In winter cyclists need external clothing for two purposes: warmth and weather protection (rain/snow and wind).
Here's my typical winter urban bike clothing set up for riding around or commuting. Colder means below -5C.
This presumes that you are cycling fast enough to be generating some body heat. Always stop and remove the optional layers when you get too warm, save them from sweat so that they are dry when you need them again for warmth or weather protection. If you dress so you are a bit cold when you start means you are perfectly dressed for when the body-generated heat from cycling does its thing and magically warms you up.
-light hiking boots or runners (urban, eh)
-Snug fitting synthetic blend "QD" cargo pants with knit cuff (the knit cuff is critical to cycling)
-If the temperature approaches -10, I add stretch patagonia capiline long johns
- Cycling shorts or cycling short-style boxers
-T-shirt or synthetic T (if colder or a longer ride)
-Sweatshirt or long sleeve synthetic zip-T (if colder or a longer ride)
-down vest (eddie bauer - bought in 1978!) or stretch fleece pullover (if colder)
-Colder is all these layers, warmer is less, more intense requires less insulation
-Co-op microfibre bike jacket, neon green, and with the pitzip always open (always!)
-v-shaped neck gaitor (velcro)
-neck tube (if colder or windier)
-polypro thin double layer tuque (ancient) fits well under helmet and unfolds for a bit more coverage when needed (it's still winter remember!) very wide temp range
-headband - optional or additional - thicker, direct ear protection and seals helmet
-silk balaclava - backup when I need a full coverage base layer (colder) or want the ninja look
-waterproof yellow rain helmet protector - hot! sweat accumulates, for rain only
-to zero: knit gloves or bike glove with liner glove underneath
- Below zero-some Yoko x-c gloves - knit cuff - liner gloves added as necessary
-Below -5 to -10 - big helly hanson nylon covered fleece mittens
-Rain: patadonia overmitts (bomber)
Extra clothes: If I start out with less clothing, rather than more, I try to ride with a couple of extras in backpack (Urban riding=backpack). I carry a neck tube a headband, and liner gloves.
Front and rear lights are installed on the bike (with fresh batteries!) A red light onmy back pack, and some reflective ankle bands are on the pack and around my ankle.