I think it's generally accepted heat and humidity are enemies of primers and powder. That said, they're still pretty resilient!
Hmm I've generally not been concerned as the temps in Western WA are usually pretty mild. What are you thoughts of buying an old mini-fridge or something and just putting it in there (with it unplugged obviously)? I feel like the insulation should help any extreme fluctuations and I don't have to explain to my wife why there's 40lbs of powder in the coat closet..