Many years ago I used to carry a pertex on every ride. It is completely windproof so if the temp drops- wearing it will stop the body temp going down. Problem is that it is not waterproof so last year I found the ideal jacket in a Hiking shop. Bit heavier than pertex- completely windproof and waterproof for all except the heaviest of rain. It does breath but that is due to the number of zippable vents fitted. Packs up to handsize so fits in a rear pocket easily.

It is ideal for those cool mornings when you know it will warm up and there is the possibility of rain. But if it is that warm and moisture arrives- then just get wet.