You lose a lot of heat through your head, especially with a well-ventilated bike helmet. Layer one of these skull caps under there and get some extra warmth to get you through the cold months. This winter hat is Pearl Izumi's warmest full-coverage hat, so you get plenty of extra insulation.
Barrier windproof front panel blocks chilly winds from your forehead and ears
P.R.O. Transfer fabric provides extra insulation with moisture-wicking properties to prevent sweat buildup