Air Density And Water Vapor
Air density describes the density of a cubic foot of air in earth's atmosphere. This value changes depending on the altitude because this higher up you are, the less atmosphere you have above you to press down. It also changes based on temperature, which is the measurement of the movement of molecules. This dives into thermal dynamics gets quite complicated but basically, the more molecules move around, the less molecules are present and the higher the temperature gets. Therefore, measuring air density is a matter of measuring the pressure and temperature.
Factoring water vapor into your reading
In an absolute state with 0% humidity, air density is just a measure of pressure and temperature. However, that does not account for other items in the atmosphere that also take up space within that cubic foot of atmosphere. Humidity or water vapor is one of those factors because it takes up space and displaces air. In engine tuning, we want to measure the amount of oxygen in the air so it is important to factor in anything that displaces that oxygen when doing measurements.
Old School Air Density
Air density gauges from 20-25 years ago did not factor in water vapor. They were simply a calculation based on temperature and pressure. This provided the density of the air in a presumed completely dry environment and did not factor in the displacement of air by water vapor.
If you raced in a place without much change in humidity, air density didn't necessarily need to include water vapor since the only factors changing will be pressure and temperature. If you raced somewhere where humidity fluctuations happened frequently, tracking water vapor as part of your air density calculation was critical.
Using this site
In the tracks section of this website, we provide air density as a percentage of total volume of a cubic foot of air - both with and without water vapor. Both values are provided for comparison purposes as another way to evaluate how much water vapor is in the air. Generally, you will want to do your tuning using the value with water vapor. That value will be closer to the actual air density including all materials in the atmosphere.