When I lived in a locale that had an actual summer, most of my riding partners and I put on tights when the temperature was below 75F. Now that I live where 75F is a warm summer day, I wear less clothing (and I have less muscle but more adipose). I'll wear shorts down to somewhere between 55F and 50F. Knickers come into play down to about 45-40F. Below that it's tights. If it's in the low 30s, then tights may be worn over knickers.

My key piece of cool weather gear is my thin wool cap. Having a warm head (and warm ears) really makes a difference, especially on descents. I also love my booties and my glove collection. Keeping those tiny bits warm really takes the edge off of otherwise chilling winds.