Nothing like a fresh winter morning to roll out. I don't have a choice, I use my bike for everything, so my "comfort zone" is before I reach a point where I can't make progress for some reason. I dress for the temperature, and I don't mind the cold so much.
Heat, on the other hand will soon make me run for cover. I carry a silverized umbrella in summer for this reason: So I can stop and be in the shade regardless of where I am. I'm very sensitive to too much heat, so I'm more likely to stay in or postpone a trip to later in the day or to the next morning because it is too hot than any other reason.