I think my limit is if my nose hairs freeze, then there's no way I'm running outside. That's just me. Kudos to those of you who are super motivated to run in the extreme cold. I have to wear layers of tights to run in the 20's and heavy tights to run in the 30s. I ran in 17 degree weather about a month ago and hated life the whole way, OMG that was the worst run ever. I just couldn't get warmed up EVER. I'd say my cutoff is about 20 degrees OR my nose hair rule of course, whichever comes first.