I just set up my tank. I have a hydrometer in there, along with substrate, two hides, and an empty water bowl (no snake yet).
Just that alone the humidity is at 50%, when the hydrometer wasn't in the tank it read 40%. So that means the substrate, which is kinda dampish, contributes 10% humidity. Now if I had a lamp beaming down on that substrate, it would dry it out, and make the humidity drop. Not a whole bunch, being that only one side would dry out, but enough to make a difference.