What does "specific gravity" refer to in terms of gas?

Specific gravity in the context of gas refers to the weight of the gas compared to the weight of air. This concept is crucial as it helps in determining whether a gas will rise or fall in the atmosphere based on its density relative to air. If the specific gravity of a gas is less than 1, it will be lighter than air and tend to rise. Conversely, if it is greater than 1, the gas will be heavier than air and will tend to settle. Understanding the specific gravity of various gases is especially important in applications such as ventilation, gas detection, and safety measures in environments where different gases may be present.

The other options relate to different physical characteristics: temperature, volume, and pressure, which do not accurately define the concept of specific gravity in the context of gases.