For winter time riding, it all starts with the base layers. You'll want to find bottoms that lock in warmth, yet have the ability to breathe since you're going to perspire while biking, no matter the temperature. On top, wind chill resistance is a variable as much as warmth and breathability, the same things concerning your legs. Generally speaking, the cycling base layers should fit relatively tight, so as to maximize keeping warmth locked in. You won't find any base layers made with cotton, the worst material in cold situations. Rather, you will find space age nylons and polyesters which have the dual property of heat containment and breathability. More specifically, biking base layers are usually a combination of polyesters, spandex, and small infusions of other materials which give optimum warmth and stretchability.