To be honest clothes make me colder so I can undress and not feel as cold... I woukd say maybe 40s I have been naked outside... Not long though...
I have been outside nude and barefoot when the temp was 32 and snowing. I walk around the backyard but only for 5 or 10 minutes.
A wood-fired sauna out in the woods of New Hampshire in January. Stepping out into the snow nude, with steam billowing off my naked body. Initially, I didn't even feel the cold.
Not sure exactly. I had a cabin in the woods, with a wood burning stove I used to heat the place. Outside I had wood to bring in. I would go outside to get wood and sometimes split some wood to use. It could be snowing, or snow on the ground. Temps in the teens were common. If just getting wood, only a minute or two, but if splitting some, I could have been out there for 10 minutes or more. In better weather with little wind, I would be out working in the 50s.
I have challenged myself to go out in below zero weather. It's perfectly fine if it is calm out. I wouldn't try it if the wind was blowing though.
One morning went swimming in the sea here and checked the temperatures - air was 35 f, and the sea was 45 f. Wasn't in the sea long (even though it felt warm compared to the air!) but it was bracing. Lovely. And yes, was naked.
When I lived in the U.P. my landlords had a sauna that we would walk out to. Subzero temps were not uncommon.
Winter before last it got really cold in the DFW area and I wanted to test my limits. It was 4 degrees one night and I went into the backyard naked and stayed for 10 minutes. It was around 1:00am so little chance of being spotted by neighbors, but I remained as quiet as possible, just in case.