As far as baselayers go, it's hard to beat the characteristics of Merino wool. Naturally breathable, very warm, and supremely comfortable, the Icebreaker 150 Zone Long Sleeve Half Zip Top is an ultra-lightweight top for heavy exertion. For ski touring, splitboarding, and other activities on the snow / sweat front lines, this 150g/m² marvel is soft enough to wear next to your skin, warm enough to keep your core temp regulated, and breathable enough to keep you comfortable all day long. With mesh cooling panels and optimized seam placement that eliminates chafing, this is long underwear from the future!