12*F this morning. I ride in below-zero temps every year. My record is -25*F.

Below about -10, you absolutely need really serious face gear. I use a balaclava plus a neoprene face mask (buy them for motorcycle riders, but you have to open up the mouth area to get more air - motorcycle riders don't need that much air), plus a pair of goggles. You need to have NO exposed skin on your face.