I remember reading this article in Runners World that answered the above question by saying never. “Too cold” for running didn’t exist as long as a runner dressed well, planned well AND didn’t stop running outdoors until the runner was back some place warm when they began to cool down.
It was the cool down that would kill a runner, reported the article, if I remember correctly.
Tuesday night I ran hill repeats starting out at 16 degrees, feeling like 5 degrees outside (see image above). I was triple layered and chilly the first mile, but toasty after that.
As a matter of fact, in my three years of spring marathon training I believe I ran once in sub-zero temperatures, with a wind chill of something like 9 below zero. I would honestly say my run in icy conditions was more treacherous than that run.
What’s your threshold for temperatures before you convince yourself to run on the dreadmill?