When it comes to dressing for the outdoors, layering is probably the most important concept one should know about. Layering refers to putting on several thin to medium-thick layers, which you can then remove or put back on as needed. The following example details this idea.
Let’s say you and your partner are out on a ski hill. It’s below zero, snowing occasionally and a bit windy. You’re wearing a base layer, a mid-layer, shell pants and a warm, lined jacket. You stay warm and cozy when you’re standing still, sitting on the lift or having an outdoor coffee, but as soon as you start going downhill and exerting yourself, you get hot — too hot — and start to sweat. But you can’t take off the jacket, because then you’ll have no protection against wind or precipitation, and removing the middle layer doesn’t help because the jacket is so warm that you still sweat.
Your partner, on the other hand, is wearing a base layer, a slightly thinner mid-layer, another mid-layer, shell pants and a shell jacket. [note: I have made this “partner” gender neutral] They also have a packable insulated jacket in their rucksack, along with the thermos and sandwiches. If they start to get cold during a coffee break or on the chairlift, they throw on the insulated jacket; when it’s time to go downhill again, they take it off and stuff it back into their rucksack. While you feel sweaty, they neither freeze during inactivity nor become excessively hot during skiing. If they still feel a little too warm, they have the option of removing one of their two middle layers.
It’s pretty clear to see who dressed smarter here. To summarize, it’s better to wear several thin and medium-thick layers of clothing under a shell than a single mid-layer under a thick jacket.