Which term refers to the weight of a gas compared to air?

The term that refers to the weight of a gas compared to air is vapour density. Vapour density is defined as the mass of a certain volume of gas compared to the mass of an equal volume of air under the same conditions of temperature and pressure. This measurement is crucial in various applications, including safety and hazard assessment in firefighting and hazardous material handling, as it helps to identify whether a gas will rise or settle in the atmosphere.

Specific gravity, on the other hand, is a broader term that compares the density of a substance (whether solid, liquid, or gas) to the density of water at a specific temperature. While specific gravity may apply to gases, it is not specifically designed for comparing gases to air. Vapour gravity and specific density are not standard terms used in this context and would not be applicable for defining the relationship between a gas and air. Understanding vapour density is pivotal for firefighters in assessing risks associated with various gases in different environments.