Winter can be cold, and it can be cold enough to freeze flesh if we add wind to the equation.
Windchill is what we got this morning. -13C temperature and 30-40 km wind (combined wind speed and bike speed) which calculates to a wind chill of -25C (-13F). That's c-c-c-cold.
Protection is important. Go here for more info about winter cycling.
Understanding the cold and windchill is a useful thing to do if you go outside and play in the winter.
The National Weather Service in the US has a nice web page and brochure on windchill here.