We don't get a lot of sub zero temps here in Columbus, but when we do it is frequently get 10-20mph winds with it. In the 15 years I've been a hot tub owner, 4F is the coldest I've soaked it. Our Sundance is currently set to 106 this time of year. Tonight it was 2F when I checked at 10pm with no wind. I opened up the hot tub and it was 106. Sitting in the tub in a totally still night, my hood was uncharacteristically quiet. The air around my head seemed to stay around 80f as there was no wind at all. I spent 30 minutes in the tub feeling totally comfortable. In that time the tub dropped to 104.
It was a really nice soak.