Winter Clothing: Shell or No Shell?

Quite a few readers have asked about winter clothing. Most of all, they seem surprised that I don’t wear windbreakers or shells unless it is very cold (way below freezing) or raining very hard. What is the best clothing for winter riding?
I think the answer depends. If you ride at a brisk pace, you tend to generate so much heat that you tend to stay warmer. The extreme example are cyclocross races. Even in 45-degree weather, I race in shorts and an extralight wool jersey with short sleeves, without being cold. Climbing mountain passes at night, we often wear just shorts and an extralight short-sleeve jersey, despite the temperatures being rather nippy (in the photo below, we ran into snow just a little higher up the climb).

During winter rides, I layer up in wool. I often wear three or even four layers, starting with a short-sleeve undershirt, then a long-sleeve base layer, followed by a long-sleeve jersey, and, if it is really cold, a thicker wool jersey on top. For my legs, wool usually tights suffice. If I add shells to this, I tend to get clammy, because the brisk pace not only generates heat, but also transpiration.
Even when it rains, I prefer to have my outer layer get wet, since even the most breathable shell tends to disrupt the moisture transfer. The heat transfer from my body outward keeps the inner layers dry. (I have to add that I use fenders that keep all spray off my body, and a handlebar bag that shields my legs from the rain.) However, if it rains so much that more moisture comes down than goes outward from my body, I use a shell to keep myself (marginally) drier.

I also use a shell for mountain descents. I don’t pedal much on long downhills, so the outward heat and moisture transfer are much-reduced, while the wind (and rain) come at much greater velocity. A shell keeps cold air from penetrating my clothing and reaching my skin.
If you pedal without generating as much heat, then a shell may be useful even while riding on the flat. As always, experiment to find out what works for you. Every rider has a unique body, so all these thoughts are just starting points for figuring out what works for you.
You also may be interested in our previous post about how to stay warm on a ride.