I need my knees covered below 65 F if it's dark or cloudy and below 60 if it's sunny. I can do knee warmers and shorts down to 50 or so, but sometimes my butt gets cold and I'll wear light tights. It was often 40-50 in the AM when I used to commute and 70-80 on the way home. I just stuffed my tights in my backpack.
Today was beautiful and 64 degrees and my legs were freezing for the first half of my ride. I was in shorts, because I knew I would feel a bit overheated eventually with knee warmers.