Naval training center, San Diego , Aka Boot Camp, in December is not warm ,
if you are in your cotton , issue , dungarees..
or at least so, in 1966.. my sample.
I have a friend who lives in San Diego. One day a few Aprils ago, she was whining on the phone how cold it was. I asked her how cold it had been. She said it went down in the 40s one night. I was barely able to sympathize, since here in Lansing it hadn't been that warm for at least five months. At least I didn't laugh at her, the way my friends in the Upper Peninsula laugh at me when I complain about the cold weather in Lower Michigan.
It's all about what you're used to...and how you're dressed.
Around -25f (-31c), with about a 15mph north wind (and I had to go north), so around a -55f windchill (figuring a 5mph average speed). That's 5 miles, or over an hour in that as I slow down the colder it is. This was 2-3 years ago. Even last winter saw a full week of -5 to -10 days. Really need a face mask as my eyes normally freeze shut on those days.
Interesting fact: The colder the air, the denser it is. The greater density is significant even at bicycle speeds. This means you work harder in cold weather to go the same speed. Or you do the same intensity of work, but your speed will be a little slower.
My low is somewhere in the 0 to -5F range, depending on which thermometer you trust.
The good thing about the consistently cool weather in this region is that I've gotten desensitized to that temperature range. There are appropriate clothing items, and I make sure that they are on my body before going out on the bike.
Living in the Dakota's it gets very cold here.
My personal "best" has been -35F with a windchill at about -45F with 30-35mph winds.
I know how to dress for it but with the wind and the icy ruts, it was not fun, just did it to do it and only about 12 miles...