When choosing garments for outdoor activities, it's more about wicking than warmth. Wicking means to pull the moisture away from your body. Choosing your base layer—the layer next to your skin—is one of the most important dressing decisions you can make before heading outdoors to recreate - especially in the winter. Everything else you're putting on after that is helping to keep you insulated. If you pick the wrong base layer, you're still going to be cold when you get wet or when you perspire.
The fundamental principle of outdoor clothing—especially winter clothing—is no cotton! A common saying to remember is "cotton kills." Cotton absorbs perspiration, but it doesn't take it away from your body. When you're moving, it's fine but as soon as you stop, you may become chilled by your wet clothes. Wet makes you feel cold, putting you at greater risk of hypothermia and pulled muscles. The days of the traditional waffle-weave cotton long johns are long gone!
Silk has always been known for its luxury and comfort. What some people may not know is that also makes a good base layer that will help keep you warm, even when you perspire. Silk long underwear and turtlenecks are light, thin and breathable, as well as sweat-wicking.
The composition of wool naturally endows it with high performance qualities for cold weather activities such as warmth and water-repellent. The interior of wool fiber can hold up to 30% of its weight in moisture without the wool fabric feeling damp or clammy, while the exterior is water repelling. Also, when moisture enters the wool fiber, energy is released in the form of heat which helps keep the outdoor enthusiast warm. The crimped structure of wool fibers creates fabrics loaded with tiny air spaces which act as nature's most efficient insulator. It is the air trapped between the wool fibers and not the fibers themselves that keep you warm. Several apparel manufacturers have created wool clothing that doesn't itch, shrink or smell like a sheep when wet.
Wool has a reputation that it'll keep you warmer if it does get wet, but synthetics aim to alleviate wetness in the first place. Synthetics dry faster, so they're good for aerobic or sweaty activities like skiing, kayaking, hiking, or biking. Wool probably holds a little more heat and is a good option for a winter hike. Synthetics covers a wide range of clothing. Polypropylene is another word for polyester. Microfiber technology has led to a smaller fiber that can be woven more densely, making it thermal, wind resistant and can wick moisture away quicker. You can also find clothing with UV protection from the sun and synthetic fleece material that is soft, comfortable, warm, and fast drying.