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

Vapor density refers to the weight of a gas compared to air, specifically the ratio of the gas's molecular weight to that of air. This concept is essential in firefighting and hazardous material response because it helps responders understand how a gas will behave in the environment. For instance, gases with a vapor density less than 1 will rise in air, while those with a vapor density greater than 1 will tend to settle. This information is critical for predicting how a gas might move and accumulate, which can influence strategy during a response to a gas leak or fire involving flammable gases.

Other terms such as specific gravity, weight ratio, and gas concentration, while related to physical properties of materials and substances, do not specifically address the weight comparison of a gas to air in the context used here. Specific gravity typically refers to the ratio of the density of a substance to the density of a reference substance (usually water), but may not focus exclusively on gases or their behavior in air.