My coldest commute was 1/14/2004, -4F/-20C. That was a 8.5 mi/13.6km each-way ride. I wore a LS Coolmax tee, a thin microfleece, wool jersey, wind vest and cape-vented Jackson-Gibbens jacket on top; tights and windpants on the bottom; wool liner socks & regular socks under ordinary winter walking boots on my feet; heavy wool mitts with leather "chopper" mitts over them on my hands; and a polypro balaclava under my helmet and ski goggles. According to my notes, I should have left the wind vest off, I was sweating too much. When it is cold enough that you have to completely cover your face, you will find your body is much warmer. To prevent chilling you never want to be dressed so warmly that you start to feel sweat rolling on your skin. One thing about riding at those temperatures, I've had 18 wheelers block intersections to let me through and one driver stopped, got out and was yelling "Bravo, Bravo" as I crept by at 10mph/16 km/h.

Yes, global warming is real, those are the last sub-zero (F) commutes I record.