My first instinct was body temperature.
But then I thought that a 40’c wind might help sweat evaporation compared to still air, so still cool you down, even though the air is still warmer than body temperature. Then there’s probably humidity as a complicating factor.
Anybody have any ideas?
I think wind will always cool you down if you are properly hydrated, up to the temperature where still air would heat you up. There must be some maximum perspiration rate that would be a limiting factor, but people can spend hours in saunas at 175F if they have enough water to drink. So the point where skin and tissue are damaged from direct exposure to air may be the actual limiting factor.
ETA: Just checked on sauna temperatures. They can reach 100C. But I don’t know how long the body could tolerate that.
Similarly, I think in the limit where humidity is 100% and air velocity approaches infinity, it would be body temperature.
The only qualification to this is that at fantastically high velocities, air creates its own heat, as in spacecraft re-entry and meteors.
I can tell you this: in Arizona, we often get very hot, dry nights (temperatures over 100 °F). I used to drive a convertible, and at those temperatures, the breeze was not the least bit cooling!
If you place a water-saturated wick around the bulb of a thermometer and blow air over it, you get the “wet bulb temperature”. If that temperature is above body temperature, the wind will heat you up. The wet bulb temperature is related to the dew point.
If the humidity is 100%, the wet bulb temperature, dew point, and dry bulb temperature are all the same.
Thus, at 100% humidity, you need a temperature greater than body temperature. For lower humidity, it needs to be hotter.
Back in the '70s we lived in Palm Springs and drove a VW bus with no AC. In the heat it was necessary to drive with the windows up as freeway speeds were painfully hot with everything open.