We know that for a closed system, isobaric heating decreases the humidity and isothermal compression increases the humidity. But assuming that we start with the volume completely filled with humid air, is it not true that any increase in temperature must also increase the pressure? If so, doesn't that imply that there is no change in temperature that could possibly decrease the humidity of such a system? Doesn't that then imply that there is no way to decrease the humidity of such a system? Of course, no molecules are entering or leaving, and the pressure cannot decrease since it starts with the volume completely filled, right? So I think that exhausts everything.