Agree with everyone else but would add that in addition if your feet actually start to go dumb due to the cold temps that your form can suffer without you knowing it. Even if my feet have been fine running in the 30's, I've noticed if its a few degrees colder to where I start having some numbness here and there that my soles tend to feel like they are on fire the rest of the day. That is on my rough road, probably not as much of an issue on smooth asphalt. But if they go numb at all I must be abrading them without realizing it on the rougher surfaces. Numb feet are dumb feet, as they say.