I think for some it is "ambient" as some mention misting to raise the humidity, but since the humidity is concentrated in the substrate and no one keeps snakes that
can fly -I'm pretty sure?- I think the biggest concern IS for the humidity reading on the floor. Are you asking because you're using some type of overhead heat
that drastically lowers the humidity near the top of the enclosure? Good question, I can't recall anyone asking that before. If we did a poll you might find the answers
that people give are rather divided, but as long as your humidity is good near the floor I wouldn't worry about it. Especially since you're talking about a ball python, not a Brazilian rainbow boa, etc.