Air is composed of 75.54% nitrogen, 23.10% oxygen, and 1.3% argon by mass. The mean molar mass is therefore

and air can be taken as a single diatomic gas with molar mass 29.0 g to a good degree of approximation. At standard temperature and pressure, the speed of sound is 331.1 m s^-1, and increases by roughly 0.61 m s^-1 for each degree celsius above 0 .

Cauchy's formula gives the index of refraction of air. The zenith optical depth of the Earth 's atmosphere from sea level is

for in m with a surface pressure of 1 atmosphere (Hansen and Travis 1974).

Atmospheric Pressure, Cauchy's Formula, Speed of Sound