The quotes around the words heavy air may be the giveaway to the answer to this question. If you asked random people they would say the air feels heavier on a very humid day. While we may be sluggish on days like this, it’s not the air that is weighing us down. Let’s break down the science of the atmosphere and solve this question.
First of all, let’s talk about the composition of our atmosphere. 99% of the atmosphere is made up of two gasses, nitrogen and oxygen. There is less than a percent of Argon while the rest is filled with trace gasses, including things like carbon dioxide, methane and water vapor.
The two main gasses of our atmosphere are quite heavy. Both of these gasses are heavy. Nitrogen has an atomic weight of 14 and oxygen 8. Both nitrogen and oxygen are diatomic which means that the molecules in our atmosphere are made of two atoms each. These atoms together have an atomic weight of 28 and 16 respectively for nitrogen and oxygen. Water vapor is a molecule which contains two atoms of hydrogen and one of oxygen.
Hydrogen is the lightest gas with an atomic weight of 1. So a water vapor molecule has a total atomic weight of 10, which is much lighter than the other main gasses in the atmospheres. Therefore, the more water vapor in the air, the lighter the air.
Therefore, if everything else in the atmosphere is the same, a golf ball will be hit farther on a humid day rather than a dry day. Same goes for a baseball. Though wind, pressure and other factors will also play a factor more than the humidity.