20F with 16mph headwind and snowing, but I was dressed for it. Except I had to take off the safety glasses I normally use when it's cold because they caked up with snow so I couldn't see with them on.

There was already 5 inches of snow on the ground but my studded tires worked like champs, though there were a half dozen times I had to put a foot down in order not to fall when my front tire wanted to go
sideways. It was slow and tedious going at times -- took about and hour and twenty minutes to go the 10 miles.

But all-in-all this was the most fun I've had on my commute in a long, long time.