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I understand that humid air is generally lighter than dry air, as water vapor molecules are lighter than nitrogen or oxygen molecules.

But... my issue is that when I test RH in a closed container (a cigar humidor), the RH is always 4 - 6% higher at the bottom.

I have verified my hygrometers are calibrated and tested this using multiple humidors and multiple hygrometers to ensure accuracy.

I live in Colorado and figured the high altitude may have something to do with it. Or is it that in a closed container, the displaced nitrogen and/or oxygen molecules have no where to escape.

Not sure why this would be.

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    $\begingroup$ Is there a temperature difference in the container from top to bottom? $\endgroup$
    – tpg2114
    Jun 1, 2017 at 20:43
  • $\begingroup$ I think so, even though it is a totally sealed environment. I'll do further testing on that asap. $\endgroup$
    – jordanG
    Jun 2, 2017 at 3:04
  • $\begingroup$ It's possible the humidor is, say, sitting on a table. In that case, heat transfer to the table is larger than that to the air above and a gradient is established. Over a long period of time, it should reach equilibrium and the humidity would probably be very close to equal. But depending on the properties of everything, that could take awhile. Try it and report back! $\endgroup$
    – tpg2114
    Jun 2, 2017 at 3:19
  • $\begingroup$ Okay so I've done the temp tests, consistently showing 3 degrees warmer at top. Humidors are not on table but set on the ground of my upstairs level and basement level. I don't think this will stabilize out, as the humidors have been set up like this for years. $\endgroup$
    – jordanG
    Jun 2, 2017 at 22:11

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So, based on the comments to the question, the answer is because of a temperature gradient within the humidor. The reason this is important is that relative humidity is based on the partial pressure of water vapor to the saturation pressure. The saturation pressure of water decreases with temperature, so colder air can hold less water vapor.

The question now -- is the total vapor content different? You would have to plug in the two temperatures and relative humidity measured to see what the partial pressure of water vapor is at both conditions. From there, you can see if you actually have more water in the air at the bottom vs the top, or if the total amount of water is the same. Relative humidity is, as the name implies, relative to the conditions.

Lastly, if it is something you are concerned about or if the air at the bottom does actually contain more water, you might want to look at adding some electric fans to provide some circulation. That will even out any stratification that occurs.

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  • $\begingroup$ That's a bit over my head but I'll try my best to make that work, thanks $\endgroup$
    – jordanG
    Aug 29, 2017 at 20:10

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