Acclimatization ... although the master keeps the studio warm, too. He actually turned the A/C on for the first time yesterday (another 90+ degree day). Still got drenched doing Tai Chi.
I'm going from A/C (bedroom/office) into non-A/C (rest of house), and I'm breaking a sweat just going to the kitchen or bathroom.
There is a pretty famous gym in texas has no a/c suppose to get hot as fack in there lot of pro's train there something to be said for training in high heat/humidty. http://www.metroflexgym.com/