Thermal underwear has the role of capturing and recycling the heat produced by your body, while wicking away moisture. Worn like a second skin, it is the first layer of your outfit, before the insulating and waterproof layers.
In winter, underwear must, more than anything, provide you with optimal thermal comfort. Depending on the temperature, which sport and how you practice it, and your resistance to the cold, choose either long or ¾ ski leggings (which will prevent wrinkles in your ski boot) and a long-sleeved t-shirt with a reasonably high collar. For maximum comfort, you can also adjust the thickness of your underwear to the other two layers you are wearing. If you’re wearing a thick, very warm second layer of insulation, such as a fleece or down jacket, it’s better to go for thinner underwear to avoid getting too hot.