If it's really hot and humid (most of the summer), our sensei has us start off doing really energetic stuff like iriminage and then tapers off later on in the class. Forward roll techniques in lines are very welcome! Or bokken/jo work. Or getting-uke's-balance exercises that don't require strenuous ukemi. Other than that, he doesn't seem to change much of the class due to heat. Our dojo is on the third floor of a non-air-conditioned old industrial building, so it gets really stifling sometimes. You don't even want to put that heavy gi on. Bleuch. I'm in TX right now visiting my family. The dojo here isn't AC-ed either..............my mom said she was sweating just watching class, she couldn't imagine how WE felt flying around the room for two hours!
