The important point is the relative density of the air.
Shrub canopy cover and density by species were converted to relative cover and relative density.
relative density index is one measurement used to quantify the densities of forest stands.
British Dictionary definitions for relative density
the ratio of the density of a substance to the density of a standard substance under specified conditions. For liquids and solids the standard is usually water at 4°C or some other specified temperature. For gases the standard is often air or hydrogen at the same temperature and pressure as the substance d See also specific gravity, vapour density