It's too cold if everyone else is wearing ski gear. Even if you run warm, protect your skin on cold, snowy days with tights.
It's ok to have goose bumps at first. Being cool when you start will help keep you from sweating too much, which can lead to abruptly cooling down. It also will prevent your heart rate from jumping too quickly. Even though it means a cold start, I wear shorts down to about 44 degrees (~4 C).
- A little chilly throughout the run? Try adding gloves and/or a warmer hat.
- Cold with goose bumps, even at the end? Move up to tights.
- Totally Goldilocks (aka Just right!)? You've found a sweet spot and can adjust accordingly as temps rise and fall.