The description in the question shows the density to measured is that of gases (upper atmosphere). The density of a gas can be described as the mass of the gas divided by the volume of the gas at a certain temperature and pressure. It's unit can be represented as lbm/ft³ or psi/ft.
NOTE:
"lbm" means "lean body mass" which is a unit for mass or weight and can be described as the pound mass
"psi" means "pound per square inch"; is also a unit for mass or weight
