Common sense tells you to be comfortable while running; this is also true in cold weather. Both body fat and clothing act as heat conserving mechanisms. High body fat is not conducive to good performance and is not common in runners, so most must learn to dress warmly. Layers of clothing trap and warm air between them to act as insulation. If clothing becomes wet either through sweating or external sources (rain, snow), it can conduct heat away from the body. Fabrics that are waterproof, but can still breathe are best for external layers. Polypropylene is excellent next to the skin as it wicks away the water and allows a warm air layer to remain. A major part of heat loss is through the head, so wear a hat or ski headband to help keep warm. Gloves are nice as well. You can remove gloves, hat or layers of clothing as you become warmer. Running with bare legs in cold weather is not advised. The red color of the skin shows that a great deal of the blood is detoured to the skin trying to keep the body warm and is not going to the exercising muscles where it is needed most. Cold muscles feel tight and are more susceptible to injury, especially pulls and strains.