I was the assistant scout master in a Boy Scout troop my son was in. One January, we had a camp-out scheduled, and the forecast was for -20 F. I had a sleeping bag rated at that level, but none of the scouts was even close to prepared, except for my son. It was a cold night, as I went around to check on the kids repeatedly. My batteries failed, but fortunately, I had some lithiums in my stash, which go lower in temps.