<P> The calculated ratio of the masses of each present in the sample is then compared to a standard, which can yield information about the temperature at which the sample was formed - see Proxy (climate) for details . </P> <P> O is two neutrons heavier than O and causes the water molecule in which it occurs to be heavier by that amount . The addition of more energy is required to vaporize H O than H O, and H O liberates more energy when it condenses . In addition, H O tends to diffuse more rapidly . </P> <P> Because H O requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H O, and the residual liquid is enriched in H O. When water vapor condenses into liquid, H O preferentially enters the liquid, while H O is concentrated in the remaining vapor . </P> <P> As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation . The precipitation removes H O, leaving progressively more H O - rich water vapor . This distillation process causes precipitation to have lower O / O as the temperature decreases . Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures . </P>

What is the ratio of hydrogen to oxygen in the oceans