The density of air, ρ (Greek: rho) (air density), is the mass per volume of Earth's atmosphere, and is a useful value in aeronautics. In the SI system it is measured as the number of kilograms of air in a cubic meter (kg/m^{3}). At sea level and at 20 °C dry air has a density of approximately 1.2 kg/m^{3}. varying with pressure and temperature. Air density and air pressure decrease with increasing altitude. Effects of temperature and pressure
The formula for the density of air is given by: where ρ is the air density, p is pressure, R is the gas constant, and T is temperature. The individual gas constant R for dry air is: Therefore:  At standard temperature and pressure (0 °C and 101.325 kPa), dry air has a density of ρ_{STP} = 1.293 kg/m^{3}).
 At standard ambient temperature and pressure (25 °C and 100 kPa) dry air has a density of ρ_{SATP} = 1.168 kg/m^{3}).
Effect of water vapor For moist air, the partial pressure of the water vapor must be considered as well. In this case, the density of the air is the sum of the density of the dry air and the density of the water vapor: The gas constant for water vapor is: Effects of altitude To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using the universal gas constant instead of the specific one:  sea level atmospheric pressure p_{0} = 101325 Pa (= 101325 (kg/m·s^{2}) = 1013.25 mbar or hPa = 101.325 kPa)
 sea level standard temperature T_{0} = 288.15 K
 Earthsurface gravitational acceleration g = 9.80665 m/s^{2}.
 dry adiabatic lapse rate L = −0.0065 K/m
 universal gas constant R = 8.31432 J/(mol·K)
 molecular weight of dry air M = 0.0289644 kg/mol
Temperature at altitude h metres above sea level is given by the following formula (only valid below the tropopause): The pressure at altitude h is given by: Density can then be calculated according to a molar form of the original formula: Importance of temperature The below table demonstrates that the properties of air change significantly with temperature. Table  speed of sound in air c, density of air ρ, acoustic impedance Z vs. temperature °C Impact of temperature  °C  c in m/s  ρ in kg/m³  Z in Pa·s/m   10  325.4  1.341  436.5   5  328.5  1.316  432.4  0  331.5  1.293  428.3  + 5  334.5  1.269  424.5  + 10  337.5  1.247  420.7  + 15  340.5  1.225  417.0  + 20  343.4  1.204  413.5  + 25  346.3  1.184  410.0  + 30  349.2  1.164  406.6 
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