Thermal underwear or clothing is generally used as a base layer for alpine adventures, hiking during winter and sometimes as everyday wear in colder climates. To ensure that you stay warm, base layers are often made with merino wool or synthetic materials such as polyester. Advantages to wearing thermal base layers include a regulated core temperature while still allowing airflow and moisture-wicking abilities to remove perspiration. While you may think that warm clothing needs to be thick or heavy, our women's base layers are also lightweight so that you can pair with a mid-layer that keeps you warm and an outer layer that is waterproof and protects you from the snow, wind and rain.