It was 12 degrees when I woke at 5am to hit the street. NOPE! I went to the gym. Below 20 degrees and you can count me out. Typically if below 20, that means real feel of 0-5 degrees due to the wind. No thanks, love my running, but it was warm in the gym and I got a better workout.
Actually below about 25 degrees and running outside isnt good for you or your body. Learned this last year after nearly getting hypothermia on a 15 mile training run. My doc said to stop being stupid and get on the treadmill.