18 degrees and blowing snow this AM = fat bike, ski helmet & goggles, insulated boots, gauntlet gloves, balaclava, polypro layer + softshell on both top and bottom. I was sweating 15 minutes into the ride given the 5 inches of crusty, slippery snow we were plowing through and the 8 psi in my tires. 30 minutes into the ride, I removed the goggles and opened the vents on my jacket. With this getup, I've ridden down to 0 degrees without issue.
In other words, I laugh heartily at some of the above "cold weather riding" in dry weather in the 40s. Those are perfect riding conditions.