I also live in a very humid climate (for example, my outdoor hygrometer is reading 93% right now), and I use sugar rolls as my go-to method of mite checks. I doubt that the humidity would have a significant effect on the roll unless your sugar is clumping as a result of the moisture. As long as your sugar is nice and powdery, I think it should be okay. Sugar rolls are not as accurate as alcohol washes, plain and simple, but I don't like to kill bees every few weeks do to a mite check I so prefer the sugar roll. To help compensate for the greater margin of error in a sugar roll, I multiply the number of mites in the roll by 1.3, on the recommendation of a very experienced member here, and I've had no trouble with accuracy since using that method.