27 g/L). At the same temperature, a column of dry air will be denser or heavier than a column of air containing any water vapor, the molar mass of diatomic Water vapor, water vapour, or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor is transparent, like most constituents of the atmosphere. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and rem The molar mass of water is 18.02 g/mol, as calculated from the sum of the atomic masses of its constituent atoms. The average molar mass of air (approx. 78% nitrogen (N2); 21% oxygen (O2); 1% other gases) is 28.57 g/mol 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 (32 °F)) the density will go down to 28.51 g/mol. STP conditions imply a temperature of 0 °C (32 °F), at which the ability of water to become vapor is very restricted. Its concentration in air is very low at 0 °C (32 °F). 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 (212 °F). However the difference in densities between air and water vapor would still exist (0.598 g/L vs. 1.27 g/L).