Can Humid air ever be heavier than dry air? 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.
 A: 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. 
