We mt bike all winter here in the North East U.S. For me, below 30 is too cold for the road bike, though I motivated and did a lot of road rides this winter when it was mid 30’s (F). As long as it wasn’t too windy, it was tolerable. 30’s F down to about 20F is good mt bike temps if we have no snow. The ground freezes and riding conditions are good as long as there’s no much. Then we skip. Unlike the south, it doesn’t get nasty hot here, maybe on occasion to 100F. Of you get out early in the AM it’s mid 80s and also tolerable. I would not want to live in the south where it’s mid. 90’s a lot plus very humid. Riding in that would suck.