Which statement about air density is correct?

The specific gravity of air is a measure that compares the density of air to a reference substance, typically dry air at a specific temperature and pressure. The statement that the specific gravity of air is always 1 is correct because specific gravity is defined based on a ratio. Since air is referenced against itself for this measurement, its specific gravity will consistently yield a value of 1.

This concept holds true regardless of changes in air temperature, altitude, or pressure. However, while the density of air does change due to variations in humidity or temperature, the specific gravity remains foundationally tied to its own density. As a result, it will not change simply because these extraneous factors fluctuate.

In terms of the other options, while humidity does influence the density of air due to the presence of water vapor, the specific gravity remains defined against itself, thus not fluctuating in the same way. Moreover, dry air is generally denser than humid air, contradicting the assertion that dry air is heavier. Comparatively, most gases have densities lower than that of air, making the notion that air density is higher than most gases incorrect.