Question

Give reasons for the following:

Humid air is lighter than dry air.

Answer 1

Humid air is lighter than dry air because of the difference in the molecular weight of the gases involved.

Dry air is made up mostly of nitrogen (N₂) and oxygen (O₂), which have relatively higher molecular weights,28 and 32, respectively. However, humid air contains water vapor (H₂O), which has a lower molecular weight of 18.

When water vapor replaces some of the nitrogen and oxygen molecules in the air, the average molecular weight of the air decreases, making humid air less dense (lighter) than dry air at the same temperature and pressure. So, despite the moisture, humid air rises more easily than dry air because it is less dense.