What is Vapor Density and Why Is It Important? Vapor Density of Air and Water (PDF)

Vapor density is a fundamental property of gases that describes how heavy or light gas is compared to the ambient air. It is commonly expressed as a ratio relative to the density of dry air at the same temperature and pressure. For example, the vapor density of a gas compared to air is less than 1 if it is lighter than air, and greater than 1 if it is heavier. As the vapor density is the ratio of two densities, it is a dimensionless quantity.