Water vapor and dry air density calculations at 0 °C:
* The molar mass of water is , as calculated from the sum of the atomic masses of its constituent atoms.
* The average molar mass of air (approx. 78% nitrogen, N; 21% oxygen, O; 1% other gases) is  at standard temperature and pressure (STP).
* Obeying Avogadro's Law and the ideal gas law, moist air will have a lower density than dry air. At max. saturation (i. e. rel. humidity = 100% at 0 °C) the density will go down to 28.51 g/mol.
* STP conditions imply a temperature of 0 °C, at which the ability of water to become vapor is very restricted. Its concentration in air is very low at 0 °C. The red line on the chart to the right is the maximum concentration of water vapor expected for a given temperature. The water vapor concentration increases significantly as the temperature rises, approaching 100% (steam, pure water vapor) at 100 °C. However the difference in densities between air and water vapor would still exist (0.598 vs. 1.27 g/L).