How cold is too cold

to run outside?  Lately, it has been around 13 degrees when I run outside.  I don't think it's bad at all as long as I dress accordingly and wear a mask.  Actually, it is beautiful outside when it is cold because ice crystals form on everything.  The other day I watched a coyote run about 600 yards through a field...in the opposite direction as me.  Either he was cold, hungry or trying to get a good cardio workout in.  I would have missed that had I let the cold air keep me inside.  I guess the key to feeling good running in the cold is dressing appropriately.