I will ride as long as its above 15° F and there is sun. (Without sun, 20-25° F is my limit.) I have a lot of experience dressing for the cold via cross country skiing and what works for one works for the other. Gore waterproof shoe liners, keeping a light down jacket in a small pack or bar bag, and an extra pair of gloves really help. Also, never use a camelbak in the winter.
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