Several of you have probably heard me say at some point that I don't bike below -5 °F. This is purely empirical -- I've never successfully biked at lower temperature, because at -5 I could no longer prevent my breath from freezing onto my glasses.
Until today. New personal best: -9 °F. I used a snorkel.
(The new helmet, which has a coverslip to close the vents, also helped. But it was mostly the snorkel, which kept my exhaled air safely behind my head. I probably looked like a steamship trundling down the street.)