I think the coldest run I've ever done was about 9 F - I had a long-sleeved icebreaker top under my ibex hoodie dress. No jacket. If it's in the 20s, I have a Brooks vest that I'll wear over a wool long-sleeved top. This year I do have a wool zip-up hoodie I might wear if it drops below 10 again, but we'll have to see. I only run when it's that cold if other people will run with me!