What is the approximate weight of water vapor compared to dry air?

The approximate weight of water vapor compared to dry air is often cited as around 5/8 as much. This comparison is based on the molecular weights of water vapor and dry air. The molecular weight of dry air averages about 29 grams per mole, while the molecular weight of water (as a vapor) is about 18 grams per mole.

When considering the composition of air, an increase in water vapor can make the air less dense than completely dry air due to the lighter weight of the water vapor molecules. In a typical atmosphere, the volume of one mole of dry air can accommodate a certain volume of water vapor, meaning that when we assess their respective contributions, water vapor tends to weigh less in comparison.

This specific ratio reflects real atmospheric conditions, such as varying humidity levels. In drier conditions, air contains less water vapor, while in more humid conditions, the weight of the vapor can be significant, but it still remains lighter on average compared to an equivalent volume of dry air.

This understanding is crucial in fields like meteorology and aviation, where humidity and air density play significant roles in flight performance and weather assessment.