The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity. At sea level and at 15°C according to ISA (International Standard Atmosphere), air has a density of approximately 1.225 kg/m3 (0.0023769 slugs/ft3).
The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:
where ρ is the air density, p is absolute pressure, Rspecific is the specific gas constant for dry air, and T is absolute temperature.
The specific gas constant for dry air is 287.058 J/(kg·K) in SI units, and 53.35 (ft·lbf)/(lbm·R) in United States customary and Imperial units.
Therefore:
The following table illustrates the air density - temperature relationship at 1 atm or 101.325 kPa:
The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive.