100% Merino Wool All-Season Short Sleeve Base Layer Tee This versatile merino tee is a true all-season all-star. Merino is naturally heat-regulating, so it will help you stay warm in winter and cool in summer. And with its natural elasticity, you'll feel comfortable all day no matter what you're doing. The benefits of merino are plentiful: it's biodegradable and renewable for lessened environmental impact; wrinkle- and UV-resistant for long-lasting low maintenance; and naturally breathable for comfort.