How does humidity affect the density of air?

Humidity affects the density of air because it is related to the composition of the air mixture. Dry air is primarily composed of nitrogen (N₂) and oxygen (O₂), which are relatively heavy gases. When water vapor (H₂O) is introduced into the air, it takes the place of some of the nitrogen and oxygen molecules. Since water vapor has a lower molecular weight (about 18 g/mol) compared to nitrogen and oxygen (approximately 28 g/mol and 32 g/mol, respectively), the overall density of the air decreases when humidity increases.

This means that humid air is less dense than dry air at the same temperature and pressure because the lighter water vapor molecules replace heavier gas molecules, ultimately leading to a reduction in the mass of the air for a given volume. Therefore, increased humidity results in lower air density, making the answer to how humidity affects the density of air clear.