when the plants are flowering the humidity will effect the thc production, but not in the vegatative phase.

more important than the humidity is the water in the soil/plants. one reason the tricombs are produced is to protect the buds/seeds from drought, so if the soil is very dry when you harvest the bud will have more thc. tricombs are also produced to block uv rays, so it is best to harvest just before the lights will come on.

i like to cut off all water a week or so before harvest to maximize thc on the plant. you must be careful not to let them wilt though.