I have been cycling year round in the Boston area for quite a few years. It gets pretty cold in the winter. For a while, I kept a log of every ride, and some of the things I recorded in the log were the temperature and what I wore. The idea was that I could look at the temperature and forecast in the morning, look at the cycling log, and know exactly what to wear each day.
Here is a summary of the log, listing what I like to wear in each temperature range.