Staying warm in the cold is rarely about one thick garment. It is about layers you can add or remove as conditions change. The classic system has three parts: a base layer, a mid layer and an outer layer, and each has a different job.
The base layer sits against your skin and moves sweat away from it. Wool and synthetic fabrics do this well. Cotton holds moisture and leaves you feeling cold once you stop moving, so most people avoid it for cold-weather activity.
The mid layer traps warm air. Fleece, light down and synthetic insulated vests are common choices. Because this layer does the main insulating, it is the one you adjust most often as the temperature or your effort changes.
The outer layer, usually your jacket, protects the other two from wind, snow and rain. A good shell lets moisture escape while keeping weather out. Put the three together and you can handle a wide range of conditions with a few well-chosen pieces.
