theorical humidity question

I have an typical 2100 Sqft. colonal home and I purchased a mid-sized humidifer. By mid-size, it's about the size of a big microwave.
In theory, does humidity seek it's own level?
In other words, if I place this humidifier at the bottom of the stairway between the main level and upper level, would the humidity in both levels rise at the same time?
Or would the level the humidifer is on get all the humidity and the upstairs be dryer?
I am running an experiment now with a few hygrometers, but it should take overnight to get the actual results. I wonder what the theorical answer is.
Thanks
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With forced air heat it will circulate rapidly, but the room the unit is in will be the highest. With hot water heat it will circulate but not completly, and slowly. The moisture is absorbed into wood and most everything.
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