In Chicago: with a suit, yes, thats what that liner is for. Without a suit underneath, I wouldn't do it, and not just for temperature or weather reasons, but fit. I have a 46R overcoat that fits great over a suit, but man those armholes look awful without one underneath, much too large. If you want to wear a coat without a jacket underneath, make sure the coat is cut for it, or size down to make it work. You'll find out quickly if you need extra layers with any coat.