We usually do the G-banding in A/C lab, so that there is not much fluctuation in humidity and temperature. I dont think humidity affects this process much. Fresh reagents, and appropriate concentration of trypsin would be sufficient to give good results.
When you have low humidity the spread of chromosome in the slides could be affected, because the evaporation of fixative is too fast and them you have remains of cytoplasm around chromosomes. It difficult the action of tripsin and you can not get good G banding. this is my point of view.