Originally Posted by Carbonfiberboy

Firstly, as said just above, on a cool day you wan to be shivering standing at the ride start. Secondly, you don't need a heavy jacket. All you need is a wind jacket that will fit in the center pocket of your jersey. If it's cool, you should also have a wind vest either in that same pocket or a side jersey pocket. Your wind jacket should be no larger than a Naval orange when crumpled up. In the pouring rain, you stay warm in your wind jacket because you have adequate insulating clothes under it. Since you can't evaporate sweat to cool you when wearing a jacket, you use the slight amount of rain water that comes through the wind shell for that purpose. It's really quite simple, just takes a few realizations to adapt to the cycling mindset.