just a thought, no real science to back this up:
Workout intensity is what drives adaptive responses, and it is a relative thing. If heat and humidity increase the intensity, it could be that the hormetic response remains the same, even though the weights you lift are lower.
So exercise until you feel the same kind of fatigue (+/- same kind of training intensity). Keith Norris at http://theorytopractice.wordpress.com/ talks about this kind of thing and calls it autoregulation (although I don't remember him using this principle with heat)
(Edit: speaking of the devil, Keith Norris just posted this about autoregulation, good stuff!):