International Standard Atmosphere
The density of air or atmospheric density, denoted "ρ" (Greek: rho), is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature and humidity. At 101.325kPa (abs) and 15°C, air has a density of approximately 1.225kg/m3 (or 0.00237slug/ft3), about 1/1000 that of water according to ISA (International Standard Atmosphere).