you stopped then that heat loss is hugely accelerated and you can quickly begin to suffer from hypothermia.
So it's important for a hiker to dress in layers that can be easily removed and put back on. Also each layer should be made of some material that will wick moisture but still keep you warm when it's a bit moist. So cotton is OUT. The saying among search and rescue teams is "the best-dressed corpses wear cotton."
Natalie and I did some things right and some things wrong on this trip. I'll show you what we wore.
The layer closest to your skin is called the "base layer". Here is a very unflattering picture of me wearing my base layer.
The two types of materials most favored for base layers are polypropylene and wool. Here I am using both. My shirt is an Icebreaker 200 merino wool top. My bottoms are polypropylene thermals. My socks are Smartwool hiking socks.
Most people are used to thinking of wool as being scratchy and not something you would want against your skin. However merino wool is much finer and softer than other wool. The icebreaker top I'm wearing is as soft to the touch as any cotton T-shirt. Wool is a great insulator and will keep you warm even if it is wet. The advantage of wool compared to synthetics is that it seems to never absorb odors. I know this sounds unlikely but I have tested it. I have worn this top on a number summer hiking trips and got pretty sweaty (particularly my back because of the pack). In between I didn't wash the top. After 5 or 6 times I was stunned that it smelled the same as it did the first day. Synthetics don't have this property as any runner (or their family) can tell you.
The polypropylene bottoms I picked up at a military surplus store. In the military they wear something like this as part of their Extended Cold Weather Clothing System (ECWCS) apparently. They are very warm.
Over your base layer you put on a layer that can be removed if you get too hot. Some people call this the "fleece layer". That's a marketing gimic made up by the people who sell fleece material. Fleece makes an excellent insulation layer and is warmer per ounce than wool. But wool has other advantages as I've mentioned. Here is Natalie and me in our Icebreaker tops that we use for insulation layer. Mine is a 260 zip top (in the picture it is zipped all the way down) and hers is a hoody. We love our Icebreaker tops but they are very pricey!
One interesting thing about my top is that it has a thumb hole in the sleeve. You can put your thumb through that hole and if you pull anything on over the top (like a jacket or gloves) the sleeve will stay down by your wrist rather than sliding up your arm. Pretty clever.
The problem with insulating materials like wool or fleece is that they are not windproof. A good stiff wind will cut right through to your skin and steal your warmth. Many people react by getting a warmer jacket when all they really need is something to stop the wind. This is my wind shirt that my lovely wife made for me using the Liberty Ridge kit from Thru-Hiker. He sells materials and patterns for backpackers who like to make their own gear (I bought the materials for my synthetic quilts from him). This top is made of a very thin but water resistant/breathable material called Momentum 90. The entire thing only weighs 2.2 ounces! It's so small that when I take it off I can put it in my pocket when I feel warm and want more ventilation.
In the picture I'm also wearing REI Sahara pants which are made of nylon. These are actually convertible pants so you can zip off the bottoms and have shorts. I have no use for that in the winter but I can't see a reason to have summer pants and winter pants. In both cases you need a wind layer on the bottom.
Now you'll notice that I'm wearing 3 layers on top and only 2 on the bottom. That's pretty common. My bottom base layer is extra warm and so it's sort of a combination base layer/insulation layer.
Now for Natalie I didn't have any insulation layer that would fit under her nylon camping pants so I had her wear these snow pants. Perhaps overkill but remember that I'm a paranoid parent. The sides do unzip (as you can see here) so she could ventilate if she needed to. Underneath she wore a thin base layer.
The mistake that Natalie and I made was to bring warm coats. I didn't even put my wind shirt on because a coat is essentially an insulation layer and a wind layer in one. But therein is the problem. A coat is not adjustable enough. I would have been better off with a down vest that would fit under my wind layer (I have a larger wind shirt I could use for that purpose). That would make one small piece that I could take on and off easily. On Saturday when we were hiking and it got above freezing it was too hot to wear a coat but not warm enough to do without it entirely.
We'll do better next time.