I voted "zero to ten above" on the basis of what I have done, and what I would do if I HAD to. But that doesn't mean I'm going to rush out and do it. I rode for about 40 minutes at a time in temperatures of around 2 degrees F. three or four times and it wasn't too bad. But give me a choice between that and fifty degrees and I'd take fifty.

Can't get out of my mind that TV special I saw once about Siberia: how the people went out and had a picnic when it reached 29 degrees F. (roughly -4 deg. C.) one day.

So much depends on what you're acclimated to.