Oxygen and Nitrogen are pulled down by the gravitational potential, and diffuse the back upwards die to entropy the atmosphere is a giant sedimentation equilibrium experiment. Assuming the earth's gravitational acceleration is uniform and the 9.802 m s-2, and calculates the pressure of nitrogen and oxygen at the elevation of a transatlantic airplane flight (10 km) when the average temperature of the atmosphere is -20°C, the density of the air is negligible, and the pressure of O2 and N2 at sea-level are 0.20 and 0.79 bar respectively.