At the same temperature, moist air can seem denser than dry air but the exact opposite is the case. Density is the mass divided by volume. Keeping volume constant, as mass increases the density will also increase. When air is more humid then there is a higher percentage of water vapor in the air. Water vapor is a relatively light gas since it has two Hydrogen atoms with only one Oxygen atom connected with the two Hydrogens. Hydrogen is the lightest element of all the elements. The Oxygen and Nitrogen that is breathed from the air is actually diatomic Oxygen and diatomic Nitrogen which means that two Oxygen atoms are connected and two Nitrogen atoms are connected. Two Oxygen or two Nitrogen atoms connected is heavier than only one Oxygen atom with two Hydrogens. Thus, Water vapor (H2O) is lighter than diatomic Oxygen (O2) and diatomic Oxygen (N2).

Why is it that more moist air can seem denser?

One reason is because the moisture in the air can make it a little more difficult to breath. The moisture can make the nose and lungs feel stuffy and this stuffy feeling can produce a feeling of heaviness.

Another reason is because sweat does not evaporate as efficiently in humid air as compared to dry air. This feeling is especially evident when working or working out outside. Sweat is produced to cool the body. The sweat is best able to cool the body when rapid evaporation can occur since evaporation is a cooling process. As the relative humidity gets closer to 100%, less moisture can be evaporated from the body. Thus, the humidity will contribute to the body feeling hotter and thus that can give a feeling of heaviness to the air.

A third reason is because air that is hotter has the capacity to contain more water vapor. When temperatures are hotter it can seem more uncomfortable outside, especially when the humidity is high. It is easiest to notice high humidity in the air when it is both warm and humid since warm/humid air will have much more moisture than cool/humid air. The uncomfortable feeling with hot and humid air can make it seem that the humid air is denser.

A final reason is because of thinking of water being liquid water instead of gaseous water. Liquid water is much denser than gaseous water thus it can seem that the air has to be denser when it contains water. However, the gaseous water is actually quite light and it is only the gasses in the air we are interested in when determining the density of air.

In conclusion, at the same temperature, humid air can feel like it must be denser but in actuality it is drier air that is denser.