I have found that setting the tub temperature to anything close or just below body temperature works better than you might think. Yes, it still feels a bit warm going in, but after just a few minutes you realize it is really very refreshing. In fact, if you get it down into the eighties (swimming pool temp) it will feel uncomfortably cold after just a few minutes. Remember, you are sitting still, not doing laps.
Most tubs can be set to temps in the mid nineties, and that's all it takes. If you have had your tub for any length of time, you will realize that one or two degrees of temp change can make a big difference - this is slightly less so at the lower temperatures, but still very adjustable!
Also, some folks will lower the water lever in warm weather so more of you is exposed in the event of any nice evening breezes. Again, doesn't take much!
