Humid air vs. dry air- which is heavier?

On these hot, sticky summer days it may seem like the air is heavy when compared to a day with lower humidity but actually the opposite is true. Humid air is actually lighter than dry air.

Dry air is made up of diatomic Oxygen and diatomic Nitrogen which means that two Oxygen atoms are connected and two Nitrogen atoms are connected. However, when the air is more humid, there is a higher amount of water vapor in the air. Water vapor is a fairly light gas since it has two Hydrogen atoms and only one Oxygen atom. Hydrogen is the lightest element of all the elements so the molecular weight of water is lower than the molecular weight of dry air.

So any mixture of dry air with water vapor will weigh less than pure dry air. The more water vapor in the air, the smaller it’s molecular weight will be and therefore it will be lighter.

There are reasons why humid air seems heavier. One reason is that the moisture in the air can make the nose and lungs feel stuffy, producing a feeling of heaviness. Another reason is because when we sweat, it’s able to cool the body more efficiently when it’s evaporated from the skin since evaporation is a cooling process. But when there is more moisture in the air, less sweat can be evaporated from the body, making it feel hotter, producing a feeling of heaviness to the air.