The molecular weight of water in comparison to that of oxygen and nitrogen the density of moist air versus that of completely dry air.

The molecular weight of water compared to oxygen and nitrogen is less, inverse to humid air versus completely dry air.

Explanation:

In the case of water, the molecular weight is approximately 12, while that of nitrogen is 14 and oxygen is 16.

When in the atmosphere the column of water vapor increases

(ie there is a more humid environment) the density also increases in relation to a dry atmosphere with little water column in the environment.