Went to Navy boot camp in Orlando in August. For intensive training(punishment) we had to exercise outside in our raincoats.
It wasn't yoga but it was hot.
Remember when people jogged in black trash bags? No cooling from sweating. I guess a little heat such as in a yoga class would be OK but not 100 degrees.
"Hot yoga is performed in a room heated to between 80 degrees and 105 degrees Fahrenheit and may be more physically rigorous than regular yoga, making practitioners susceptible to dehydration and muscle injuries."
I'd say 80 degrees might be OK. Might be good if you are training for the Peachtree Road Race.