It's tempting to reach for cotton thermals, but they're one of the biggest mistakes you can make on the mountain. Cotton has poor moisture wicking ability—it absorbs your sweat and holds it right against your skin instead of pulling it away to evaporate. You might feel warm at first, but once that fabric gets damp, it stays damp.
The real danger hits when you stop moving. Wet skin loses heat 25 times faster than dry skin, so that sweaty cotton turns icy cold the moment you're sitting on a lift or taking a break. Even on frigid days, you sweat while skiing—especially during runs, hiking to terrain, or just working hard in heavy gear.
Instead, look for base layers rated "Good" or "Excellent" for moisture wicking. Synthetics like polyester pull sweat away fastest, while merino wool absorbs some moisture but still keeps you from feeling clammy. Either option keeps you dry and warm, which cotton simply cannot do.