If you wear your cotton base layer out on a ski trip, you're going to have a bad time. Skiing is a high-intensity winter sport, which means your clothing needs to be designed to handle perspiration in cold temperatures without leaving you wet and hypothermic. The best thermals for skiing are quick drying, warm when damp and close fitting for comfort on your longest days on the slopes.
A thermal or base layer is your first protection against a chill when you're in the backcountry during winter. Thermals fit underneath your insulated clothing and waterproof shell, and fit next to your skin for comfort. The main purpose of a thermal is to wick moisture off of your skin as you perspire during high-intensity activity. This makes a quick-drying fabric ideal. The best thermals use flat seams to prevent any rubbing from the layers you wear over them. They can consist of a single piece or a separate top and bottom, and might include features such as a zip neck, zip ankles, a drawcord waist or pockets.
The best thermals for especially wet conditions are made from synthetic petroleum-based fibers such as polyester and nylon. These fibers are essentially made of extremely thin plastic and do not hold any moisture. Moisture can still be trapped in the spaces between the fibers of a synthetic thermal but will quickly evaporate from your body heat. Synthetic thermals stay warm when wet, unlike cotton fibers, which can be essential for preventing hypothermia out in a snowy mountain pass. Higher quality synthetic thermals have special treatments to provide some odor resistance.
Better than a Wool Sweater
Merino Wool is a special variety of wool fiber from merino sheep. These fibers are much thinner than traditional wool, making a merino wool base layer much less itchy than a common wool sweater. Unlike synthetics, merino wool fibers have a scaly appearance under a microscope and can absorb up to 30 percent of their weight in water. This water is held in the core of the fiber, so a wet wool garment won't feel clammy. A thermal made from merino wool is also much more odor-resistant than synthetic, so you can come off of your ski trip without smelling like a locker room, making it the best choice for longer ski days. Wool is a natural resource that is harvested without any harm to the sheep and will break down naturally over time, making it an eco-conscious choice.
The best thermal depends on the conditions on the mountain and your personal preference for comfort, but other alternatives to wool and synthetics are also available. Silk is a natural fabric that has better odor resistance and thermal properties than a synthetic, with slightly less moisture wicking and drying performance. Silk layers are lightweight, making them the best choice for ounce-counting backcountry skiers who want high performance in a more compact package.